The Lone Ranger
11-28-2008, 04:20 AM
An Introduction to Zoology
Chapter Nine: The Radiata: Phylum Cnidaria:
The Eumetazoans:
The eumetazoans, as you recall, are the “true animals.” Unlike the parazoans (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=8#content_start), eumetazoan animals possess true tissues (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start), and most of them have organs (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start) as well. Also unlike the parazoans, all of the eumetazoans have some degree of body symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=2#content_start), and all eumetazoans undergo embryonic development (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start).
Another thing that distinguishes the eumetazoans is that most of them have two types of tissues that are found in no other living organisms. These tissue types that are unique to eumetazoan animals are neural (nervous) tissue (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=8#content_start) and muscular tissue (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=9#content_start). In most eumetazoans, muscle tissue is attached to some sort of supportive skeleton – either an internal endoskeleton or an external exoskeleton.
The Radiata and the Bilateria:
The eumetazoans, as you no-doubt recall, are divided into two major taxa: the Radiata and the Bilateria. The radiates are named for the fact that most of them are radially-symmetrical (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start). Radiate animals are diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5), meaning that their bodies develop from only two embryonic tissue layers. Since they have no mesodermal tissue, radiates have no mesodermal organs, and so have relatively simple bodies.
This does not mean that radiates have no organs; some of them do. Nonetheless, the body of a radiate is typically much less complex than is the body of a bilaterian.
By contrast with the radiates, all of the Bilateria are triploblastic, meaning that their bodies develop from three embryonic tissue layers. Because these animals possess mesodermal organs, most of them have much more complex bodies than do radiate animals. As the name for the taxon implies, the great majority of the bilaterians have bilateral symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=4#content_start), at least embryonically.
[b]The Cnidaria:
There are two phyla of radiate animals, the Phylum Cnidaria and the Phylum Ctenophora. In this chapter, we’ll deal with the Cnidaria. The cnidarians include such familiar animals as jellies (“jellyfish”), sea anemones, and corals.
Though cnidarians [the “c” is silent] are more complex than are poriferans (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=11#content_start), they are nonetheless very simple animals. Most cnidarians are sessile; those that are not, such as jellies, can swim only very slowly. So cnidarians cannot chase down their prey. All in all, you’d think these soft-bodied, slow-moving, brainless animals would be easy prey for other animals.
Surprisingly, though, cnidarians are quite adept predators, and they frequently capture and eat animals that are far faster, more complex, and more intelligent than themselves. Their “secret weapon” is that they have tentacles armed with a unique structure known as a nematocyst.
A nematocyst, when stimulated, rapidly ejects a barbed thread that can penetrate flesh. When it penetrates the flesh of a victim, the thread injects a paralyzing toxin. So any small animal that’s unlucky-enough to brush up against a cnidarian will immediately be speared by hundreds or even thousands of nematocysts and quickly immobilized. The hapless victim is then pulled into the cnidarian’s mouth and digestive cavity.
http://www.freethought-forum.com/forum/gallery/files/5/0/anemonefood.jpg
This sea anemone (Phylum Cnidaria, Class Anthozoa)
has captured a fish (Phylum Chordata), and is eating it.
[b]Cnidarian Characteristics:
The members of the Phylum Cnidaria share a number of characteristics, but their defining characteristic is that they possess specialized cells known as cnidocytes. In most cases, the cnidocytes contain stinging organelles known as nematocysts.
Cnidarians are entirely aquatic. Most species live in the ocean, though there are some freshwater species.
Cnidarians have radial symmetry. As such, a cnidarian has no anterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) end and no posterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) end, and has no head. The side on which the mouth is located is known as the oral surface and the opposite side is known as the aboral surface.
Cnidarians have two basic body plans. A polyp is usually sessile and attached to a hard surface, and the oral surface faces upward. A medusa is free-swimming, and the oral surface faces downward.
Cnidarians are diploblastic, and an adult’s body therefore consists of only two tissue layers. The outer tissue layer is the epidermis, and it is derived from the embryonic ectoderm. The inner tissue layer is the gastrodermis, and it is derived from the embryonic endoderm.
A non-living, jelly-like substance known as mesoglea fills the space between the epidermis and the gastrodermis. (There may be living cells embedded within it.) The mesoglea helps provide buoyancy, and is the “jelly” of a “jellyfish.”
A cnidarian has an incomplete gut, meaning that there is only a single opening into it. This opening serves as both the mouth and the anus. The gut of a cnidarian is known as its gastrovascular cavity. Food is digested in the gastrovascular cavity, then absorbed by the surrounding cells. Undigestible matter is expelled through the mouth/anus.
Extensions of the body wall known as tentacles surround the mouth in most species. Cnidocytes at the tips of the tentacles allow the animals to capture prey. The tentacles push captured prey into the cnidarian’s gastrovascular cavity for digestion.
These are the simplest animals with muscle cells, though they have no true muscles. Contractions of muscle cells allow the animals to move, and some can even swim. While their movements are slow and clumsy compared to those of, say, vertebrates, most cnidarians are far more mobile than are poriferans.
Cnidarians are also the simplest animals with well-developed sense organs. Sense organs found in cnidarians include statocysts that respond to gravity, allowing the animals to distinguish “up” from “down.” Many cnidarians have light-sensitive organs known as ocelli, and some even have fully-functional eyes.
Cnidarians are the simplest animals with neurons. While cnidarians don’t have true nerves, much less brains, the neurons of a cnidarian are sometimes organized into a nerve net that allows the animal to coordinate its movements.
Cnidarians are capable of both sexual (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start) and asexual reproduction (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start). Many species are capable of asexual reproduction through budding (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=3#content_start) or fission (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=3#content_start), and these species often form colonies of cloned individuals. Other species reproduce sexually. The larval (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start) form of a cnidarian is very distinctive and is known as a planula.
A cnidarian neither needs nor possesses a respiratory system. The function of a respiratory system is to transport oxygen to an animal’s body cells in order to support cellular respiration, and to transport the poisonous CO2 produced as a waste product of cellular respiration out of the animal’s body. The reason that a cnidarian doesn’t need a respiratory system is because every cell in its body is in direct contact with the surrounding water. This means that oxygen can diffuse directly from the water and into a cnidarian’s body cells. Similarly, the CO2 produced in the animal’s cells is easily disposed of, because it diffuses directly into the surrounding water and is then carried away by currents.
For the same reason that it doesn’t need a respiratory system, a cnidarian neither needs nor possesses an excretory system. The function of an excretory system is to remove poisonous metabolic waste products such as urea and ammonia from an organism’s body. Since the metabolic wastes produced by a cnidarian can simply diffuse away into the surrounding water, a cnidarian has no need of an excretory system.
A cnidarian has no coelomic cavity (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start). A coelomic cavity is a fluid-filled, pressurized body cavity that can function as a hydrostatic skeleton. The lack of a coelomic cavity limits the efficiency with which a cnidarian can move, since there’s no coelomic cavity to redirect the force generated by contraction of muscle cells. The lack of a coelomic cavity also means that a cnidarian has no way to resist outside pressure. (Because water cannot be compressed, a pressurized internal cavity gives its possessor the ability to resist external pressure.) Even so, some cnidarians can close their mouths and use their gastrovascular cavities as a sort of simple coelomic cavity.
Despite the fact that cnidarians don’t have any sort of coelomic cavity, we don’t normally refer to them as “acoelomates (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start).” The reason is because cnidarians are diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5#content_start). A pseudocolom or a true coelom is, by definition, partially or completely surrounded by mesodermal tissue, and cnidarians don’t have mesodermal tissue. Accordingly, the terms “acoelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start),” “pseudocoelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start),” and “coelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start)” don’t apply to such animals.
