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Cnidarians (anthoza) Aquarium.Net Jan 97

In this issue Rob continues his series on invertebrate zoology with cnidarians, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

 

A Reefkeeers Guide to Introductory Invertebrate Zoology

By Rob Toonen

 

Part 3: Cnidarians (Anthozoa).

In past articles I have attempted to give a ridiculously brief overview of some basic transport and maintenance information for the focus group. Obviously that is impossible to do with the Anthozoans; there are dozens, if not hundreds, of entire volumes dedicated to providing detailed information on the identification, selection and husbandry of anemones and corals (e.g., Veron 1991, Moe 1993, Delbeek & Sprung 1994, Bornemann & Puterbaugh 1996). Actually, it is just as impossible to provide detailed or widely useful information in an article of this length for sponges or jellies, but I included some rudimentary information in those articles because so little is generally available for those groups. I will not belittle the efforts of those before me by trying to give a thumbnail "how to" section for the care of anthozoans in a captive environment. Instead, I thought I would focus on some interesting and probably widely unknown aspects of anthozoan biology, ecology and systematics this month.

I will start with the same format that I have followed in the other articles and describe what makes an animal a Cnidarian. Although the Cnidarians are defined by a number of shared traits, I will discuss only two here. First, they all possess a special "poison harpoon" structure called a cnida (cnida is singular, cnidae is plural). Cnidae are specialized capsules ( organelles ) produced by cells ( cnidocytes ) unique to the Cnidarians, although many other groups (nudibranchs for example) often steal these organelles from their prey and use the stinging capsule for their own defense. Cnidae come in many different "flavors," the most common of which are the nematocysts . Nematocysts are a hollow spear used to inject a toxin from the cnidarian into the prey (or whatever is being "stung"). Nematocysts can be found on the tentacles, the mouth, the walls of the gut, and even on the flat surfaces of mushroom polyps. These cells are often clustered into wart-like structures referred to as nematocyst batteries all across the surface of the animal. With a good eye, or the aid of a hand-lens, you will probably be able to see the tiny dots lining the surface of any species with relatively clear tentacles. Although nematocysts are the most common, there have been at least three dozen types of cnidae described in the scientific literature. These types fall into three main groups: 1) nematocysts , which have a hollow thread or tube, often covered with barbs or hooks, to deliver a protein and/or phenol-based toxin to their prey. Nematocysts are found in all classes of the Cnidaria; 2) spirocysts , which contain a muco- or glycoprotein to form a long sticky thread which adheres to the surface of the prey rather than penetrating it. These sticky threads can be used to attach the animal if it is dislodged from it's perch as well. These cnidae are only found in Anthozoans; and 3) ptychocysts which are only found in the tube-building anemones, and are used to "spin" the fiberglass-like tube in which these animals live. Although cnidae are typically thought of as automatic, it has been demonstrated that the animals have some active control over their discharge. For example, discharge of nematocysts in one area leads to a similar response from surrounding areas, and starved anemones fire their nematocysts more often and in response to less stimulation than ones that are well fed. Discharge can occur simply in response to touch, but most require both a mechanical and a chemical stimulus to discharge. An individual cnida can only ever be fired once. The cell that manufacturers the capsule can replace the organelle once discharged, but they are all single-use structures. Three hypotheses have been proposed to explain the mechanism of firing: 1) the capsule becomes permeable to water just before firing, and the rapid influx of water to the capsule results in the explosive release of the thread; 2) the manufacture of the capsule leads to a high internal pressure and that pressure drives the thread out of the capsule and into the prey; and 3) the cnidocyte contracts violently to cause the thread to be discharged by squeezing the capsule. The small size of the cnidae and the extreme rapidity of the discharge (it is over within a few milliseconds) make testing these hypotheses very difficult. Although it seems the most reasonable of the three, the last hypothesis is almost certainly incorrect, because cnidae "stolen" from cnidarians for self-defense by other animals (called kleptocnidae ) require only the capsule itself, and not the living cell (none of these animals that steal cnidae from cnidarians can ever produce more once the capsule is fired - they must consume new prey to regain functional cnidae). Recent work with high-speed microcinematography suggests that both the first two hypotheses may play a role in firing the cnidae.

