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20. Siphonophorae (Cnidaria) of the Gulf of Mexico
- Texas A&M University Press
- Chapter
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The siphonophorae are a subclass of the class Hydrozoa , phylum Cnidaria. They are highly polymorphic and complex animals, and their zooids, both medusoid and polypoid, have become specialized to carry out various functions, such as swimming, feeding, reproduction, and protection. Although phylogenetically they originated as colonies, structurally, physiologically, and embryologically they qualify as an organism (Wilson 1975), and they are sometimes referred to as “superorganisms” (Mackie 1963). They are exclusively marine, and the vast majority of species are pelagic. The only exceptions are a family of physonect siphonophores, the Rhodaliidae, that use their tentacles to attach themselves to the bottom substrate, and the only well-known species, the Portuguese Man o’ War Physalia physalis, which is pleustonic, floating at the surface with its tentacles hanging down many tens of meters into the water below. Siphonophores occur in a bewildering array of shapes and sizes, and can vary in length from a few millimeters to several tens of meters; some are undoubtedly the longest animals in the world (Robison 2004). They can be found throughout the world’s oceans, and throughout the water column down to a depth of at least 4500 m, although individual species are restricted in their geographical and depth distributions to a lesser or greater extent. Classically, siphonophores have been split into 3 orders, based first on the presence (Cystonectae and Physonectae ) or absence (Calycophorae) of an apical, gasfilled float (pneumatophore). The orders whose species possess a pneumatophore are split according to whether asexual swimming bells (nectophores), arranged in a biserial or spiral series immediately below the float, are present (Physonectae) or absent (Cystonectae). However , recent molecular studies (Dunn, Pugh, and Haddock 2005) have shown that although the Cystonectae are monophyletic and sister to all other siphonophores, the Physonectae are paraphyletic and gave rise to the Calycophorae . Dunn, Pugh, and Haddock (2005) suggested that these 2 latter groups should be collectively called the Codonophora. The consequence of this study is that the traditional taxonomy of the Physonectae will have to be 395 20 Siphonophorae (Cnidaria) of the Gulf of Mexico Philip R. Pugh and Rebeca Gasca Siphonophora. After Brusca and Brusca 2003, modified by F. Moretzsohn. 396 ~ Siphonophorae (Cnidaria) is largely a reflection of sampling effort, and one would expect to find all the species throughout the area. Most of the 13 species found exclusively in the SE quadrant were recently collected by the submersible Johnson-Sea-Link, and the only quadrant where net sampling took place at depths greater than 500 m was the SW one, where 9 species were exclusively found. Abbreviations In the checklist, the families, and the genera and species within each, are listed in alphabetical order for each of the orders—Cystonectae, Physonectae, and Calycophorae , respectively. Under the heading of Habitat-Biology, each species is considered to be either neritic (ner) or deep-sea (dps), and, for the pelagic species, their depth distribution is categorized as epipelagic (epp = 0–ca. 300 m), mesopelagic (mep = ca. 300–1000 m), and/or bathypelagic (bap = >1000 m). The latter categories refer the usual depth ranges where the species occur (Pugh, unpublished data), which may not be reflected in the records from the Gulf of Mexico itself. One species is pleustonic (ple). Under the heading Overall Geographic Range, the geographical distribution of a species is considered in 3 ways. First, whether the species has a widespread distribution in the world’s oceans (world) or is restricted to either the Atlantic (Atl), Indian (Ind), or Pacific (Pac) Oceans, or to the Mediterranean (Med) Sea. For rarer species, where there is no clear-cut pattern of distribution, they are considered to have sporadic occurrences. Second, within these distributional zones, the waters where the species is most abundant, when known, are considered broadly as either tropical (trop), subtropical (subtrop), temperate (temp), or boreal. It should be recognized, however, that these are very coarse categories and that their boundaries vary spatially. Third, the relative abundance of each species is indicated by the categories: v. common, common, uncommon, rare, and v. rare. ThespecimensfromwhichthenewrecordsfortheGulf of Mexico are derived are currently housed in the National Oceanography Centre Collections (NOCC). The numbers refer to the Johnson-Sea-Link dives during which the specimens were collected. At some time in the future, once the specimens have been fully examined and described, it is intended to transfer them to the U.S. National Museum of Natural History (Smithsonian Institution). Abbreviations used in the Habitat-Biology column of the checklist include bap = bathypelagic; dps = deep sea...