[b]Cnidocytes and Nematocysts:
The name for the Phylum Cnidaria is derived from the Greek root “knide” (meaning “nettle”) and the Latin root “aria” (meaning “like” or “connected with”). In other words, the cnidarians are named for the fact that they possess cnidocytes. Those cnidocytes allow cnidarians to be effective predators, despite their very simple bodies.
A cnidocyte is a specialized type of cell that’s usually found in the ectodermal cells of a cnidarian, especially near the tips of the tentacles. Occasionally, cnidocytes can be found in endodermal cells as well.
Each cnidocyte produces one of over 30 different types of distinctive organelles known as cnidae. A cnida is a cup-shaped organelle that can contain any of several different substances. In addition to a cnida, each cnidocyte has a modified cilium that acts as a “trigger.” When something brushes against a cnidocyte’s trigger, the cnida rapidly ejects whatever substance it contains to the outside of the cell. Since each cnidocyte can discharge only once, it is absorbed and replaced after it discharges.
Broadly speaking, cnidocytes can be divided into four different categories, depending on what the cnidae contain and eject. A penetrant is a cnidocyte that ejects a harpoon-like structure that penetrates the skin of potential attackers or prey. A glutinant ejects a sticky fluid onto the surface of the cell, which traps prey. A volvent ejects a lasso-like thread that wraps around and captures prey. A ptychocyst is a unique type of cnidocyte found in burrowing (tube) anemones. Lacking a coelom (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start) of any sort, a cnidarian cannot burrow, at least not directly, but a tube anemone’s ptychocysts eject sticky threads that the animal can use to construct its burrow.
One particular type of penetrant cnida is known as a nematocyst. A nematocyst contains a hollow, coiled, threadlike filament inside itself. This filament is often barbed. When triggered, a nematocyst ejects the filament with enough force to penetrate the skin of a small animal. This ejection is one of the fastest biological processes known; it takes much less than one millisecond. When it penetrates its victim’s skin, the filament injects a paralyzing toxin. Between the barbed filaments and the venom they inject, any small animal unfortunate enough to come into contact with a cnidarian’s tentacles is unlikely to escape.
http://www.freethought-forum.com/forum/gallery/files/5/0/cnidocyte.jpg
The structure and function of a cnidocyte armed with a nematocyst.
[b]Cnidarian Body Plans:
Any species that exhibits more than one type of body plan is said to be polymorphic, and this is true of most cnidarians. Specifically, most cnidarians are dimorphic, because there are two basic body plans.
The two basic body plans in cnidarians are polyps and medusae. In most cnidarian species, an individual spends part of its life as a polyp and part of its life as a medusa. In some species, though, after the larva develops into either a polyp or a medusa, the animal retains that body type for the rest of its life. In both polyps and medusae, extensions of the body wall known as tentacles surround the mouth/anus. These tentacles usually have cnidocytes at their tips, and they’re used for capture of prey.
In a polyp, the oral surface and tentacles face upward, and the aboral surface faces downward. In most species, a polyp’s body is more or less tubular in shape, and the aboral surface is attached to a hard surface. A polyp is usually sessile; if it is capable of locomotion, it crawls along surfaces, because it cannot swim. The polyps of many species are capable of asexual reproduction through budding, fission, or pedal laceration. Pedal laceration occurs when something tears off a portion of a polyp’s base, and that tissue grows into a new polyp. In most cnidarian species, polyps do not produce spermatozoa or ova, and so cannot reproduce sexually.
http://www.freethought-forum.com/forum/gallery/files/5/0/polyp_original.jpg
The “polyp” body type in a cnidarian.
In a medusa, the oral surface and tentacles face downward. The body of a medusa is typically bell-shaped or umbrella-shaped, and medusae are free-swimming. In a typical medusa, the margins of the “bell” extend to form a shelf called the velum, which partially closes the open side of the bell. Contraction of muscle cells alternately empties and fills the bell. As water is expelled from the bell, the animal is pushed forward, aboral side first, with a weak form of “jet propulsion.” In most species, medusae have gravity-sensing structures known as statocysts, which allow them to distinguish “up” from “down.” Many have light-sensitive ocelli that allow them to swim toward or away from light. A few species even have fully-functional eyes. In most cnidarian species, an individual medusa produces either spermatozoa or ova, and so medusae are capable of sexual reproduction.
http://www.freethought-forum.com/forum/gallery/files/5/0/medusa.jpg
The “medusa” body type in a cnidarian.
If you’re thinking that a polyp looks like an upside-down medusa and vice versa, you’re exactly right. In those species in which both body types are found, a polyp transforms into a medusa by detaching from whatever it’s attached to, flipping over, and reshaping its stalk into a bell. Similarly, a medusa can transform into a polyp by flipping over, reshaping its bell into a stalk, and attaching to a surface.
[b]Cnidarian Life Cycles:
The polyps and medusae play very different roles in the life of a cnidarian. Because one body type is much more mobile than the other, the same species can occupy both open water habitats and the ocean floor.
In most cnidarians, the embryo develops into a very distinctive larval form known as a planula. A planula is flat, two cell layers thick (endoderm and ectoderm), and is usually ciliated. Its cilia allow a planula to swim about until it finds a suitable place to settle down.
http://www.freethought-forum.com/forum/gallery/files/5/0/planula.jpg
The planula larva of a cnidarian.
In cnidarian species with both body types, the planula ultimately settles onto a surface and undergoes metamorphosis (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start) to form a polyp. Polyps can reproduce asexually, and may form large colonies of clonal individuals.
Polyps can produce medusae through various means, including budding. Each medusa is either male or female. A species that reproduces sexually, but in which individuals produce both spermatozoa and ova is said to be hermaphroditic or monoecious (from the Greek “mono,” meaning “single” and “oikos,” meaning “house”). A sexually-reproducing species is dioecious (“di” = “two”) if individuals produce either spermatozoa or ova, but not both.
When spermatozoa from a male medusa fertilize ova from a female medusa, they produce a zygote. The zygote undergoes cleavage (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start) to ultimately produce a planula, and the life cycle is complete.
http://www.freethought-forum.com/forum/gallery/files/5/0/i10-82-cnidaria_original.jpg
The life cycle of a typical cnidarian.
[b]Cnidarians and Zooxanthellae:
Like some poriferans, many cnidarians form mutualistic relationships (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=20) with zooxanthellae (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=20). Polyps are especially likely to have photosynthetic algae living inside their bodies. Because the polyps absorb some of the food the zooxanthellae produce, their need to feed is considerably reduced. In fact, some cnidarian species don’t need to feed at all, because the algae living in their tissues provide all the food they need.
http://www.freethought-forum.com/forum/gallery/files/5/0/zooxanthellae.jpg
Zooxanthellae living within the tissues of a coral polyp.
[b]Cnidarian Diversity:
There are four generally-recognized classes within the Phylum Cnidaria. They are the Class Hydrozoa, the Class Scyphozoa, the Class Cubozoa, and the Class Anthozoa.
Most species in the Class Hydrozoa are marine, though there are some freshwater species. Most hydrozoans are colonial. The typical life cycle of a hydrozoan includes an asexual polyp stage and a sexual medusa stage.
The Class Scyphozoa includes most of the larger species of jellies (“jellyfishes”). A few species can have bell diameters in excess of 2 meters and tentacles that extend for 60 meters or more. Like hydrozoans, most scyphozoans have both asexual polyps and sexual medusae, but in scyphozoans, the medusa stage is dominant. In fact, some scyphozoan species are not known to have a polyp stage at all.