Second, cnidarians have no head, no centralized nervous system and no distinct excretory, circulatory or gas exchange systems. The gut may be branched, but forms a blind sac, with the opening to this sac serving as both the mouth and the anus. This feature of the digestive system leads to the common observation that healthy anemones and large-polyped corals occasionally contract, sometimes violently, and expel a viscous and sometimes noxious mucus from the mouth. Because there is no anus through which these animals may expel waste, any indigestible material must eventually pass out of the sac in the same way in which it entered: through the mouth. The frequency of this occurrence will be a function of the number of times and amount of food the animal is receiving. Another often mentioned characteristic of the cnidarians (and one which I do not use, but will mention) is the alternation of generations. Many cnidarians have both an asexual polyp stage and a sexual medusa (jellyfish) stage. The reason I do not like this distinction, however, is that more cnidarians violate this characteristic than follow it.

As I said last month, there are four classes of Cnidaria: the Hydrozoa (hydroids and hydromedusae), the Cubozoa (box jellies or sea wasps), the Scyphozoa (true jellyfish) and the Anthozoa (sea anemones, corals and sea pens). I have covered the first three Classes of the Cnidaria in as much detail as I intend to in this series last months, and will spend the remainder of this month's article discussing the Class of primary interest to most reef keepers -- Anthozoa. Of about 10,000 living species of Cnidarians, nearly 6,000 are in the Class Anthozoa, which is, in turn, divided into three subclasses. The first is the Alcyonaria (or Octocorallia), the second is the Zoantheria (or Hexacorallia), and the last is the Ceriantipatharia. The diversity of these groups precludes any attempt at covering any but the most basic characters of each subclass herein. If there turns out to be a significant amount of interest, I would be happy to cover more detail on the taxonomic basis and biology of these groups in a future article (interested readers should probably consult a source such as Delbeek & Sprung for a much more in-depth treatment of the subject).

Members of the Subclass Octocorallia (this subclass is referred to as Alcyonaria in many texts - the names are synonymous), as their name implies, possess polyps which almost without exception bear eight tentacles. These tentacles, in turn, typically are covered with small projections called pinnules, giving the tentacles a "feathery" appearance. The polyp morphology of the subclass is fairly uniform, and the diversity of the group is instead expressed in terms of the structure and organization of the colony, and variation in the spicules (skeletal elements). This subclass includes the well-known gorgonians and sea fans, as well as such groups as the 'pulsing corals' ( Xenia ) and 'clove corals' ( Clavularia ) . Much of the classification of the octocorals is essentially subjective due to the lack of basic knowledge concerning the degree to which environmental versus genetic sources influence variation among colony forms. If your animal possesses some multiple of eight tentacles, and those tentacles appear feather-like rather than whip-like, you can be relatively secure that it is an Octocoral.

Hexacorallia (=Zoantharia), again as the name suggests, tend to have hexamerous (multiples of 6, rather than 8 as in the Octocorallia) symmetry, especially in respect to their internal structure. I say especially in respect to their internal structure because many species have significant increases (e.g., Stochiactus ) or reduction (e.g., Actinodiscus ) in the number of tentacles on the polyps. Their skeleton (if they possess one) may either be calcareous or horny, but is never composed of spicules, which members of this subclass lack. This group includes the true anemones (Order Actiniaria), zoanthids (Order Zoanthidea), mushroom polyps (Order Corallimorpharia), and the true corals (Order Scleractinia).

Finally, Ceriantipatharians include both the "black corals" (Order Antipatharia) and the "tube anemones" (Order Ceriantharia). The black corals are typically seen as pieces of jewelry more often than as aquarium specimens, and are often confused with gorgonians by untrained observers. Unlike gorgonians, the polyps of Antipatharians have six non-retractile, unbranched tentacles occurring across the surface of the colony. Ceriantharians can be differentiated from true anemones by the possession of a fiberglass-like tube encasing the body of an anemone-like polyp. They typically live buried in sand flats, with the tube at least partially submerged in the substrate. The tube is contructed of a combination of ptychocysts (the unique cnidae of this group mentioned above), mucus and various detrital materials accumulated through time. The coloration of these "anemones" is highly variable, and many of these animals are primarily or exclusively nocturnal (expand for feeding at night).

