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Cnidarians Are Some of the Most Conspicuous and Colorful Tidepool Animals, Yet They Are Often Overlooked and Unappreciated

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Cnidarians Are Some of the Most Conspicuous and Colorful Tidepool Animals, Yet They Are Often Overlooked and Unappreciated Page 1 of 1 Biology 122L – Invertebrate zoology lab Cnidarian diversity lab guide Author: Allison J. Gong Figure source: Brusca and Brusca, 2003. Invertebrates, second edition. Sinauer Associates, Inc. INTRODUCTION Cnidarians are some of the most conspicuous and colorful tidepool animals, yet they are often overlooked and unappreciated. To the untrained eye their radial symmetry makes cnidarians seem more plantlike than animallike, and the colonial forms of hydroids often have a "bushy" appearance that reinforces that mistaken first impression. However, cnidarians are indeed animals, and a closer examination of their bodies and behaviors will prove it to you. The generalized body plan of a cnidarian consists of an oral disc surrounded by a ring (or rings) of long tentacles, atop a column that contains the two-way gut, or coelenteron. This body plan can occur in either of two forms: a polyp, in which the column is attached to a hard surface with the oral disc and tentacles facing into the water; and a medusa, in which the column is (usually) unattached and the entire organism is surrounded by water. In the medusa phase the column is flattened and generally rounded to form the bell of the pelagic medusa. Cnidarian tentacles are armed with cnidocytes, cells containing the stinging capsules, or cnidae, that give the phylum its name. Cnidae are produced only by Page 2 of 2 cnidarians, although they can occasionally be found in the cerata of nudibranch molluscs that feed on cnidarians: through a process that is not understood, some nudibranchs are able to ingest cnidae from their prey and sequester them, unfired, in their cerata for defense against their own predators. Cnidarian polyps and medusae are passive predators that hang cnidocyte-laden tentacles in the water column and catch whatever prey blunder into them. Most cnidarians feed on small prey – copepods, worms, larvae, etc. – but some can immobilize large animals such as fishes. In fact, one of the deadliest toxins in the animal kingdom is produced by the cubomedusa Chironex fleckeri; every year on the Great Barrier Reef a few people are killed by the sting of these sea wasps. There are currently three recognized classes of cnidarians. The Class Hydrozoa contains the freshwater Hydra as well as marine hydroids and hydromedusae; the Class Scyphozoa contains the large marine medusae, or jellies. The Class Anthozoa is the most diverse class and contains the corals, sea anemones, soft corals, and gorgonians. Page 3 of 3 CLASS HYDROZOA Individual hydrozoan polyps are usually very small, on the order of a few millimeters, but colonies can be quite large, consisting of hundreds of genetically identical polyps. Similarly, a single polyp is morphologically very simple, comprising a Page 4 of 4 mouth on a stalk (manubrium) surrounded by one or more rings of filiform or capitate tentacles. If a colony is reproductive the polyps may carry medusa buds or gonophores, which are degenerated medusae that are retained on the polyp. The distal end of a hydroid polyp, which bears the mouth and feeding tentacles, is called a hydranth. Hydranths are usually raised above the level of the basal stolons by a length of hydrocaulus. All hydroid colonies contain some amount of perisarc or periderm, a chitinous exoskeleton that surrounds the living tissue, or coenosarc, of the stolons and stalks. The perisarc may be transparent and difficult to distinguish from the coenosarc, or it may be a golden-brown color and have a noticeably tougher texture than the coenosarc. In thecate hydroids, the perisarc extends beyond the end of the stalk to form a cup, or theca, into which the hydranth can withdraw; some hydrothecae are equipped with an operculum that can be pulled down to shut the hydranth inside the theca. Obelia and Orthopyxis are local examples of thecate hydroids. Thecae are usually completely transparent, and may not be visible unless you get the light shining at exactly the right oblique angle. They may be wineglass-shaped or ovoid. In athecate hydroids there is no theca and the hydranth is continually exposed. Tubularia is an athecate hydroid with large pink hydranths bearing two rings of filiform tentacles and long (up to 10 cm) stalks. Sarsia is another athecate hydroid and is much smaller; its hydranths are stubby and bear capitate tentacles. Colonies, on the other hand, can be quite complex, growing horizontally over surface nooks and crannies or vertically to produce bushy upright forms. All of the polyps in a colony are connected by a shared gastrovascular cavity (GVC). Food ingested by one or a few polyps is circulated and shared by all polyps in the colony. The sharing of food resources has allowed some