Members of the Class Cubozoa, like scyphozoans, have large medusae as their dominant life stages. In many cubozoan species, polyps are not known to occur. These animals are commonly known as “box jellies,” because of their distinctive, cube-shaped bells.
Members of the Class Anthozoa are known as the “flower animals,” because of their flower-like appearance. Anthozoans have no medusa stage, and the polyps can reproduce sexually. Anthozoans include sea anemones and corals.
[break=Class Hydrozoa]
[b]Class Hydrozoa:
In hydrozoans (from the Greek “hydro,” meaning “water” and “zoa,” meaning “animals”), the polyp is the dominant life stage. An individual polyp is known as a hydroid. Most species live in colonies of clonal individuals and cooperate to capture prey. Individual hydroids are generally quite small, usually only a few centimeters tall at most.
The medusae produced by hydrozoans are generally tiny, usually no more than a few millimeters in diameter. These disperse to new habitats, produce spermatozoa and ova, then die. (Each individual medusa is either a male or a female, you recall.) From the zygote produced by union of a spermatozoan and an ovum, a planula develops. The planula metamorphoses into a polyp, and the cycle is complete.
http://www.freethought-forum.com/forum/gallery/files/5/0/obelia.jpg
The life cycle of Obelia, a common colonial marine hydrozoan.
Hydra is a genus of common freshwater hydrozoans that is named for the Hydra, the many-headed monster of Greek mythology. Unlike most hydrozoans, Hydra are solitary, rather than colonial. Also unlike most hydrozoans, Hydra do not have a medusa stage; polyps can reproduce either sexually or asexually. Another thing that distinguishes Hydra from most other hydrozoans is that the polyp form (the only form in Hydra, of course) is not sessile. If the animal is threatened, it can detach from whatever it’s attached to and move away by “cartwheeling.”
Hydra can be found on the undersides of lily pads or on the bottoms of cool, clean freshwater ponds throughout the world. An individual Hydra might be 30 millimeters or so long, and so is (barely) visible to the naked eye. If you’ve ever had an Biology course, chances are good that you’ve seen Hydra.
http://www.freethought-forum.com/forum/gallery/files/5/0/hydra2_original.jpg
Several specimens of Hydra attached to a substrate.
[break=Physalia]
A most unusual species of hydrozoan is Physalia physalis, the Portuguese Man-of-War. A man-of-war is not an individual animal; it’s actually a colony of polyps. A man-of-war consists of a large, gas-filled float (or “sail”) with long “tentacles” made of thousands of polyps hanging beneath it. The toxins produced by the nematocysts in the “tentacles” are so strong that they’re dangerous even to animals as large as humans.
http://www.freethought-forum.com/forum/gallery/files/5/0/physalia.jpg
Physalia physalis, the Portuguese Man-of-War
It’s pretty, but best left untouched.
[break=Class Scyphozoa]
[b]Class Scyphozoa:
When you think of a “jellyfish,” you’re probably thinking of a member of the Class Scyphozoa. In scyphozoans, the medusa stage is dominant, and in some species, the polyp stage is not known to occur. The body is distinctly bell- or umbrella-shaped, and there is a relatively large amount of mesoglea, which provides buoyancy. It’s because of the abundance of jelly-like mesoglea that these animals are commonly referred to as “jellyfishes.”
The nerve net in a scyphozoan is relatively well-developed, as are its muscle cells. These adaptations make them much more mobile than are most other cnidarians. As you might expect, most scyphozoans have well-developed statocysts and ocelli, and a few even have functioning eyes. In most species, four extensions of the body wall known as oral arms surround the mouth. These oral arms are used to capture prey and then push it into the gastrovascular cavity.
Not too many scyphozoans form mutualistic relationships with zooxanthellae, but there is at least one species that does. Cassiopeia andromeda, the “Upside-Down Jellyfish,” can be found in warm, shallow ocean waters. There it lies on the bottom or floats in the water column with its oral surface facing upward. This ensures that the zooxanthellae living in its tissues get the maximum exposure to sunlight, so that they can perform photosynthesis.
http://www.freethought-forum.com/forum/gallery/files/5/0/giant_jelly_fish.jpg
Despite their simplicity, some scyphozoans can grow to truly impressive sizes.
http://www.freethought-forum.com/forum/gallery/files/5/0/aurelia_aurita.jpg
The common scyphozoan Aurelia aurita, commonly known as the “Moon Jelly.”
The four white rings are the animal’s gonads. The four oral arms are also easily visible.
http://www.freethought-forum.com/forum/gallery/files/5/0/aurelia_life_cycle.jpg
The life cycle of Aurelia aurita.
http://www.freethought-forum.com/forum/gallery/files/5/0/cassiopeia.jpg
Cassiopeia andromeda, the “Upside-Down Jellyfish”
http://www.freethought-forum.com/forum/gallery/files/5/0/mawsonites.jpg
The cnidarians are an ancient group. This is a fossil named Mawsonites;
it appears to be a scyphozoan, and it lived some 600 million years ago.
[b]Class Cubozoa:
Cubozoans are superficially similar to scyphozoans in that the medusa stage is dominant. As in the scyphozoans, the medusae of cubozoans grow to relatively large sizes, while their polyps are tiny – and in some species, polyps are not known to occur at all.
The easiest way to distinguish a cubozoan from a scyphozoan is that whereas a scyphozoan has a round bell, a cubozoan has a bell that is square or rectangular in shape. This explains the common name for these animals – “Box Jellies.”
In cubozoans, the tentacles tend to be bunched together at the corners of the bell. Though cubozoans tend to smaller than scyphozoans, their nematocysts often have extremely toxic venom, and so they’re very efficient predators upon fishes and other small sea creatures. Some have venom so toxic that they’re capable of killing humans. Chironex fleckeri, for instance, is commonly known as a “Sea Wasp”; its stings are extremely painful, and can kill a human in only 3 minutes’ time.
Unlike most other cnidarians, cubozoans not only possess gravity-sensing statocysts and light-sensing ocelli, they many also have fully-functional eyes. These are true, camera-type eyes; each has a cornea and a lens, and can form images.
http://www.freethought-forum.com/forum/gallery/files/5/0/carybdea_sivickisi.jpg
The cubozoan Carybdea sivickisi, a typical box jelly.
The roughly square bell is apparent, as is the fact that the
tentacles are clustered at the corners. The colored structures
near the top of the bell are the animal’s gonads.
http://www.freethought-forum.com/forum/gallery/files/5/0/chironex.jpg
The cubozoan Chironex fleckeri, commonly known as
a “Sea Wasp.” It’s beautiful, but very dangerous.
http://www.freethought-forum.com/forum/gallery/files/5/0/tripedalia_cystophora.jpg
The cubozoan Tripedalia cystophora.
Three of its eyes are visible; they are the
dark structures near the bottom of the bell.
[b]Class Anthozoa:
The anthozoans are known as the “flower animals,” because their numerous tentacles give them flower-like appearances. These animals exist only as polyps. The most distinctive feature of anthozoans is that their body walls are folded inward in most species. These infolding of the body wall are known as septae, and they effectively subdivide an anthozoan’s body into many different partitions. There are two major groups of anthozoans: sea anemones and corals.
http://www.freethought-forum.com/forum/gallery/files/5/0/septae.jpg
A cross-section through the body of a Sea Anemone,
showing how the body is subdivided by septae.
[b]Sea Anemones:
In terms of size, anyway, the sea anemones are the largest anthozoans. Though most people think of sea anemones as sessile and permanently attached to the substrate, many of them can move. When threatened, many anemones are capable of detaching from the substrate and then rolling away. Some can even swim, albeit poorly.
