The Porifera. General Features. Feeding

Transcription

1 The Porifera General Features You are probably aware that we have reached the Kingdom Animalia. But just like the Fungi, the Animalia are opisthokont eukaryotes. We will not be through with the opisthokonts until we reach the plants. The 10,000 or so species of sponges are among the simplest animals. They are the only animals we will study that are asymmetrical. Sponges have no organ systems, nerves or muscles. They have some specialized cells, but these are only weakly dependent on one another. A sponge is more a colony than a multicellular animal. Thus they do not qualify for inclusion in the true multicellular animals, the Eumetazoa. The colonial nature of sponges was confirmed by a spectacular early experiment. An investigator squeezed a sponge through a silk cloth, disassociating it into small clumps of cells. In a short time, the cells reassembled themselves into several smaller sponges. Sponges are considered to be different from all other animals mainly because their a) cells are so primitive. b) cells are nearly independent of one another. c) nutrition is neither hetertrophic nor autotrophic. d) number of species is so great. Sponge cells may be unspecialized, but they are otherwise like typical animal cells, and they are clearly heterotrophic. The Porifera doesn't have a remarkable number of species either. The sponge difference lies in the fact that sponge cells practically live as a colony of individual cells rather than as an integrated, multicellular animal. Feeding The poriferans are filter feeders, getting their food by drawing large volumes of water into a central cavity (the spongocoel) through pore cells and then out of the body through the osculum. The water is moved by choanocytes, flagellated cells with fine microvilli "collars" around their flagella. Some large sponges can pump a volume of water equal to that of the sponge per minute. As the water passes through the collars, the microvilli trap bacteria and other small particles. You may remember that the Choanoflagllata (which were opisthokonts) were a protistan group whose cells were strikingly similar to collar cells. This is one reason we believe animals evolved from an opisthokont protistan ancestor, and are most closely related to choanoflagellates and fungi. After the particles are captured by the microvilli, wandering cells called amoebocytes pick up partially digested food from the choanocytes and digest, store, and transport the food throughout the body. The filtering by the choanocytes is very efficient. In one study on a Jamaican reef, 80% of the particles captured were too small to be resolved with light microscopy. While filter feeding is employed by the vast majority of sponges, in 1995 French researchers reported that a deep-water sponge named Asbestopluma was a carnivore! It extends filaments that capture small crustaceans. Then, over the course of a day, more filaments overgrow it and pull it into the sponge's body for digestion. In 2012 another carnivorous sponge called a harp sponge (Chrondrocladia lyrae) was discovered in deep water off the northern California coast. The feeding strategy of this animal is similar to that of Asbestopluma. The harp sponge catches small crustaceans on its spicules and then digests them.

2 Anatomy The simple sponge body wall consists of an inside and an outside layer of cells, with a gelatinous area called the mesohyl between them. This is where spicules are found. Spicules are one of the distinctive features of sponges, and their only fossils. These structures are composed of calcium carbonate, silica, or a rubbery protein called spongin, and they support and protect the animal. Asbestopluma also uses spicules to capture its prey. Another, puzzling part of sponge anatomy is an enormous number of bacteria--maybe up to 40% of the sponge volume may be prokaryotic cells. These species of bacteria are not found in the surrounding water, but are confined to sponges, and seem to be the same all over the world. We don t know what the bacteria do for the sponges or vice versa. Reproduction and Gas Exchange Sponges reproduce both sexually and asexually. Eggs and sperm are usually derived from collar cells. The zygote develops into a free-swimming microscopic larva which eventually settles down onto a substrate and forms the familiar sessile creature. Sponges can also reproduce asexually. Most sponges can reproduce asexually by fragmentation simply breaking off and then growing and developing. Freshwater sponges can produce gemmules a dormant aggregation of cells that can germinate and produce a new colony under favorable conditions. Sponges exchange gases and wastes with their environment by simple diffusion. Because of their loose and open structure, no cell is very far removed from the environment. Simple diffusion across cell membranes removes wastes and provides enough oxygen to maintain cellular metabolism. Like the protozoa, sponge cells maintain water balance with contractile vacuoles. You are hanging onto a sponge spicule while amoebocytes cluster around you, trying to digest you. You are closest to (or in) the a) gastrodermis. b) central cavity. c) osculum. d) mesohyl. The spicules and ameobocytes are in the mesohyl, between the inner and outer cell layers. Review sponge anatomy by pressing the buttons below. Ecology All sponges are aquatic, and the vast majority live in marine environments. Because of their filtering lifestyle, they cannot live in waters with much suspended sediment. They prefer quiet, clear water, and in crowded spots like coral reefs, there is often deadly competition for growing space between different species of sponges and between sponges and other encrusting animals like bryozoans. The spreading growth of a sponge has been successfully modeled using fractal simulations. Taxonomy There are three classes in the Porifera: Calcarea, Hexactinellida, and Demospongiae. Calcarea, or calcareous sponges. The Calcarea's notable feature is spicules made of calcium carbonate. All species are marine. However, molecular data indicates that the Calcarea may not descend from the same ancestor as the glass sponges and demosponges. They may have evolved from a more complex animal by loss of advanced features.

