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Euphausiids are exclusively marine holoplanktic crustaceans ; they are distributed in all latitudes and from the surface layers to the great oceanic depths. They are generically known as “krill,” a term originally used in connection with the euphausiids of the cold waters of the North Atlantic (Brinton 1962). They are the second most abundant group of the crustacean zooplankton, after the copepods , but they are frequently more relevant than copepods in terms of biomass and number, especially at greater depths and in polar regions (Mauchline 1980, Antezana and Brinton 1981). Euphausiids can form overwhelmingly dense swarms in cold and temperate areas (Mauchline 1980, Antezana and Brinton 1981). Overall, they constitute 5–10% of the total biomass of the zooplankton and approximately 30% of the crustacean biomass (Mauchline and Fisher 1969). Longhurst (1985) stated that euphausiids alone represent 20–40% of the biomass available in the world ocean. At higher latitudes these crustaceans are a key intermediate link between the planktonic primary producers and the upper trophic levels (Brinton 1975), and are part of the diet of fish and whales (Boden, Johnson , and Brinton 1955, Mauchline and Fisher 1969, Antezana 1970, Brinton 1996). Euphausiids are distinguished from other pelagic crustaceans by the presence of external gills, a reduced maxillar exopod, and a characteristic larval development. There are taxonomically relevant characters in the carapace (lateral spines, shape of keel, rostrum), the thoracic legs, and the eyes. The structure of the genital apparatus is particularly relevant in recognizing species within some genera. The length of most euphausiids varies between 10 and 30 mm; the smallest forms have a length of 5.5 to 7.8 mm (Stylocheiron suhmi). Some species can be as long as 150 mm (Thysanopoda spinicaudata) (Antezana and Brinton 1981, Baker, Boden, and Brinton 1990). Their current taxonomy was developed by Boden, Johnson, and Brinton (1955), Mauchline and Fisher (1969), Mauchline (1980), Baker, Boden, and Brinton (1990), and Brinton (1962, 1975, 1996), among other researchers. The relatively low number of species of Euphausiacea and the well-defined distributional patterns of many species make them potential indicators of oceanographic conditions (Brinton and Townsend 1980, LavaniegosEspejo , Lara-Lara, and Brinton 1989, López-Cortés 1990, Lavaniegos 1994, Brinton 1996, Biggs et al. 1997). Recent studies in tropical areas (Gómez-Gutiérrez et al. 1995, Biggs et al. 1997, Castellanos and Gasca 1999, Gasca, Castellanos, and Biggs 2001) prove that the structure of the euphausiid communities are sen1013 58 Euphausiacea (Crustacea) of the Gulf of Mexico Iván A. Castellanos and Eduardo Suárez-Morales  Euphausiacea. After Rupert, Fox, and Barnes 2004, modified by F. Moretzsohn. 1014 ~ Euphausiacea (Crustacea) abundant species within a Gulf anticyclone. Castellanos and Suárez-Morales (2002) provided a list of 34 species for the Gulf and provided data on their vertical distribution . Worldwide revision works by Brinton (1962, 1975), Mauchline and Fisher (1969), Mauchline (1980), and Reid et al. (1978) include some data referred to the Gulf of Mexico. Taxonomy and Number of Species Described The euphausiids represent a well-defined group within the subclass Eucarida. They are divided into 2 families: Bentheuphausiidae, which is monotypic (Bentheuphausia amblyops), and Euphausiidae, which contains all the other species known; they are currently distributed among 10 genera. Mauchline (1980) recognized 85 species in the order Euphausiacea. After Mauchline’s account, the number of nominal species increased to 87 because of the reinstating of the taxonomically questioned Stylocheiron armatum on the basis of specimens collected in the Arabian Sea (Mathew 1980) and the description of the deepliving Thysanopoda minyops (Brinton 1987). However, Baker, Boden, and Brinton (1990) suggested that Euphausia similis var. armata is not a species-level taxon; hence, the number of euphausiid species considered in our analysis is 86. The 34 species recorded in the Gulf of Mexico represent nearly 40 percent of all known species, a relatively high percentage for a single basin. This number of species is higher than that known in adjacent tropical and subtropical oceanic areas such as the Caribbean Basin and the Florida Current (see taxon summary). Up to 19 species known in the Gulf waters are epipelagic forms; 12 species are mesopelagic, and 3 are bathypelagic. Representatives of the 2 euphausiid families, Euphausiidae and Bentheuphausiidae, have been found in the Gulf. At the generic level, only 4 (Meganyctiphanes, Nyctiphanes, Pseudeuphausia , and Tessarabrachion) out of the 11 genera currently recognized have not been recorded in the Gulf of Mexico or in adjacent waters (see taxon summary). Aside from the potential discovery of many...

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