For a creature with no brain, an anemone shows surprisingly complex behavior. If you observe an anemone, you’ll see that most of its tentacles are perfectly ordinary-looking; these are known as feeding tentacles, and as you’d imagine, they’re used for capturing prey. In some species though, some of the tentacles are enlarged, white, and packed with nematocysts. These are known as fighting tentacles (acrorhagi). If an anemone encounters an unrelated anemone close to itself, it will lean over and sting it with its acrorhagi. In other words, anemones fight over territory. Usually, the smaller anemone is forced to retreat; it detaches from the substrate and rolls away.
Sea anemones are typically found in shallow marine waters, and many species live in tubes that they secrete. When threatened, the animal contracts retractor muscles that pull it down into the protection of the tube.
http://www.freethought-forum.com/forum/gallery/files/5/0/anemone.jpg
A typical sea anemone
http://www.freethought-forum.com/forum/gallery/files/5/0/fighters.jpg
Two sea anemones engaging in a territorial dispute.
The anemone on the left is leaning over to attack the one on the right.
http://www.freethought-forum.com/forum/gallery/files/5/0/fighting.jpg
A close-up view of a sea anemone, showing the white “fighting tentacles.”
Sea anemones are famous for forming mutualistic relationships with other organisms. Many species host zooxanthellae, but they’re most famous for providing shelter to various species of fishes, especially Clownfishes. These fishes somehow avoid being stung by the anemones (exactly how is still unclear), and live among their tentacles. The anemone provides the clownfish with protection, and the clownfish keeps the anemone free of sediment and debris. There’s some evidence that the small fishes that live in the anemone’s tentacles will actually lure larger fishes to the anemone, which are then captured and eaten.
Some crabs attach anemones to their shells. This is a mutually-beneficial arrangement because the anemone feeds on scraps of the crab’s meals, and the anemone’s stinging nematocysts protect the crab from would-be attackers.
http://www.freethought-forum.com/forum/gallery/files/5/0/clown-anemonefish.jpg
A Clown Anemonefish (Phylum Chordata) and its anemone host.
http://www.freethought-forum.com/forum/gallery/files/5/0/hermit_original.jpg
This is an Anemone Hermit Crab (Phylum Arthropoda);
it has attached several sea anemones to its shell.
http://www.freethought-forum.com/forum/gallery/files/5/0/anemone-crab.jpg
Of course, not all relationships between species are mutualistic.
This anemone has captured a crab and is eating it.
[break=Corals]
[b]Corals:
Corals are very similar to sea anemones, but individual animals are typically much smaller. A great many coral species are colonial, and many (but by no means all) species secrete protective exoskeletons (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=9#content_start) of calcium carbonate, into which they can retreat when threatened. The colonial species build coral reefs. Each generation builds on the skeletons of the previous generation, and so coral reefs can grow to immense size over time. (The Great Barrier Reef off the coast of Australia is generally believed to be the largest structure on the planet built by living things; it dwarfs anything built by humans.)
Like many other cnidarians, corals often form mutualistic relationships with zooxanthellae. This is one reason why most corals are found in clear, shallow water – in those species that form mutualistic relationships with zooxanthellae, it’s vital to the survival of the zooxanthellae (and, therefore, the coral animal) that they receive sufficient sunlight.
Soft corals do not produce hard exoskeletons, and so do not form reefs. Reef-building corals are known as hard corals. Basically, a hard coral is a miniature sea anemone that lives inside of a secreted calcium carbonate tube.
http://www.freethought-forum.com/forum/gallery/files/5/0/softcoral.jpg
A soft coral. You can see that this is not a single animal,
but a colony containing hundreds of individual animals.
[break=Coral Reefs]
Coral reefs are among the most diverse of all marine ecosystems. The hard coral skeletons provide scarce shelter, and a great many species of fishes, crabs, shrimp, and other marine animals are found nowhere else. Given how depended many hard coral species are on zooxanthellae, it shouldn’t surprise you to discover that coral reefs are generally found in warm, shallow, and clear waters.
There are some deep-sea corals, incidentally. These species, of course, do not form mutualistic relationships with zooxanthellae.
Individual coral animals are tiny, usually only a few millimeters long at most. So as you would imagine, coral reefs grow very slowly. Any sizable reef will be thousands of years old at least, and it will take many years for the coral to repair any damage that is done to the reef. Even so, given enough time, they can form very large structures.
In addition to reefs, hard corals form atolls. An atoll is an island formed by hard corals. An atoll begins as a coral reef growing in shallow water around an island. If the island begins to sink, so long as it sinks slowly-enough, the coral reef grows upward at a pace sufficient to keep it near the water’s surface. If the island sinks completely beneath the waves, its former position is marked by a reef surrounding its former position. A subsequent drop in sea level can expose the reef, forming an atoll. This process may take as long as 30 million years. (Here’s a piece of trivia for you; the first person to describe how atolls form was Charles Darwin.)
http://www.freethought-forum.com/forum/gallery/files/5/0/coral04a_480_original.gif
How an atoll forms. A coral reef (in pink/purple) grows in
shallow water around an island. If the island sinks, so long
as it sinks slowly enough, the reef can grow upward fast-enough
to keep the coral in sunlit water. Eventually, the island may
sink entirely, leaving a coral reef surrounding its former position.
If the sea level drops, the reef is exposed, forming an atoll.
Zooxanthellae are very temperature-sensitive, and a change in water-temperature of only a few degrees will kill them. Similarly, anything that clouds the water will deprive the zooxanthellae of needed sunlight, killing them. If the zooxanthellae die, so does the coral.
In tropical seas all over the Earth, a phenomenon known as coral bleaching is occurring. Corals are dying, leaving their white, bleached skeletons behind. As the coral dies, the ecosystems that depend on them collapse. It is widely believed that there are two principle reasons for coral bleaching. First, the waters of the Earth’s oceans are warming as the Earth’s average temperature is increasing. Second, water pollution (from dumping of sewage into the oceans, for instance) clouds the water and deprives the zooxanthellae of the sunlight they need to survive.
http://www.freethought-forum.com/forum/gallery/files/5/0/coral_reef1.jpg
A healthy coral reef is an exceptionally diverse ecosystem.
http://www.freethought-forum.com/forum/gallery/files/5/0/partially_bleached_coral_original.jpg
A partially-bleached hard coral. As the zooxanthellae die, so do the coral
animals, and as they die, the skeletons of previous generations are exposed.
[break=Finding Nemo]
Collection of corals and fishes for marine aquaria is another serious threat to coral reefs. “Live coral” is frequently collected by dynamiting reefs and then collecting the shattered remains to sell to marine aquarium enthusiasts. Needless to say, this practice does tremendous damage to the reef ecosystems.
Many of the fishes that live in coral reefs are brightly-colored and so sought for marine aquaria. The problem is that these fish seek shelter within the coral, and so are difficult to catch. So, a frequently-used method for capturing them is to squirt cyanide into the spaces in the reef where the fishes hide. The cyanide stuns the fish, which can then be captured by ripping apart the reef with a crowbar. Of course, the cyanide has a nasty tendency to kill lots of fish in the process, not to mention the coral polyps.
Ironically, since the success of the movie Finding Nemo, demand for Clownfish has increased greatly. This has resulted in an increased rate of destruction of coral reefs as collectors attempt to capture fish to fill the demand.
http://www.freethought-forum.com/forum/gallery/files/5/0/dynamite.jpg
Dynamite is often used to collect “live coral” for the marine aquarium
trade. The practice does tremendous damage to coral reef ecosystems.
http://www.freethought-forum.com/forum/gallery/files/5/0/nemo.jpg
Sure, he’s cute, but if you must have a marine aquarium,
get your fish from a reputable breeder. Otherwise, you’re
contributing to the destruction of coral reef ecosystems.