3 Hexactinellida, or glass sponges. The Hexactinellida are exclusively marine, and primarily deep ocean. They have beautiful, intricate body forms composed of silica. Demospongiae, or demosponges. The Demospongiae may be found in marine, brackish, or freshwater environments at all depths. Some have both silica spicules and spongin fibers, but the familiar bath sponge has only spongin. Importance to Humans In times past, Poriferans were an important source of the sponges used for cleaning a function largely lost to synthetic sponges. The major importance of sponges today is as chemical factories. As a matter of fact, sponge researchers today are more likely to publish in the Journal of Organic Chemistry than in any zoological journal. Sponges produce an amazing number of biotoxins and antimicrobial agents important features for an organism that can't run or hide and that has an extremely high surface area in a wet environment. Many natural chemicals produced by sponges are being studied by scientists for their pharmaceutical possibilities. Often these compounds are named after the sponge. For example, the sponge Microscleroderma produces some anti-fungal hexapeptides called microsclerodermins. Compounds with respiratory, cardiovascular, gastrointestinal, anti-inflammatory, antitumor, antibiotic and even anti-hiv and insecticidal activities have been identified from many marine sponges. Evolution It used to be thought that sponges were among the earliest multicellular animals, and arose from the choanoflagellate opisthokont ancestor that gave rise to both the animals and the fungi. Now some researchers are suggesting that at least the calcareous sponges evolved from more complex animals by loss of features, and that the ctenophores may have been the earliest animals. This controversy will have to be resolved by future data.

4 The Cnidaria General Features The Cnidaria includes about 10,000 species of hydras, jellyfish, sea anemones and corals. Unlike the Porifera, the Cnidaria are truly multicellular eumetazoans. They have specialized tissues and primitive organs, radial symmetry, and a digestive system. However, the Cnidaria are the most primitive of the Eumetazoa. Because they are radially and not bilaterally symmetrical, they do not show any tendency towards cephalization (an important role for a head containing a brain). Their digestive system is incomplete, and they are diploblastic (containing two rather than three germ layers). Body Plan Perhaps because of their lack of mesoderm, the cnidarian body plan is simple. A cnidarian is basically a sac two cell layers thick. The outside cell layer is the epidermis, and the inside one is the gastrodermis. A mouth at one end is fringed with tentacles. The space inside the sac is used for digesting food and for circulation of water that transports food and oxygen. Because of this double function, the space is called a gastrovascular cavity. The space between the two cell layers is filled with a gelatinous secretion called mesoglea. Mesoglea is the "jelly" from which jellyfish get their name. The mesoglea may or may not have cells in it. Excited by what you're learning in Biology 1110, you go home during Spring Break and try to explain cnidarian anatomy to your little brother. You take a plastic sandwich bag and put some grape jelly in it. Then you put a second bag inside the first and expand it with you fist so that the jelly spreads between the two bags. "OK, I get it," your little brother says. "The inner bag is the..., the jelly is the..., the outer bag is the..., and the bag opening is the...." a) epidermis... mesohyl... gastrodermis... mouth b) gastrodermis... mesoglea... epidermis... mouth c) epidermis... mesoglea... gastrovascular cavity... anus d) endoderm... mesoderm... ectoderm... blastopore The two cnidarian cell layers are the two plastic bags. The jelly between them is mesoglea. The fact that the bags both only have one opening is fine because the cnidarian gut is incomplete. By the way, mesohyl is found in sponges, and cnidarians don't have mesoderm because they're diploblastic. In the little-brother-friendly analogy in the previous question, where is the gastrovascular cavity? a) Around outside the opening of the outer bag. b) Between the inner bag and the outer bag. c) Inside the inner bag. d) The gastrovascular cavity was left out of this simple demonstration. The gastrovascular cavity is analogous to the stomach, so it is logical that the mouth should lead into it. Cnidarians have no cavity between their epidermis and gastrodermis because they have neither a coelom nor a pseudocoelom. Polyps and Medusae Two body forms are common among cnidarians - the medusa form and the polyp form. These two forms alternate in most cnidarian life cycles.