However the systematics I have just presented are far from certain. The Scleractinian corals have always been notoriously difficult to classify (see, for example, Eric Bornemann's article in the Novmeber issue of Aquarium Net, inspired by a talk of coral expert Dr. Charles Veron at MACNA), and there have been several major rearrangements of Scleractinian systematics in recent memory. Well, after we all became used to the last major reorganization of the stony corals, there is about to be another. Earlier this year, molecular systematic work with the scleractinian corals suggests the suborders erected in the last major reorganization (based on similarities in skeletal morphology) are not only polyphyletic (include species from multiple, unrelated higher taxonomic groups -- an extreme example would be to include birds and butterflies in the group "Fliers" because both groups can fly), but that the Scleractinia itself is not monophyletic (composed of a single hypothetical ancestor and all the decedent taxa which have originated from that ancestor and are, therefore related). There are at least two distinct and evolutionarily independent lineages included within the scleractinian corals -- Romano named these groups the "Robust" (e.g., Fungia , Leptastrea , Euphyllia , Lobophyllia , etc...), and the "Complex" (e.g., Acropora , Tubastraea , Goniopora , Porites , Montipora , etc...) stony corals (Romano & Palumbi 1996). The fact that apparently related corals from within former suborders turn out to be completely unrelated (virtually all suborders have members that fall into both groups) suggests that there has been considerable convergent evolution among the corals, and that the morphological diversity observed in present-day scleractinian corals is most likely a reflection of multiple evolutionary origins rather than a "true" diversification or radiation of these animals. There are a few cases in which the fossil evidence conflicts with the molecular evidence, but both sets of data clearly support multiple independent origins of the stony corals (Romano & Palumbi 1996). There is good reason to believe that the fossil evidence may be unreliable in this case, however, because (using a conservative "molecular clock") the molecular data indicate that the split between the two "robust" and "complex" corals predates the origin of "typical" scleractinian skeletons, and is therefore invisible in the fossil record. There is naturally some debate about this validity of these data and the importance of taxonomy to coral reef ecology in general (e.g., Sale 1994, Knowlton & Jackson 1994) right now, but don't be surprised if the next edition of Veron's book has a very different set of relationships outlined among the stony corals than the one we've just recently come to accept.

Preservation of coral reefs is a high priority of many nations and environmental groups, and this, along with the fascination many people hold for these beautiful and incredibly diverse systems, provides the impetus for constant new research into the biology of corals (in order to save time and space, I will not try to provide a suitable list of references in this article, but rather I will give a recent example from the scientific literature which interested readers can use as a starting point to gather relevant references). Over the last decade or so, bleaching of coral reefs has attracted worldwide attention, and a considerable amount of effort has been expended in the attempt to discover more about the relationship between corals and their algal symbionts and what causes this relationship to break down. It has been widely assumed that because the relationship between hermatypic (reef building) corals and their zooxanthellae is so important and apparently fragile, the relationship must be a tightly coevolved one with each coral having a specific type of zooxanthellae (e.g., Travis 1996). The way in which juvenile colonies become colonized by these specific zooxanthellae was questioned, and subsequent studies (e.g., Glynn et al. 1994) showed that all developmental stages of the planula (the swimming larval form of anthozoans) contained numerous zooxanthellae in a number of Porites and Acropora species. This work further supported the notion that there was a special and highly specific relationship between corals and the zooxanthellae in thier tissues.

That work, however, determined the identity of zooxanthellae only by their association and/or morphology, which are notoriously insensitive methods for determining species identity among marine invertebrates (Knowlton 1993). Recent work with highly sensitive molecular genetic markers indicates that there are only a few zooxanthellar strains, and that similar strains of zooxanthellae are found in every coral tested so far in the waters around Panama (Rowan & Knowlton 1995). As yet unpublished new research in Dr. Knowlton's lab has found that there are at least four primary strains of zooxanthellae found in all the Panamanian corals, and that these strains live in areas of similar light exposure or orientation rather than in different corals (N. Knowlton, pers. comm.). Several large coral heads (e.g., Montipora ) were found to house all the identified strains of zooxanthellae in different locations across the same coral colony. Obviously, if all identified strains are found within a single coral colony, the relationship between the corals and the zooxanthellae cannot be as specialized as has been traditionally thought. Corals can simply be described as the landscape on which these symbiotic algae grow rather than invoke some complex and highly specific coevolutionary relationship between specific corals and the type of zooxanthellae contained within thier tissues (although there is definitely a strong and necessary symbiotic relationship between zooxanthellae and many cnidarians -- I'll get back to this shortly).

[an error occurred while processing this directive] However, the generality of these discoveries is still a topic of considerable debate, and data from some of the most recent research is apparently in opposition to the information I have just outlined. For example, one study presented at the Coral Reef Symposium this past summer in Panama indicated that some Acropora species lacked zooxanthellae in their larvae, and that those larval recruits would only become "infected" with zooxanthellae if transplanted among adults of the same species (Stan Brown, pers. comm.). Given that cryptic species (otherwise indistinguishable groups of animals which are incapable of interbreeding under natural conditions) are a common and serious problem with marine research in general it will take some time yet before these issues are decided.