[break=What Comes Next]
The cnidarians are a familiar group of radiate animals. The other radiate phylum is the Phylum Ctenophora, the “Comb Jellies.” These animals are much less familiar to most people than are the cnidarians. They will be the subject of the next chapter.
Chapter Nine: The Radiata: Phylum Cnidaria:
The Eumetazoans:
The eumetazoans, as you recall, are the “true animals.” Unlike the parazoans (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=8#content_start), eumetazoan animals possess true tissues (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start), and most of them have organs (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=6#content_start) as well. Also unlike the parazoans, all of the eumetazoans have some degree of body symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=2#content_start), and all eumetazoans undergo embryonic development (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start).
Another thing that distinguishes the eumetazoans is that most of them have two types of tissues that are found in no other living organisms. These tissue types that are unique to eumetazoan animals are neural (nervous) tissue (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=8#content_start) and muscular tissue (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=9#content_start). In most eumetazoans, muscle tissue is attached to some sort of supportive skeleton – either an internal endoskeleton or an external exoskeleton.
The Radiata and the Bilateria:
The eumetazoans, as you no-doubt recall, are divided into two major taxa: the Radiata and the Bilateria. The radiates are named for the fact that most of them are radially-symmetrical (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=3#content_start). Radiate animals are diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5), meaning that their bodies develop from only two embryonic tissue layers. Since they have no mesodermal tissue, radiates have no mesodermal organs, and so have relatively simple bodies.
This does not mean that radiates have no organs; some of them do. Nonetheless, the body of a radiate is typically much less complex than is the body of a bilaterian.
By contrast with the radiates, all of the Bilateria are triploblastic, meaning that their bodies develop from three embryonic tissue layers. Because these animals possess mesodermal organs, most of them have much more complex bodies than do radiate animals. As the name for the taxon implies, the great majority of the bilaterians have bilateral symmetry (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=4#content_start), at least embryonically.
[b]The Cnidaria:
There are two phyla of radiate animals, the Phylum Cnidaria and the Phylum Ctenophora. In this chapter, we’ll deal with the Cnidaria. The cnidarians include such familiar animals as jellies (“jellyfish”), sea anemones, and corals.
Though cnidarians [the “c” is silent] are more complex than are poriferans (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=11#content_start), they are nonetheless very simple animals. Most cnidarians are sessile; those that are not, such as jellies, can swim only very slowly. So cnidarians cannot chase down their prey. All in all, you’d think these soft-bodied, slow-moving, brainless animals would be easy prey for other animals.
Surprisingly, though, cnidarians are quite adept predators, and they frequently capture and eat animals that are far faster, more complex, and more intelligent than themselves. Their “secret weapon” is that they have tentacles armed with a unique structure known as a nematocyst.
A nematocyst, when stimulated, rapidly ejects a barbed thread that can penetrate flesh. When it penetrates the flesh of a victim, the thread injects a paralyzing toxin. So any small animal that’s unlucky-enough to brush up against a cnidarian will immediately be speared by hundreds or even thousands of nematocysts and quickly immobilized. The hapless victim is then pulled into the cnidarian’s mouth and digestive cavity.
http://www.freethought-forum.com/forum/gallery/files/5/0/anemonefood.jpg
This sea anemone (Phylum Cnidaria, Class Anthozoa)
has captured a fish (Phylum Chordata), and is eating it.
[b]Cnidarian Characteristics:
The members of the Phylum Cnidaria share a number of characteristics, but their defining characteristic is that they possess specialized cells known as cnidocytes. In most cases, the cnidocytes contain stinging organelles known as nematocysts.
Cnidarians are entirely aquatic. Most species live in the ocean, though there are some freshwater species.
Cnidarians have radial symmetry. As such, a cnidarian has no anterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) end and no posterior (http://www.freethought-forum.com/forum/showthread.php?t=17203&garpg=2#content_start) end, and has no head. The side on which the mouth is located is known as the oral surface and the opposite side is known as the aboral surface.
Cnidarians have two basic body plans. A polyp is usually sessile and attached to a hard surface, and the oral surface faces upward. A medusa is free-swimming, and the oral surface faces downward.
Cnidarians are diploblastic, and an adult’s body therefore consists of only two tissue layers. The outer tissue layer is the epidermis, and it is derived from the embryonic ectoderm. The inner tissue layer is the gastrodermis, and it is derived from the embryonic endoderm.
A non-living, jelly-like substance known as mesoglea fills the space between the epidermis and the gastrodermis. (There may be living cells embedded within it.) The mesoglea helps provide buoyancy, and is the “jelly” of a “jellyfish.”
A cnidarian has an incomplete gut, meaning that there is only a single opening into it. This opening serves as both the mouth and the anus. The gut of a cnidarian is known as its gastrovascular cavity. Food is digested in the gastrovascular cavity, then absorbed by the surrounding cells. Undigestible matter is expelled through the mouth/anus.
Extensions of the body wall known as tentacles surround the mouth in most species. Cnidocytes at the tips of the tentacles allow the animals to capture prey. The tentacles push captured prey into the cnidarian’s gastrovascular cavity for digestion.
These are the simplest animals with muscle cells, though they have no true muscles. Contractions of muscle cells allow the animals to move, and some can even swim. While their movements are slow and clumsy compared to those of, say, vertebrates, most cnidarians are far more mobile than are poriferans.
Cnidarians are also the simplest animals with well-developed sense organs. Sense organs found in cnidarians include statocysts that respond to gravity, allowing the animals to distinguish “up” from “down.” Many cnidarians have light-sensitive organs known as ocelli, and some even have fully-functional eyes.
Cnidarians are the simplest animals with neurons. While cnidarians don’t have true nerves, much less brains, the neurons of a cnidarian are sometimes organized into a nerve net that allows the animal to coordinate its movements.
Cnidarians are capable of both sexual (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start) and asexual reproduction (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=2#content_start). Many species are capable of asexual reproduction through budding (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=3#content_start) or fission (http://www.freethought-forum.com/forum/showthread.php?t=17235&garpg=3#content_start), and these species often form colonies of cloned individuals. Other species reproduce sexually. The larval (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start) form of a cnidarian is very distinctive and is known as a planula.
A cnidarian neither needs nor possesses a respiratory system. The function of a respiratory system is to transport oxygen to an animal’s body cells in order to support cellular respiration, and to transport the poisonous CO2 produced as a waste product of cellular respiration out of the animal’s body. The reason that a cnidarian doesn’t need a respiratory system is because every cell in its body is in direct contact with the surrounding water. This means that oxygen can diffuse directly from the water and into a cnidarian’s body cells. Similarly, the CO2 produced in the animal’s cells is easily disposed of, because it diffuses directly into the surrounding water and is then carried away by currents.
For the same reason that it doesn’t need a respiratory system, a cnidarian neither needs nor possesses an excretory system. The function of an excretory system is to remove poisonous metabolic waste products such as urea and ammonia from an organism’s body. Since the metabolic wastes produced by a cnidarian can simply diffuse away into the surrounding water, a cnidarian has no need of an excretory system.