5 The medusa floats in the water like an upside-down, tentacle-fringed bell. Pulsations of the bell allow it to swim. A jellyfish is a typical medusa. The polyp is usually a tube-shaped organism that is attached to a solid surface at one end and has a tentacle-fringed mouth at the other end. Hydra is a typical polyp, and sea anemones are larger polyps. The next screen shows diagrams of these two forms. Polyp (left) and medusa (right). For comparison, the medusa is drawn upside down. The black area in each drawing is jellylike mesoglea. The layer of cells inside the mesoglea is gastrodermis and the layer outisde the mesoglea is epidermis. The medusa is normally the motile part of the life cycle and the polyp is the non-motile or sessile part. However, some polyps can move. Hydras can somersault (very slowly, to be sure), and even some sea anemones can glide slowly on their bases, turn upside down and walk on their tentacles, or even swim by lashing their tentacles. Your little brother is hungry for more details on cnidarian anatomy. He wants you to present your plastic bag as a medusa. What would be the best way to do that? a) Turn both bags upside down and cut some strips from the lower edge to simulate a fringe of tentacles. b) Leave the outer bag opening-up, but invert the inner one so the opening is pointing down. c) Remove the inner bag and place it beside the outer bag. The analogy would be even better if you packed more jelly into the space between the two bags and expanded the lip of the outer one into a bell. Feeding A unique feature of all cnidarians is the cnidocyte, or stinging cell. These contain nematocysts, or stinging capsules, and are especially common in the tentacles, although they are even found in the gastrodermis in some cnidarians. When a prey organism comes nearby, a combination of chemical cues and contact with the nematocyst trigger causes the nematocyst to shoot out a thread containing poison. Other kinds of threads stick to the prey or entangle it. After capture, chemicals leaking from the wounded prey stimulate the cnidarian to slowly move the prey to its mouth and then stuff it into the gastrovascular cavity (GVC). The gastrovascular cavity, where extracellular digestion begins, is lined with glands that produce mucus and enzymes. Intracellular digestion is completed within food vacuoles. Undigested wastes are expelled through the mouth. As you might expect from this style of feeding, most cnidarians are carnivores. An interesting puzzle in cnidarian evolution is that the cnidarians existed long before their present-day prey (mostly small crustaceans). What did cnidarians eat before the crustaceans evolved? Not all cnidarians are predators, however. A few medusae and polyps are suspension feeders. They catch small particles in mucus on their tentacles and then move the mucus to their mouths with flagella. Gas Exchange and Reproduction Cnidarians acquire oxygen from their environment in the same way as sponges by diffusion across cell membranes.

