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The sea stars and their relatives, comprising the phylum Echinodermata, are universal symbols of the ocean realm. They are strictly marine, living in all seas and at all depths; very few species are capable of tolerating the reduced salinity of estuarine waters. Where habitats are suitable, the echinoderms can form populations of enormous size, and they can dramatically affect the general economy of the benthos. There are approximately 6700 living species, and about 13,000 fossil species are known, ranging from the lower Paleozoic. Five classes of echinoderms are recognized today: the feather stars and sea lilies (class Crinoidea , 700 species); sea stars (class Asteroidea, 1800 species ); brittle stars (class Ophiuroidea, 2000 species); sea urchins (class Echinoidea, 900 species); and sea cucumbers (class Holothuroidea, 1300 species). Echinoderms are typically benthic animals, with a very few spectacular exceptions in the form of swimming holothurians (Miller and Pawson 1990). The feather stars and sea lilies are suspension feeders, extending their feeding arms to capture small drifting plankton organisms or particulate matter, and then passing their prey along food grooves to their mouths. Sea lilies and feather stars usually attach themselves to firm substrates; the feather stars may be epizoic, clinging with their prehensile cirri to sponges, sea fans, sea whips, corals, or other coelenterates. Most sea stars are epibenthic, but numerous species burrow into soft substrates, for at least part of the time. Sea stars forage on the seabed for food, eating live prey and carrion with equal alacrity. Some sea stars are capable of using their suctorial tube feet to open oysters, clams, and scallops. Others (order Brisingida) suspension feed by raising their arms into the water, superficially resembling feather stars in their feeding posture. Brittle stars are also scavengers and predators; some form aggregations and suspension feed with upraised arms, capturing prey with their sticky tube feet. The basket stars, with branching arms, are also efficient suspension feeders; they usually attach themselves to hard substrates before extending their feeding nets. Sea urchins include the more-or-less spherical “regular” urchins, which are epibenthic and generally feed on grass and weed and small associated organisms . The “irregular” bilaterally symmetrical sea urchins include the familiar discoidal sand dollars, which live partially or completely buried and ingest sand and sediment, and the infaunal heart urchins, which remain buried and feed on detritus and mud. Sea cucumbers may be epiben1177 71 Echinodermata of the Gulf of Mexico David L. Pawson, Doris J. Vance, Charles G. Messing, Francisco A. Solís-Marin, and Christopher L. Mah  Echinodermata. After Pratt 1916. 1178 ~ Echinodermata noderms. Harrison and Chia (1994) treat the microscopic anatomy of the phylum. Lawrence (1987) considers functional biology. A. H. Clark (1954) noted that the first published record of an echinoderm from the Gulf of Mexico was that of Seba (1759), who described and figured Stella marine polyactis seu Luna marina (= possibly the feather star Nemaster grandis) from New Spain (presumed to be Mexico). In the 1954 volume on the Gulf of Mexico, the echinoderms other than holothurians were listed, without additional comment, by A. H. Clark (1954), and the holothurians were dealt with in some detail by Deichmann (1954). A total of 434 species were listed (Crinoidea, 38 species; Asteroidea, 81 species; Ophiuroidea, 182 species; Echinoidea , 66 species; Holothuroidea, 67 species). Both Clark and Deichmann included in their lists species that were as yet unknown from the Gulf of Mexico, but that were expected to occur there. Consequently, it is difficult to compare, in terms of absolute numbers, the current listing (see the taxon summary) with those of Clark (1954) and Deichmann (1954). Suffice to say that, with 512 species now known from the Gulf of Mexico in the strict sense, there has been a significant increase in our knowledge of the Gulf echinoderm fauna over the past 50 years. These 512 species comprise approximately 7% of the echinoderm fauna of the world. Diversity of echinoderms in the Gulf of Mexico is relatively high, and compares with the New Zealand echinoderm fauna of 535 species (D. Gordon , personal communication); the New Zealand area is more than twice as large as the Gulf of Mexico. The Gulf of Mexico echinoderm fauna is in itself not distinctive, as evidenced by the low (6%) endemism rate. The Gulf fauna is a typical western central Atlantic fauna, most taxa being shared with the general Caribbean area, or thic or infaunal. They may selectively or nonselectively ingest sediment with the aid of their well-developed feeding tentacles, or...

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