A cnidarian has no coelomic cavity (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start). A coelomic cavity is a fluid-filled, pressurized body cavity that can function as a hydrostatic skeleton. The lack of a coelomic cavity limits the efficiency with which a cnidarian can move, since there’s no coelomic cavity to redirect the force generated by contraction of muscle cells. The lack of a coelomic cavity also means that a cnidarian has no way to resist outside pressure. (Because water cannot be compressed, a pressurized internal cavity gives its possessor the ability to resist external pressure.) Even so, some cnidarians can close their mouths and use their gastrovascular cavities as a sort of simple coelomic cavity.
Despite the fact that cnidarians don’t have any sort of coelomic cavity, we don’t normally refer to them as “acoelomates (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start).” The reason is because cnidarians are diploblastic (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=5#content_start). A pseudocolom or a true coelom is, by definition, partially or completely surrounded by mesodermal tissue, and cnidarians don’t have mesodermal tissue. Accordingly, the terms “acoelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start),” “pseudocoelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start),” and “coelomate (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start)” don’t apply to such animals.
[b]Cnidocytes and Nematocysts:
The name for the Phylum Cnidaria is derived from the Greek root “knide” (meaning “nettle”) and the Latin root “aria” (meaning “like” or “connected with”). In other words, the cnidarians are named for the fact that they possess cnidocytes. Those cnidocytes allow cnidarians to be effective predators, despite their very simple bodies.
A cnidocyte is a specialized type of cell that’s usually found in the ectodermal cells of a cnidarian, especially near the tips of the tentacles. Occasionally, cnidocytes can be found in endodermal cells as well.
Each cnidocyte produces one of over 30 different types of distinctive organelles known as cnidae. A cnida is a cup-shaped organelle that can contain any of several different substances. In addition to a cnida, each cnidocyte has a modified cilium that acts as a “trigger.” When something brushes against a cnidocyte’s trigger, the cnida rapidly ejects whatever substance it contains to the outside of the cell. Since each cnidocyte can discharge only once, it is absorbed and replaced after it discharges.
Broadly speaking, cnidocytes can be divided into four different categories, depending on what the cnidae contain and eject. A penetrant is a cnidocyte that ejects a harpoon-like structure that penetrates the skin of potential attackers or prey. A glutinant ejects a sticky fluid onto the surface of the cell, which traps prey. A volvent ejects a lasso-like thread that wraps around and captures prey. A ptychocyst is a unique type of cnidocyte found in burrowing (tube) anemones. Lacking a coelom (http://www.freethought-forum.com/forum/showthread.php?t=17220&garpg=7#content_start) of any sort, a cnidarian cannot burrow, at least not directly, but a tube anemone’s ptychocysts eject sticky threads that the animal can use to construct its burrow.
One particular type of penetrant cnida is known as a nematocyst. A nematocyst contains a hollow, coiled, threadlike filament inside itself. This filament is often barbed. When triggered, a nematocyst ejects the filament with enough force to penetrate the skin of a small animal. This ejection is one of the fastest biological processes known; it takes much less than one millisecond. When it penetrates its victim’s skin, the filament injects a paralyzing toxin. Between the barbed filaments and the venom they inject, any small animal unfortunate enough to come into contact with a cnidarian’s tentacles is unlikely to escape.
http://www.freethought-forum.com/forum/gallery/files/5/0/cnidocyte.jpg
The structure and function of a cnidocyte armed with a nematocyst.
[b]Cnidarian Body Plans:
Any species that exhibits more than one type of body plan is said to be polymorphic, and this is true of most cnidarians. Specifically, most cnidarians are dimorphic, because there are two basic body plans.
The two basic body plans in cnidarians are polyps and medusae. In most cnidarian species, an individual spends part of its life as a polyp and part of its life as a medusa. In some species, though, after the larva develops into either a polyp or a medusa, the animal retains that body type for the rest of its life. In both polyps and medusae, extensions of the body wall known as tentacles surround the mouth/anus. These tentacles usually have cnidocytes at their tips, and they’re used for capture of prey.
In a polyp, the oral surface and tentacles face upward, and the aboral surface faces downward. In most species, a polyp’s body is more or less tubular in shape, and the aboral surface is attached to a hard surface. A polyp is usually sessile; if it is capable of locomotion, it crawls along surfaces, because it cannot swim. The polyps of many species are capable of asexual reproduction through budding, fission, or pedal laceration. Pedal laceration occurs when something tears off a portion of a polyp’s base, and that tissue grows into a new polyp. In most cnidarian species, polyps do not produce spermatozoa or ova, and so cannot reproduce sexually.
http://www.freethought-forum.com/forum/gallery/files/5/0/polyp_original.jpg
The “polyp” body type in a cnidarian.
In a medusa, the oral surface and tentacles face downward. The body of a medusa is typically bell-shaped or umbrella-shaped, and medusae are free-swimming. In a typical medusa, the margins of the “bell” extend to form a shelf called the velum, which partially closes the open side of the bell. Contraction of muscle cells alternately empties and fills the bell. As water is expelled from the bell, the animal is pushed forward, aboral side first, with a weak form of “jet propulsion.” In most species, medusae have gravity-sensing structures known as statocysts, which allow them to distinguish “up” from “down.” Many have light-sensitive ocelli that allow them to swim toward or away from light. A few species even have fully-functional eyes. In most cnidarian species, an individual medusa produces either spermatozoa or ova, and so medusae are capable of sexual reproduction.
http://www.freethought-forum.com/forum/gallery/files/5/0/medusa.jpg
The “medusa” body type in a cnidarian.
If you’re thinking that a polyp looks like an upside-down medusa and vice versa, you’re exactly right. In those species in which both body types are found, a polyp transforms into a medusa by detaching from whatever it’s attached to, flipping over, and reshaping its stalk into a bell. Similarly, a medusa can transform into a polyp by flipping over, reshaping its bell into a stalk, and attaching to a surface.
[b]Cnidarian Life Cycles:
The polyps and medusae play very different roles in the life of a cnidarian. Because one body type is much more mobile than the other, the same species can occupy both open water habitats and the ocean floor.
In most cnidarians, the embryo develops into a very distinctive larval form known as a planula. A planula is flat, two cell layers thick (endoderm and ectoderm), and is usually ciliated. Its cilia allow a planula to swim about until it finds a suitable place to settle down.
http://www.freethought-forum.com/forum/gallery/files/5/0/planula.jpg
The planula larva of a cnidarian.
In cnidarian species with both body types, the planula ultimately settles onto a surface and undergoes metamorphosis (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=12#content_start) to form a polyp. Polyps can reproduce asexually, and may form large colonies of clonal individuals.
Polyps can produce medusae through various means, including budding. Each medusa is either male or female. A species that reproduces sexually, but in which individuals produce both spermatozoa and ova is said to be hermaphroditic or monoecious (from the Greek “mono,” meaning “single” and “oikos,” meaning “house”). A sexually-reproducing species is dioecious (“di” = “two”) if individuals produce either spermatozoa or ova, but not both.
When spermatozoa from a male medusa fertilize ova from a female medusa, they produce a zygote. The zygote undergoes cleavage (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=11#content_start) to ultimately produce a planula, and the life cycle is complete.
http://www.freethought-forum.com/forum/gallery/files/5/0/i10-82-cnidaria_original.jpg
The life cycle of a typical cnidarian.
[b]Cnidarians and Zooxanthellae:
Like some poriferans, many cnidarians form mutualistic relationships (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=20) with zooxanthellae (http://www.freethought-forum.com/forum/showthread.php?t=18438&garpg=20). Polyps are especially likely to have photosynthetic algae living inside their bodies. Because the polyps absorb some of the food the zooxanthellae produce, their need to feed is considerably reduced. In fact, some cnidarian species don’t need to feed at all, because the algae living in their tissues provide all the food they need.
http://www.freethought-forum.com/forum/gallery/files/5/0/zooxanthellae.jpg
Zooxanthellae living within the tissues of a coral polyp.