6 The bodies of most cnidarians are composed of very thin tissues or else they have a very large surface area. Gas exchange occurs directly across the epidermis and into the water circulating through the gastrovascular cavity. Cnidarians reproduce both sexually and asexually. Asexual budding occurs in both polyps and medusae. In sexual reproduction, cnidarians are hermaphroditic for the most part and fertilization may be internal or external. When both polyp and medusa are present in the life cycle, the medusa creates the gametes, and the zygote becomes a larva and then a polyp that buds off more medusae. But in life cycles with only polyps, sexual reproduction still occurs and the polyp releases a ciliated larva that settles down to become a new polyp. Diversity There are four classes of Cnidarians: Hydrozoa Scyphozoa Cubozoa Anthozoa Hydrozoa. Hydrozoans alternate between polyp and medusa forms, but the polyp form is the dominant organism. The common hydra is unusual in that it is one of the few freshwater cnidarians. Many hydrozoans live as colonial polyps, and much of the growth on docks that is dismissed as "seaweed" is actually colonial hydrozoans. These colonies may consist of polyps specialized for feeding (gastrozooids), defense (dactylozooids) and reproduction (gonozooids). The familiar Portuguese man-of-war is a hydrozoan, but it is not a medusa. Instead, it is a colony of specialized polyps suspended from a float derived from a medusa. Scyphozoa. Scyphozoans, which are all marine, include the jellyfish. In this class, the medusa is dominant, and tends to be rather substantial because of a thick, firm mesoglea. The inconspicuous sexually-produced polyp form produces the medusae by asexual budding. In some pelagic (open ocean) schphozoans, the polyp has been eliminated entirely. Some jellyfish have bells up to 2 m in diameter and tentacles 30 m long! Cubozoa. These are the box jellyfish, similar to the Scyphozoa, but square when viewed from above. They include the sea wasps, jellyfish so poisonous that their stings can kill a human within minutes. Sea wasps may have the most powerful venom in the animal kingdom. Rather than trailing their tentacles in the water and waiting for prey to blunder into them, the Cubozoa actively swim after prey at m/sec, which would be a good speed for a human swimmer. However, in terms of body lengths/sec, the human would have to swim at 160 mph to equal their speed. They even have a system of 24 eyes for visual hunting, and probably can form images of prey. An odd feature of the Cubozoa is that in Australian waters, they go to the bottom and seem to sleep from 3 AM until dawn every day. Anthozoa. Sea anemones, corals, and sea pens belong to the Anthozoa. Anthozoans are all polyps, and have a complex system of radial tissue walls called "mesenteries." These are illustrated on a diagram to the right. The medusa stage has been lost. Instead, the zygote develops into a ciliated planula larva that swims out of the parent and finds a suitable spot to settle. The most important anthozoans are undoubtedly the stony corals. Their external skeletons of calcium carbonate form the largest structures ever built on earth by living things. One quarter of all marine species may depend on coral reefs.

7 Despite the massiveness of coral reefs, keep in mind that the living part is a thin film of polyps, like tiny anemones 1-3 mm in diameter. The polyps nestle in holes in the skeleton and lay down a new layer of calcium carbonate each year. If danger threatens, they withdraw into the skeleton. The polyps feed on small crustaceans, and often have symbiotic dinoflagellates (zooxanthellae) in their gastrodermis to help with their nutrition. Bleaching due to death or expulsion of zooxanthellae is a major sign of coral stress. Bleached coral become malnourished and die. In recent decades, coral reefs have been declining sharply because of pollution and global warming. For example, one study found that Caribbean corals declined by 80% between 1970 and Like sponges, stony corals often engage in fierce competition for space on the well-lit, shallow parts of reefs. Human Impacts Aside from the ecological importance of corals and the dangers they create for shipping in tropical seas, cnidarians (the scyphozoans particularly) are notable for eating fish larvae, clogging the intake manifolds of ships and stinging people. Studies of jellyfish have yielded information not only on how to treat their stings, but also on how nerve cells function. By the way, if you get stung by a Portuguese man-of-war, treat the sting with vinegar or urine. If you get stung by a sea nettle or baby lion's mane, treat the sting with a paste of water and baking soda. Evolution The presence of the medusa and the polyp in cnidarian life cycles causes us to wonder which one evolved first. The medusa may have originated as a ciliated, swimming larva called a planula. The planula larva then became sessile and greatly extended its lifespan and became a polyp. After both forms were established, the medusa was gradually suppressed. According to this theory, Hydra and the sea anemones are advanced cnidarians because they have no medusa stage at all.

8 Biology 1110 Porifera/Cnidaria Porifera 1. The sponges are the only kind of animal excluded from the Eumetazoa. What makes the sponges so different from all other animals? 2. Why do we think that sponges are descended from a group of opisthokont protists called the choanoflagellates? 3. What do most sponges eat? What startling exception to these feeding habits was recently discovered? 4. You are reduced to microscopic size and are fated to be eaten by a sponge. Tell your story. Where (and why) would you be pulled into the sponge? Where would you be captured? How would you be digested? If you teeny-tiny iphone was fated to pass out of the sponge, where would it go? 5. Where in a sponge would you find choanocytes, amoebocytes, and spicules? What is the function of each of them? 6. Recently, a very slowly-moving sponge was found. However, most sponges are completely sessile, so how do they disperse into new habitats? Hint: Consider how sponges reproduce. 7. How do sponges get oxygen and get rid of waste products? 8. What kind of habitat is best for sponge growth? 9. What are the three main classes of Porifera, and how can we distinguish between them?