As I mentioned above, there is a strong and necessary relationship between zooxanthellae and their coral hosts in most cases, however the unique nature of the algae in each coral has apparently been greatly overexaggerated (this also appears to be the case with Tridacna clams). At this point any generalizations are purely speculative, but further research may yet demonstrate that all zooxanthellae interchangably serve the same purpose given suitable environmental conditions. Regardless of the exact nature of the association, it is obvious that corals rely heavily on their algal symbionts for several reasons. First, it is known that the algae release a significant proportion of photosynthate in response to coral metabolites, and that this release ceases when the coral-produced compounds are absent (Hyman 1940). Whether corals are actually capable of getting all their nutritional requirements from the autotrophic activity of their symbiotic algae is as yet an open question. There are certainly some Anthozoans (e.g., Xenia hicksoni and Clavularia hamra among the Octocorals, and Zoanthus sociatus among the Hexacorals) that have developed structural and behavioral modifications for more-or-less complete nutritional dependence upon their contained zooxanthellae; however, the vast majority of corals are "superbly efficient and voracious carnivores that will accept practically any kind of particulate food" (Goreau et al. 1971). While it is true that many corals certainly obtain some large proportion of their energetic requirements from zooxanthellae (for example, researchers in Bermuda measured coral energetic requirements in excess of ten times higher than the amount possible to obtain from plankton caught in fine nets above the reef), it is generally believed that no coral is truly autotrophic (can produce 100% of its nutritional requirement independently). While some researchers and hobbyists have claimed to show coral growth in the complete absence of particulate food, careful study of the feeding activities of reef corals, both in the aquarium and in nature, has demonstrated that corals are: 1) efficient predators capable of capturing and consuming primarily zooplankton prey, through such mechanisms as ciliated currents and mucus, direct transfer of prey from the tentacles to the mouth, or extracoelenteric (digesting prey outside the body) feeding by mesenterial filaments ('strings', typically nematocyst laden, from the gut lining that can be extruded through temporary openings at virtually any place on the colony surface); 2) unspecialized detritus feeders, using a wide range of organic matter of animal and bacterial origin; and 3) capable of the direct uptake of dissolved or colloidal organic matter by the epidermis (e.g., Goreau et al. 1971). These feeding specializations do not suggest a lifestyle of complete independence from external sources of nutrients.

Second, it is known that although corals are only the latest in a series of reef-building organisms in the history of the Oceans, nearly all reef building species, whether molluscs, cnidarians, etc., have had an association with symbiotic algae (Vermeij 1978). The association between reef-building groups and symbiotic algae is thought to stem from the fact that photosynthetic activity of the zooxanthellae greatly enhances the calcification rate of corals today (and presumably of the now extinct reef building animals of the past). If however, the calcification rate of corals is enhanced by the photosynthetic activity of zooxanthellae, why does the white tip of stony corals such as Acropora (which lack zooxanthellae) grow so quickly? Studies comparing white-tipped (without zooxanthellae) branches to the brown stalk below it found that calcification rate was actually faster in the white-tip than the rest of the branch. In experiments with Acropora hyacinthus and A. formosa , Chinese researchers found that the corals had extremely elevated levels of ATP (ATP is a basic fuel of cellular metabolism) in the white-tipped branches relative to the brown stalk (Fang et al. 1989). The high intracellular concentration of ATP in these white tips provided the coral polyps the vitality to maintain the energy output required to sustain high calcification rates in the absence of photosynthetic activity. They concluded that although no active transport mechanisms are known for movement of products from one polyp to another in scleractinians, photosynthetic products must have been transported from the brown stalk to provide the energy required in the tip (most likely by facilitated diffusion). It still seems that the energetic requirement of calcification on the scale observed among hermatypic (reef-building) species is prohibitively high in the absence of photosynthetic aid.