[b]Cnidarian Diversity:
There are four generally-recognized classes within the Phylum Cnidaria. They are the Class Hydrozoa, the Class Scyphozoa, the Class Cubozoa, and the Class Anthozoa.
Most species in the Class Hydrozoa are marine, though there are some freshwater species. Most hydrozoans are colonial. The typical life cycle of a hydrozoan includes an asexual polyp stage and a sexual medusa stage.
The Class Scyphozoa includes most of the larger species of jellies (“jellyfishes”). A few species can have bell diameters in excess of 2 meters and tentacles that extend for 60 meters or more. Like hydrozoans, most scyphozoans have both asexual polyps and sexual medusae, but in scyphozoans, the medusa stage is dominant. In fact, some scyphozoan species are not known to have a polyp stage at all.
Members of the Class Cubozoa, like scyphozoans, have large medusae as their dominant life stages. In many cubozoan species, polyps are not known to occur. These animals are commonly known as “box jellies,” because of their distinctive, cube-shaped bells.
Members of the Class Anthozoa are known as the “flower animals,” because of their flower-like appearance. Anthozoans have no medusa stage, and the polyps can reproduce sexually. Anthozoans include sea anemones and corals.
[break=Class Hydrozoa]
[b]Class Hydrozoa:
In hydrozoans (from the Greek “hydro,” meaning “water” and “zoa,” meaning “animals”), the polyp is the dominant life stage. An individual polyp is known as a hydroid. Most species live in colonies of clonal individuals and cooperate to capture prey. Individual hydroids are generally quite small, usually only a few centimeters tall at most.
The medusae produced by hydrozoans are generally tiny, usually no more than a few millimeters in diameter. These disperse to new habitats, produce spermatozoa and ova, then die. (Each individual medusa is either a male or a female, you recall.) From the zygote produced by union of a spermatozoan and an ovum, a planula develops. The planula metamorphoses into a polyp, and the cycle is complete.
http://www.freethought-forum.com/forum/gallery/files/5/0/obelia.jpg
The life cycle of Obelia, a common colonial marine hydrozoan.
Hydra is a genus of common freshwater hydrozoans that is named for the Hydra, the many-headed monster of Greek mythology. Unlike most hydrozoans, Hydra are solitary, rather than colonial. Also unlike most hydrozoans, Hydra do not have a medusa stage; polyps can reproduce either sexually or asexually. Another thing that distinguishes Hydra from most other hydrozoans is that the polyp form (the only form in Hydra, of course) is not sessile. If the animal is threatened, it can detach from whatever it’s attached to and move away by “cartwheeling.”
Hydra can be found on the undersides of lily pads or on the bottoms of cool, clean freshwater ponds throughout the world. An individual Hydra might be 30 millimeters or so long, and so is (barely) visible to the naked eye. If you’ve ever had an Biology course, chances are good that you’ve seen Hydra.
http://www.freethought-forum.com/forum/gallery/files/5/0/hydra2_original.jpg
Several specimens of Hydra attached to a substrate.
[break=Physalia]
A most unusual species of hydrozoan is Physalia physalis, the Portuguese Man-of-War. A man-of-war is not an individual animal; it’s actually a colony of polyps. A man-of-war consists of a large, gas-filled float (or “sail”) with long “tentacles” made of thousands of polyps hanging beneath it. The toxins produced by the nematocysts in the “tentacles” are so strong that they’re dangerous even to animals as large as humans.
http://www.freethought-forum.com/forum/gallery/files/5/0/physalia.jpg
Physalia physalis, the Portuguese Man-of-War
It’s pretty, but best left untouched.
[break=Class Scyphozoa]
[b]Class Scyphozoa:
When you think of a “jellyfish,” you’re probably thinking of a member of the Class Scyphozoa. In scyphozoans, the medusa stage is dominant, and in some species, the polyp stage is not known to occur. The body is distinctly bell- or umbrella-shaped, and there is a relatively large amount of mesoglea, which provides buoyancy. It’s because of the abundance of jelly-like mesoglea that these animals are commonly referred to as “jellyfishes.”
The nerve net in a scyphozoan is relatively well-developed, as are its muscle cells. These adaptations make them much more mobile than are most other cnidarians. As you might expect, most scyphozoans have well-developed statocysts and ocelli, and a few even have functioning eyes. In most species, four extensions of the body wall known as oral arms surround the mouth. These oral arms are used to capture prey and then push it into the gastrovascular cavity.
Not too many scyphozoans form mutualistic relationships with zooxanthellae, but there is at least one species that does. Cassiopeia andromeda, the “Upside-Down Jellyfish,” can be found in warm, shallow ocean waters. There it lies on the bottom or floats in the water column with its oral surface facing upward. This ensures that the zooxanthellae living in its tissues get the maximum exposure to sunlight, so that they can perform photosynthesis.
http://www.freethought-forum.com/forum/gallery/files/5/0/giant_jelly_fish.jpg
Despite their simplicity, some scyphozoans can grow to truly impressive sizes.
http://www.freethought-forum.com/forum/gallery/files/5/0/aurelia_aurita.jpg
The common scyphozoan Aurelia aurita, commonly known as the “Moon Jelly.”
The four white rings are the animal’s gonads. The four oral arms are also easily visible.
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The life cycle of Aurelia aurita.
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Cassiopeia andromeda, the “Upside-Down Jellyfish”
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The cnidarians are an ancient group. This is a fossil named Mawsonites;
it appears to be a scyphozoan, and it lived some 600 million years ago.
[b]Class Cubozoa:
Cubozoans are superficially similar to scyphozoans in that the medusa stage is dominant. As in the scyphozoans, the medusae of cubozoans grow to relatively large sizes, while their polyps are tiny – and in some species, polyps are not known to occur at all.
The easiest way to distinguish a cubozoan from a scyphozoan is that whereas a scyphozoan has a round bell, a cubozoan has a bell that is square or rectangular in shape. This explains the common name for these animals – “Box Jellies.”
In cubozoans, the tentacles tend to be bunched together at the corners of the bell. Though cubozoans tend to smaller than scyphozoans, their nematocysts often have extremely toxic venom, and so they’re very efficient predators upon fishes and other small sea creatures. Some have venom so toxic that they’re capable of killing humans. Chironex fleckeri, for instance, is commonly known as a “Sea Wasp”; its stings are extremely painful, and can kill a human in only 3 minutes’ time.
Unlike most other cnidarians, cubozoans not only possess gravity-sensing statocysts and light-sensing ocelli, they many also have fully-functional eyes. These are true, camera-type eyes; each has a cornea and a lens, and can form images.
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The cubozoan Carybdea sivickisi, a typical box jelly.
The roughly square bell is apparent, as is the fact that the
tentacles are clustered at the corners. The colored structures
near the top of the bell are the animal’s gonads.
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The cubozoan Chironex fleckeri, commonly known as
a “Sea Wasp.” It’s beautiful, but very dangerous.
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The cubozoan Tripedalia cystophora.
Three of its eyes are visible; they are the
dark structures near the bottom of the bell.
[b]Class Anthozoa:
The anthozoans are known as the “flower animals,” because their numerous tentacles give them flower-like appearances. These animals exist only as polyps. The most distinctive feature of anthozoans is that their body walls are folded inward in most species. These infolding of the body wall are known as septae, and they effectively subdivide an anthozoan’s body into many different partitions. There are two major groups of anthozoans: sea anemones and corals.
