Zooplankton of the Atlantic and Gulf Coasts: A Guide to Their Identification and Ecology

Cnidarians: Anemones, Jellyfishes, and Related Metazoans

CNIDARIANS Anemones, Jellyfishes, and Related Metazoans The Phylum Cnidaria, sometimes called Coelenterata, contains the simplest of the true metazoans. These radially symmetrical animals have a central mouth, usually surrounded by tentacles. The simple tissues, an outer epidermis and inner gastrodermis, surround a layer of gelatinous mesoglea. Cnidarians are distinguished by having unique stinging cells, cnidoblasts, which are largely responsible for the remarkable success of the phylum. Many cnidarians have two distinct body forms in their life cycle, a sessile polyp stage and swimming medusa stage.The classes Hydrozoa, Scyphozoa, and Cubozoa have planktonic medusoid stages. The Staurozoa are sessile medusoids, and theAnthozoa (e.g., anemones, corals, sea whips) are typically sessile polyps. Only the larvae of the latter two are planktonic. DISTINGUISHING CHARACTERISTICS OF PLANKTONIC CNIDARIANS Hydrozoans (Class Hydrozoa) Hydromedusae have a thin, transparent bell usually <2 cm in diameter. The mouth extends into a narrow tube but typically lacks oral arms. A thin velum extends inward from the edge of the bell. A tubelike manubrium may extend from the stomach. Porpitids (Velella, the by-the-wind sailor, and Porpita, the blue button) are colonial, open-ocean animals that consist of a flattened rigid or semirigid disc surrounded by tentacle-bearing dactylozooids extending downward from the subsurface. Porpitids are a distinctive bluish color and float on the surface, often in aggregations. Siphonophores, comprising a distinct order of Hydrozoa, have one or more elongated swimming bells that trail a string of tentacles. An exception is the Portuguese manof -war, which has a large float and many long tentacles. Scyphozoans (Class Scyphozoa) are large “jellyfishes” with a diameter often exceeding 6 cm. Oral arms surround the mouth, and tentacles may or may not also be present. Cubozoans (box jellies, Class Cubozoa) are jellyfishes with a squarish bell and with tentacles attached at the corners. The inclusion of large and often surprisingly fast-swimming scyphozoan and cubozoan medusae among the zooplankton is often debated. Since adults are often caught in collections along our coasts, are readily observable, begin life as much smaller forms, and consume many types of smaller zooplankton, we include them in this guide. There are relatively few species of large planktonic hydrozoans but many species of small hydromedusae. ...