Third, corals "bleach" (the symbiotic zooxanthellae either die or are expelled from the coral polyp tissues, leaving the almost clear coral polyps showing the white calcareous skeleton rather than the brilliant colors of the symbiotic algae) when they are under significant stress. This loss of the symbiotic algae not only robs corals of their vibrant colors, but, if the algae do not quickly recolonize the bleached polyps, eventually kills the animal. Coral bleaching is of major concern not only because the loss of corals, but because there is wide speculation that the rapid increase in the frequency of observed coral bleaching is a sign of global warming. This speculation is based on the knowledge that unusually high temperatures frequently lead to coral bleaching, apparently because of some breakdown in the symbiotic relationship between the coral polyps and their zooxanthellae (Travis 1996). Recently researchers at Tel Aviv University isolated an infectious Vibrio strain that is responsible for bleaching in the Mediterranean coral Oculina patagonica (Kushmaro, et al. 1996). In a series of experiments with the coral and the isolated bacterium, the researchers found that adding bacteria to the surface of a coral colony maintained an an aquarium at 25 C (slightly higher than 'normal'), resulted in every colony bleaching within 6-8 days. As the temperature decreased, so did the rate of bleaching, until no bleaching was observed at 16 C (the lowest temperature to which the coral is exposed in nature). Bleaching could also be prevented by the application of antibiotics (although they will not release the name or recipe for the antibiotics just yet), and no other bacteria isolated from bleached and unbleached colonies of wild-collected corals produced bleaching in captive corals, although the mechanism of infection and bleaching is still unknown. Again, however, other researchers are skeptical about whether the finding is conclusive, arguing that populations of the bacteria naturally increase when growing on a coral already stressed by warm aquarium water and subsequent bleaching (G.W. Smith, as quoted in Travis 1996).

Given that cnidarian taxonomy is constantly under revision, and that the exact nature of the association between corals and their symbiotic algae is currently unknown, I doubt that any source can claim to be "authoritative" at this time. Scientists are constantly striving to increase our understanding of the relationships among and between taxonomic groups, but this is one phylum in which I think the nomenclature and the relationships among the groups are far from being decided.

 

Literature Cited:

Bornemann, E. 1996. Montipora and The Trouble withTaxonomy , Aquarium Net Cyberzine, November issue.

Borneman, E. & E. Puterbaugh. 1996. A Practical Guide to Corals for the Reef Aquarium , Crystal Graphics:Lexington, KT.

Delbeek, J.C. & J. Sprung. 1994. The Reef Aquarium: a comprehensive guide to the identification and care of tropical marine invertebrates. , Ricordea Publishing: Coconut Grove, FL.

Fang, L.-s., Y.-w. J. Chen & C.-s. Chen. 1989. Why does the white tip of stony coral grow so fast without zooxanthellae? Mar. Biol. 103:359-363.

Glynn, P.W., S.B. Colley, C.M. Eakin, D.B. Smith, J. Cortés, N.J. Gassman, H.M. Guzmán, J.B. Del Rosario & J.S. Feingold. 1994. Reef coral reproduction in the eastern Pacific: Costa Rica, Panamá, and Galápagos Islands (Ecudor). II. Poritidae. Mar. Biol. 118:191-208.

Goreau, T.F., N.I. Goreau & C.M. Yonge. 1971. Reef Corals: Autotrophs or heterotrophs? Biol. Bull. 141:247-260.

Hyman, L. 1940. The invertebrates: Volume 1, Protozoa through Ctenophora . McGraw-Hill Publishers, New York, NY.

Knowlton, N. 1993. Sibling species in the sea. Ann. Rev. Ecol. Syst. 24:189-216.

Knowlton, N. & J.B.C. Jackson. 1994. Taxonomy and coral reef ecology -- Reply. Trends Ecol. Evol. 9:398.

Kushmaro, A., Y. Loya, M. Fine & E. Rosenberg. 1996. Bacterial infection and coral bleaching. Nature 380:396.

Moe, M.A., Jr., 1993. The Marine Aquarium Reference: Systems and Invertebrates , 5th Printing. Green Turtle Publications. Plantation, FL. 512 pp.

Romano, S. & S. Palumbi. 1996. Evolution of scleractinian corals inferred from molecular systematics. Science 271:640-642.

Rowan, R. & N. Knowlton. 1995. Intraspecific diversity and ecological zonation in coral algal symbiosis. Proc. Nat. Acad. Sci. U.S.A. 92:2850-2853.

Sale, P. 1994 Taxonomy and coral reef ecology. Trends Ecol. Evol. 9:398.

Travis, J. 1996. Bleaching Power: Marine bacteria rout coral's colorful algae. Science News 149:379.

Vermeij, G. 1978. Biogeography and Adaptation: patterns of marine life. Harvard University Press, Cambridge, Mass.

Veron, J.E.N. 1991. Corals of Australia and the Indo-Pacific , Angus and Robertson Publishers., North Ryde, Australia.

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