9 10. What is the major importance of sponges to man today? Cnidaria 11. In what ways are cnidarians more advanced animals than sponges? In what ways are they more primitive than most other animal phyla? 12. Why is "gastrovascular cavity" a good name for the space inside a cnidarian? 13. Where in a cnidarian body would you find epidermis, gastrodermis and mesoglea? 14. Contrast the structure of a polyp and the structure of a medusa. 15. Distinguish between a cnidocyte and a nematocyst. How does a nematocyst operate? 16. What do most cnidarians eat? How do they obtain this food? 17. What is an incomplete gut? Contrast the digestive process of a sponge and a cnidarian. 18. Now you are reduced to the size of a small shrimp and are fated to be eaten by a Hydra. Once again, tell how you would be captured, where you would go then, and how you would be digested. If your teeny-tiny iphone escapes destruction again, how would it exit the Hydra? 19. Contrast the gas exchange and waste removal processes of a sponge and a cnidarian. 20. In cnidarian life cycles with both polyp and medusa present, what role do each of them play in the life cycle?

10 21. In general, what distinguishes a hydrozoan from other cnidarians? What are dactylozooids, gonozooids and gastrozooids? 22. In general, what distinguishes a scyphozoan from other cnidarians? What distinguishes a cubozoan from a scyphozoan? 23. In general, what distinguishes an anthozoan from other cnidarians? 24. Describe the structure of a stony coral. In what ways does the growth of stony corals resemble the growth of trees? 25. What causes "bleaching" of coral? Why does it often lead to the death of the affected coral? 26. In what ways are cnidarians important to humans? 27. How did cnidarians evolve? Relate this evolution to the fact that some life cycles have both polyps and medusae, some have only polyps, and some have only medusae.

11 Biology 1110 Porifera/Cnidaria 1. Sponges are considered to be more colonial than truly multicellular because 1. all their cells are identical, and show no specialization. 2. they have no organs and their cells do not have a strong functional relationship to each other. 3. each cell in a sponge reproduces individually; there is no such thing as a reproductive unit that came from the whole sponge. 4. All of these. 2. Most sponges feed on.... This food is obtained by 1. small particles in the water... the microvilli of the choanocytes. 2. dissolved organic matter... axopodia of the amoebocytes. 3. mats of algae and bacteria covering the substrate on which the sponge grows... extracellular digestion by the amoebocytes. 4. protozoa and small metazoans that swim into the sponge... the adhesive proteins of the mesohyl. 3. A gemmule is used 1. in asexual reproduction. 2. for dispersal. 3. for resisting unfavorable conditions. 4. All of these. 4. Sponges have no circulatory system, but they still must bring in oxygen, get rid of wastes and transport food to all parts of their body. They accomplish these tasks with...,... and..., respectively. 1. diffusion... active transport... porocytes 2. choanocytes... flame cells... astrocytes 3. diffusion... diffusion... amoebocytes 4. ciliated epithelium... the mesohyl... gastrocytes 5. A sponge that lived in deep water and had a beautiful, lacy, siliceous skeleton would probably be a member of the 1. Demospongiae. 2. Hexactinellida 3. Calcarea 4. Sclerospongiae. 6. The most active area of sponge research today is 1. using sponges to produce genetically engineered molecules. 2. the messenger molecules that govern sponge development. 3. the mrna primers that sponges use to turn genes on and off. 4. toxins and antibiotics produced by sponges. 7. Cnidarians are a considered to be more advanced than sponges because they...; on the other hand, they are considered more primitive than most animals because of the fact that they have true tissues and organs... cannot reproduce sexually. 2. are radially symmetrical... lack specialized cells. 3. are radially symmetrical... are diploblastic. 4. are bilaterally symmetrical... show no signs of cephalization. 8. If you put a typical cnidarian in a dissecting pan and pierce it with a scalpel, the order of structures you will encounter, from outside to inside, will be 1. epidermis... mesoglea... gastrodermis... gastrovascular cavity. 2. capsule... mesohyl... cnidodermis... gastrovascular cavity. 3. ectodermis... mesodermis... endodermis... archenteron. 4. capsule... cnidodermis... endodermis... gastrocoel. 9. A cnidarian that has just reduced its last meal to some indigestible fragments will next 1. store the fragments in the mesohyl. 2. expel the fragments through the mouth. 3. expel the fragments through the anus. 4. coat the fragments with collagen.

