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Twenty-five named species of nemerteans and at least another 15 undescribed species are known from the Gulf of Mexico (GMx). Worldwide, there are about 1100 validly named species of nemerteans, with the vast majority known from littoral and near-shore marine habitats, while about another 100 are deep-sea pelagic forms, and a handful are terrestrial and freshwater forms (see also Gibson 1995, Crandall and Norenburg 2001). Most have been described from relatively small geographical regions, often the product of a single set of collections or of a single investigator. The nemertean fauna of most of the world is poorly known. Undocumented species are common even in well-studied regions, as is the case for the Gulf of Mexico (see the following). Adult worms range from 1 mm to over 30 m long, and thickness ranges from 0.1 mm to about 5 cm for the stoutest benthic forms. Nemerteans are unusual among macrobenthos in that their outer surface is a fully ciliated epithelium, which is highly glandular and overlies several cylinders of muscles containing a regionated gut, gonads, and the diagnostic hallmark for the phylum —the rhynchocoel-proboscis complex. The proboscis is used primarily in prey capture, but in a number of species it also can be used for locomotion and facilitating rapid escape. Although there are observations and anecdotal comments for relatively few species, it seems that most nemerteans are more or less predatory. Worms of the orders Paleonemertea and Heteronemertea most commonly seem to feed on other worms, including other nemerteans , but their diets also include bivalves, gastropods, and crustaceans, and some also scavenge. The proboscis in the order Hoplonemertea, with the exception of worms in the genus Malacobdella, bears armature in the form of one or a cluster of sharp stylets. These worms seem to have relatively narrow prey preferences—most often crustaceans —but also annelids. Worms of the genus Malacobdella have become obligate commensals in the mantle cavity of bivalves. They have lost the proboscis armature and become filter-feeding kleptovores. Hoplonemerteans of the genus Carcinonemertes are ectocommensals of many decapod crustaceans, but they continue to be predators . Developing crab embryos are their only known solid food, but they also may derive sustenance from dissolved organic matter leaked by their hosts (Crowe et al. 1982). The classification of nemerteans has changed relatively little in the last hundred years, despite several significantly different evolutionary hypotheses. The most extensive 553 29 Nemertea of the Gulf of Mexico Jon L. Norenburg  Nemertea. After Hyman 1951, modified by F. Moretzsohn. 554 ~ Nemertea fixing specimens in 10 volumes of 95% ethyl alcohol or in a solution of NaCl + DMSO (Dawson, Raskoff, and Jacobs 1998) to preserve DNA. There is scant literature relating to taxonomy or distribution of nemerteans in the GMx. In fact, only 10 papers formally address the taxonomy of GMx nemerteans ; foremost among these still is Coe’s (1954) contribution to Fishery Bulletin Volume 55, which mentioned 17 species, all apparently identified by him from various unspecified collections. However, he did not include the Florida Keys in his GMx account, thereby leaving out Cerebratulus leucopsis and Paradrepanophorus crassus (see Coe 1951a). Corrêa (1961) added 2 species, Ototyphlonemertes evelinae and Tetrastemma worki, both from southwest Florida. Only 4 species have been added since that time: Heteroenopleus enigmaticus, described recently (Wern 1998); Gorgonorhynchus bermudensis (see Gibson 1974); and range extensions for Ototyphlonemertes cf. duplex and Ototyphlonemertes cf. fila (see Envall and Norenburg 2001). In comparison, 108 nemertean species are known from the Atlantic coast of the United States (Crandall and Norenburg 2001). Three species reported by Coe (1954) now are hidden by synonymies: Cerebratulus ater (now Dushia atra), Micrura leidyi (now Fragilonemertes rosea), and Lineus socialis (now Rhamphogordius sanguineus). Euborlasia maxima Coe, 1905, has been reported as occurring in the Gulf of Mexico (Gibson 1995, Crandall and Norenburg 2001) but actually occurs in the Gulf of California. Gibson and Crandall (1989) listed Amphiporus cruentatus as nomen dubium, and A. ochraceus and A. texanus as species inquirendae. Those opinions are rejected here, as all 3 have been readily identified by Coe and the author among other U.S. East Coast specimens . Two recent papers deal with nemerteans of nearby molecular phylogeny to date (Thollesson and Norenburg 2003) confirms other studies suggesting that Paleonemertea minimally is a paraphyletic basal assemblage and that the 2 most species-rich groups, Heteronemertea and Hoplonemertea, accounting for about 900 species, are respectively monophyletic. This phylogeny’s major break with traditional classification is in...

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