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A cross-section through the body of a Sea Anemone,
showing how the body is subdivided by septae.
[b]Sea Anemones:
In terms of size, anyway, the sea anemones are the largest anthozoans. Though most people think of sea anemones as sessile and permanently attached to the substrate, many of them can move. When threatened, many anemones are capable of detaching from the substrate and then rolling away. Some can even swim, albeit poorly.
For a creature with no brain, an anemone shows surprisingly complex behavior. If you observe an anemone, you’ll see that most of its tentacles are perfectly ordinary-looking; these are known as feeding tentacles, and as you’d imagine, they’re used for capturing prey. In some species though, some of the tentacles are enlarged, white, and packed with nematocysts. These are known as fighting tentacles (acrorhagi). If an anemone encounters an unrelated anemone close to itself, it will lean over and sting it with its acrorhagi. In other words, anemones fight over territory. Usually, the smaller anemone is forced to retreat; it detaches from the substrate and rolls away.
Sea anemones are typically found in shallow marine waters, and many species live in tubes that they secrete. When threatened, the animal contracts retractor muscles that pull it down into the protection of the tube.
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A typical sea anemone
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Two sea anemones engaging in a territorial dispute.
The anemone on the left is leaning over to attack the one on the right.
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A close-up view of a sea anemone, showing the white “fighting tentacles.”
Sea anemones are famous for forming mutualistic relationships with other organisms. Many species host zooxanthellae, but they’re most famous for providing shelter to various species of fishes, especially Clownfishes. These fishes somehow avoid being stung by the anemones (exactly how is still unclear), and live among their tentacles. The anemone provides the clownfish with protection, and the clownfish keeps the anemone free of sediment and debris. There’s some evidence that the small fishes that live in the anemone’s tentacles will actually lure larger fishes to the anemone, which are then captured and eaten.
Some crabs attach anemones to their shells. This is a mutually-beneficial arrangement because the anemone feeds on scraps of the crab’s meals, and the anemone’s stinging nematocysts protect the crab from would-be attackers.
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A Clown Anemonefish (Phylum Chordata) and its anemone host.
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This is an Anemone Hermit Crab (Phylum Arthropoda);
it has attached several sea anemones to its shell.
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Of course, not all relationships between species are mutualistic.
This anemone has captured a crab and is eating it.
[break=Corals]
[b]Corals:
Corals are very similar to sea anemones, but individual animals are typically much smaller. A great many coral species are colonial, and many (but by no means all) species secrete protective exoskeletons (http://www.freethought-forum.com/forum/showthread.php?t=17155&garpg=9#content_start) of calcium carbonate, into which they can retreat when threatened. The colonial species build coral reefs. Each generation builds on the skeletons of the previous generation, and so coral reefs can grow to immense size over time. (The Great Barrier Reef off the coast of Australia is generally believed to be the largest structure on the planet built by living things; it dwarfs anything built by humans.)
Like many other cnidarians, corals often form mutualistic relationships with zooxanthellae. This is one reason why most corals are found in clear, shallow water – in those species that form mutualistic relationships with zooxanthellae, it’s vital to the survival of the zooxanthellae (and, therefore, the coral animal) that they receive sufficient sunlight.
Soft corals do not produce hard exoskeletons, and so do not form reefs. Reef-building corals are known as hard corals. Basically, a hard coral is a miniature sea anemone that lives inside of a secreted calcium carbonate tube.
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A soft coral. You can see that this is not a single animal,
but a colony containing hundreds of individual animals.
[break=Coral Reefs]
Coral reefs are among the most diverse of all marine ecosystems. The hard coral skeletons provide scarce shelter, and a great many species of fishes, crabs, shrimp, and other marine animals are found nowhere else. Given how depended many hard coral species are on zooxanthellae, it shouldn’t surprise you to discover that coral reefs are generally found in warm, shallow, and clear waters.
There are some deep-sea corals, incidentally. These species, of course, do not form mutualistic relationships with zooxanthellae.
Individual coral animals are tiny, usually only a few millimeters long at most. So as you would imagine, coral reefs grow very slowly. Any sizable reef will be thousands of years old at least, and it will take many years for the coral to repair any damage that is done to the reef. Even so, given enough time, they can form very large structures.
In addition to reefs, hard corals form atolls. An atoll is an island formed by hard corals. An atoll begins as a coral reef growing in shallow water around an island. If the island begins to sink, so long as it sinks slowly-enough, the coral reef grows upward at a pace sufficient to keep it near the water’s surface. If the island sinks completely beneath the waves, its former position is marked by a reef surrounding its former position. A subsequent drop in sea level can expose the reef, forming an atoll. This process may take as long as 30 million years. (Here’s a piece of trivia for you; the first person to describe how atolls form was Charles Darwin.)
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How an atoll forms. A coral reef (in pink/purple) grows in
shallow water around an island. If the island sinks, so long
as it sinks slowly enough, the reef can grow upward fast-enough
to keep the coral in sunlit water. Eventually, the island may
sink entirely, leaving a coral reef surrounding its former position.
If the sea level drops, the reef is exposed, forming an atoll.
Zooxanthellae are very temperature-sensitive, and a change in water-temperature of only a few degrees will kill them. Similarly, anything that clouds the water will deprive the zooxanthellae of needed sunlight, killing them. If the zooxanthellae die, so does the coral.
In tropical seas all over the Earth, a phenomenon known as coral bleaching is occurring. Corals are dying, leaving their white, bleached skeletons behind. As the coral dies, the ecosystems that depend on them collapse. It is widely believed that there are two principle reasons for coral bleaching. First, the waters of the Earth’s oceans are warming as the Earth’s average temperature is increasing. Second, water pollution (from dumping of sewage into the oceans, for instance) clouds the water and deprives the zooxanthellae of the sunlight they need to survive.
http://www.freethought-forum.com/forum/gallery/files/5/0/coral_reef1.jpg
A healthy coral reef is an exceptionally diverse ecosystem.
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A partially-bleached hard coral. As the zooxanthellae die, so do the coral
animals, and as they die, the skeletons of previous generations are exposed.
[break=Finding Nemo]
Collection of corals and fishes for marine aquaria is another serious threat to coral reefs. “Live coral” is frequently collected by dynamiting reefs and then collecting the shattered remains to sell to marine aquarium enthusiasts. Needless to say, this practice does tremendous damage to the reef ecosystems.
Many of the fishes that live in coral reefs are brightly-colored and so sought for marine aquaria. The problem is that these fish seek shelter within the coral, and so are difficult to catch. So, a frequently-used method for capturing them is to squirt cyanide into the spaces in the reef where the fishes hide. The cyanide stuns the fish, which can then be captured by ripping apart the reef with a crowbar. Of course, the cyanide has a nasty tendency to kill lots of fish in the process, not to mention the coral polyps.
Ironically, since the success of the movie Finding Nemo, demand for Clownfish has increased greatly. This has resulted in an increased rate of destruction of coral reefs as collectors attempt to capture fish to fill the demand.
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Dynamite is often used to collect “live coral” for the marine aquarium
trade. The practice does tremendous damage to coral reef ecosystems.
http://www.freethought-forum.com/forum/gallery/files/5/0/nemo.jpg
Sure, he’s cute, but if you must have a marine aquarium,
get your fish from a reputable breeder. Otherwise, you’re
contributing to the destruction of coral reef ecosystems.
[break=What Comes Next]
The cnidarians are a familiar group of radiate animals. The other radiate phylum is the Phylum Ctenophora, the “Comb Jellies.” These animals are much less familiar to most people than are the cnidarians. They will be the subject of the next chapter.