12 10. You are on a ferry boat passing through some deep water, and as you stroll to the rail, a friend tells you, "Oh, you should have been here a second ago. We just passed a whole bunch of jellyfish. They were swimming really gracefully." Your best guess would be that the organisms your friend saw were... of the class medusae... Anthozoa 2. medusae... Scyphozoa 3. polyps... Scyphozoa 4. polyps... Hydrozoa 11. Then the ferry boat reaches the opposite shore. As it approaches the dock, you look at the wharf pilings and see that each piling has skirt of frilly growth perhaps six inches wide. If you're told that the growth is mainly cnidarians, your best guess would be that it would be... of the class medusae... Anthozoa 2. medusae... Scyphozoa 3. polyps... Scyphozoa 4. polyps... Hydrozoa 12. How do you think those small cnidarians on the pilings are making their living? 1. They are secreting neurotoxins into the water and paralyzing small fish. 2. They are phagocytizing small organisms on the wood around them. 3. They are capturing small, swimming crustaceans with their nematocysts. 4. They are mainly living off the photosynthate secreted by symbiotic algae. 13. A nutritional difference between sponges and cnidarians is that cnidarians use... and sponges do not. 1. ingestive nutrition 2. absorptive nutrition 3. food vacuoles 4. extracellular digestion 14. You are studying a cnidarian that has both a polyp and medusa stage in the life cycle, and you need some sperm. You should look for it in the... stage. 1. polyp 2. medusa 3. planula 15. You were told that an unusual feature of a Portuguese man-of-war is that while it looks like a..., it is really a medusa... colony, mostly of polyps 2. polyp... non-motile medusa 3. sea anenome... sedentary jellyfish 4. jellyfish... motile sea anenome 16. The Cubozoa 1. are the most poisonous cnidarians. 2. actively pursue their prey. 3. have eyes that can form images. 4. All of these. 17. One of the distinctive anatomical features of the Anthozoa is the presence of 1. cnidocytes on the tentacles. 2. mesenteries in the gastrovascular cavity. 3. eyespots on the medusa. 4. a complete gut. 18. Humans use coral reefs as diving sites. How does the coral use the stony part of the coral reef? 1. for support and protection 2. as a source of photosynthesized sugar 3. as a storage depot for paralyzed prey 4. All of these.

13 Biology 1110 Porifera/Cnidaria No. Sponges have several types of specialized cells, like amoebocytes, porocytes and choanocytes. 2. Correct. 3. No. Remembers gemmules and larvae? Correct. They are the ultimate filter feeders in that their whole body is a filtering matrix Correct Correct. Oxygen diffuses from the flow of water through the body and wastes diffuse into it. The amoebocytes visit the choanocytes, take partially digested food from them, and take it to the rest of the sponge Correct. It is a glass sponge Correct. 75% of all articles on sponges are published in chemical or medical journals and deal with the isolation, structure, or therapeutic efficacy of sponge metabolites No. They certainly can reproduce sexually. 2. No. They do have specialized cells, like cnidocytes and cells secreting digestive enzymes. 3. Correct Correct Correct. Animals with incomplete guts ingest and egest through the mouth No. Anthozoa don't have medusae. 2. Correct. 3 and 4. No. Polyps wouldn't be swimming and 2. No. Medusae would not be attached to the piling. 3. No. These exist, but would be very small. 4. Correct Correct No, they both are ingestive. 4. Correct. Sponges have only intracellular digestion. Cnidarians secrete enzymes into the gastrovascular cavity and digest extracellularly Correct. The medusa is the sexual part of the life cycle when both polyp and medusa are present. The polyp asexually buds medusae Correct. Perhaps a hundred highly specialized polyps hang from one float Correct. The Cubozoa have several remarkable "claims to fame." No. All cnidarians have cnidocytes on the tentacles. 2. Correct. 4. No, all cnidarians have incomplete guts Correct. 2. No. Zooxanthellae are important, especially for corals, but they're in the gastrodermis. 3. No. Cnidarians don't store paralyzed prey.