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The subphylum Medusozoa is a group of solitary radiate metazoans. Their anatomical and physiological simplicity , their variety of reproductive strategies, and their capacity to produce nematocysts make them highly successful . Medusozoans are found in all of the oceans, at all latitudes and all depths. Only a few species of hydromedusae are adapted to live in brackish or fresh waters. Sexes are almost always separate. The Medusozoa are distinguished by alternations of stages between a benthic asexually-reproducing modular polyp and an individual planktonic sexually-reproducing medusa, although in some species either the medusa or polyp stages have been evolutionarily lost (Cornelius 1992, Boero et al. 1997). Algal symbionts are present in a few species. Medusozoa are seasonal top carnivores with an important role in the structure of pelagic ecosystems (Matsakis and Conover 1991). Their role as prey for fishes (Arai 1988), sea turtles, marine birds, and mammals has long been recognized (Ates 1991). They are also important to human economic activities such as tourism (Williamson, Fenner, and Burnett 1996), fisheries (Omori and Nakano 2001), and various coastal industries (Rajagopal, Nair, and Azariah 1989). During the half century after the appearance of the short papers by Sears (1954) and Hedgpeth (1954), several exhaustive works on Medusozoa have been published . The monograph by Kramp (1961) is particularly relevant because it contains brief diagnoses for the 900 known species, including distribution data and a complete list of literature published since 1910. Russell (1970) published his classical monograph on the Medusae of the British Isles, including detailed descriptions, illustrations, and much information on the biology of these species. An illustrated guide to Medusozoa from Canadian Atlantic waters was provided by Shih (1977). Wrobel and Mills (1998) have published an excellent illustrated field guide to the common gelatinous zooplankton from the Pacific coast, including medusozoans. An analysis of the biology of Scyphozoa compiled over the previous 35 years was provided by Arai (1997). The first comprehensive study oriented toward a unified system of hydroids and medusae was published by Naumov (1960). Since then, considerable progress has been made to fill in the large gaps in the knowledge of life histories involving both hydroids and medusae (Brinckmann-Voss 1970, Petersen 1990, Boero and Bouil369 18 Medusozoans (Cnidaria: Cubozoa, Scyphozoa, and Hydrozoa) of the Gulf of Mexico Lourdes Segura-Puertas, Laura Celis, and Luciano Chiaverano  Medusozoa. After Mayer 1906. 370 ~ Medusozoans (Cnidaria: Cubozoa, Scyphozoa, and Hydrozoa) ing around 1000 species. Of that number, 79 species (70 hydromedusae, 8 scyphomedusae, and 1 cubomedusae) were listed from the Gulf of Mexico up until 1954. In the last 50 years, major sampling efforts in coastal and shelf regions have resulted in a list of 117 species, including 98 (84%) hydromedusae, 16 (14%) scyphomedusae, and 3 (2%) cubomedusae, arranged in 81 genera and 47 families (see the species checklist). Of these, 72 (62%) species were recorded in estuarine-neritic waters, 27 (23%) in the oceanic region, and 18 (15%) in deep water. The vast majority of species (58) were recorded in the southeastern quadrant, mainly in the eastern Campeche Bank. Fortyfour species are known from the northeastern, 17 from the northwestern, and 45 from the southwestern sections. Only 11 species were recorded throughout the Gulf. It is clear that large gaps in information result from our inability to effectively sample large areas in the open ocean. Endemism seems to be absent from the Gulf. The occurrence of Laodicea brevigonia Allwein only in the northern Gulf may represent a “relict” population in that region (Phillips 1972). Medusozoa recorded from the Gulf include 13 (11%) species of medical importance. There only exists one documented human death resulting from envenomation by Chiropsalmus quadrumanus (Müller) (Bengston et al. 1991). Previous new records from the Gulf of Mexico were excluded from the tabular species checklist, given that they are contained in the checklist published by Segura-Puertas, Suárez-Morales, and Celis (2003). The sequence of higher taxa follows the phylogenetic arrangement proposed by Marques and Collins (2004). Abbreviations Families, genera, and species are arranged alphabetically. Under the heading of Habitat-Biology, the following strata descriptors are used: est = estuarine; ner = neritic; osp = oceanic/epipelagic; and dps = deep sea. Other descriptors in that column include: sym = symbiotic; sft = soft substrates /mud; nid = nonindigenous to Gulf of Mexico; and pth = potentially pathogenic. Depth ranges are restricted to the Gulf of Mexico records. Under the heading of Overall geographic range, the following abbreviations are used: ETP = Eastern Tropical Pacific; GMx = Gulf of Mexico. Abbreviations used in the...

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