polyps to specialize in functions other than feeding. This remarkable phenomenon is known as polymorphism. In polymorphic colonies, non-feeding polyps are morphologically and functionally differentiated to carry out duties such as defense (dactylozooids) and reproduction (gonozooids). You may be able to observe polymorphism in some of the colonies in today's lab. If so, how many types of polyps can you identify? You should note, however, that not all hydroid colonies are polymorphic. Page 5 of 5 Hydroid colonies are formed by a clonal process called budding. A primary polyp buds to produce other polyps, which may arise from a stolon, a hydrocaulus, or the junction between a stolon and a hydrocaulus. Colony growth forms can be broadly described as either stoloniferous or upright. Stoloniferous colonies branch in only one plane, the basal plane along which the stolons grow; these colonies tend to grow as encrusting mats over surfaces. Upright colonies branch in the basal plane, but also branch up and away from the basal surface; these species form the bushy colonies that superficially resemble plants in their morphology and growth. A given species has either Page 6 of 6 a stoloniferous or an upright morphology. Although some details of colony growth can be modified experimentally by pruning, the overall growth pattern is consistent within a species. Polyps represent the asexual, or clonal, phase of the general cnidarian life cycle. The sexual phase is represented by a pelagic medusa. In some species, the sexual phase of the life cycle is retained by the polyp stage, either as the sessile gonophores of Tubularia or the polymorphic gonozooids of Hydractinia. In species that have a planktonic medusa stage, medusae are produced by polyps in structures called medusa buds. In the Monterey Bay area, medusae of the thecate hydroid Obelia are found year- round in coastal plankton samples, and the hydroid itself can usually be found on pilings and docks in local harbors. CLASS SCYPHOZOA Scyphozoans are the large pelagic medusae that often occur in swarms (called "smacks") in coastal waters. The medusa may be the conspicuous stage for these animals, but it plays a relatively minor role in the life cycle. Medusae are ephemeral, appearing in the water column on a seasonal basis, but the benthic polyp phase of the Page 7 of 7 scyphozoan life cycle is a persistent stage that produces medusae intermittently for many years. Scyphozoan polyps, or scyphistomae, can sometimes be found on the undersides of floating docks, where clones can cover large areas despite the polyps' small body size. In lecture we discussed the scyphozoan life cycle and the relationship between polyp and medusa. Keep this relationship in mind as you observe the different life history stages of the moon jelly, Aurelia sp. The scyphistoma of Aurelia is a small funnel- shaped polyp, usually 3-10 mm long, attached at its basal end by a few stolons. Unlike hydroid polyps, scyphistomae do not have a protective coating of periderm and can "walk" around a bit by selectively detaching and re-attaching individual stolons. Scyphistomae have filiform tentacles that are well armed with batteries of cnidocytes, which might be visible as tiny bumps on the fine tentacles. If you can look down onto the oral side of a relaxed scyphistoma, you might be able to see into the GVC. The GVC is a two-way gut whose single opening functions both as a mouth and an anus. The interior cavity of the GVC is partially divided by septa into four compartments; these are homologous to the gastric pouches seen in the medusae of Aurelia. Scyphistomae are capable of two clonal activities: budding and strobilation. Budding creates additional polyps and strobilation produces medusae. Scyphistomae bud on a more or less regular basis which can vary with food availability; strobilation appears to be a more seasonal phenomenon, although some clones of polyps strobilate at any time of the year. With any luck I will be lucky enough to have strobilating polyps (strobilae) and ephyrae for you to observe. A strobila is a polyp that has temporarily stopped feeding and is undergoing the process of producing medusae. It divides transversely to form a stack of saucer-shaped ephyrae, which break off individually from the distal end of the polyp and swim off into the plankton. After all of the ephyrae are released, the polyp regrows its tentacles and begins feeding again. The ephyrae of Aurelia are 8-armed creatures with a mouth and four gastric pouches. Contraction of the bell propels the animal through the water and simultaneously traps prey under the subumbrellar surface where they can be caught and eaten. This method of locomotion and prey capture continues as the ephyra grows into a sexually mature medusa. The cnidocytes of Aurelia are not particularly powerful, and this animal can subdue and eat only small prey such as brine shrimp. The bell of the medusa is ringed with a fringe of fine tentacles that are heavily laden with cnidocytes.
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