Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 68

Intracellular parasites of marine and freshwater vertebrate ectotherms, annelids, bryozoa, and sipunculids. Two classes and two orders. About 2200 species.

Open‐ocean species are often not as well understood or well described as nearshore species because of insufficient access to specimens. This is particularly true of the Cnidaria. Taxonomic keys and good drawings of species are limited, and many of the best treatments of cnidarian groups are quite old. For decades, the oceanographer’s gold standard for identifying medusae resided in the works of two authors, F. S. Russell (Medusae of the British Isles, volumes I and II) and P. L. Kramp (“The Hydromedusae of the Atlantic Ocean and Adjacent Waters”; Medusae of the World). Both authors built on the work of luminaries preceding them, most notably A. G. Mayer’s three‐volume Medusae of the World. Recently, the benchmark works of Kramp and Russell have been supplemented with a new, multiauthor, treatment on the South Atlantic Zooplankton (Boltovskoy 1999) that gives a very nice summary of the systematics of the Cnidaria, though as in all things systematic it is not one that is universally accepted. Classification within the Cnidaria is very well treated by Mills et al. (2007) in the new Light and Smith’s Manual (J. T. Carleton, ed.).

The classification scheme above includes recent consensus views on major groups within the Cnidaria and how they are related. Two groups stand out as unusual within the hydromedusae: the siphonophores and the “by‐the‐wind‐sailors” (sometimes known as the chondrophores). We will treat them separately because their biology is different from the remainder of the hydromedusae, even though they are now considered to be an order (Siphonophorae) and a family (Porpitidae) within the hydromedusae.

If all the classification schemes are considered together, including those of Kramp and Russell, five basic divisions within the medusae of the hydromedusae are apparent: the Anthomedusae, the Leptomedusae, the Limnomedusae, the Trachymedusae, and the Narcomedusae. Species in the first three groups have a polyp stage in most instances and thus show alternation of generations. The second two groups show direct development. We will consider the medusae within the Hydromedusae to have the five basic divisions noted above, which most closely approximate the classical literature. Whether those divisions are orders or suborders is less important than the fact that the hydromedusae can be readily segregated morphologically using those divisions, and they have been for the better portion of 100 years.