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Marine worms of the phylum Sipuncula are found in all of the world’s oceans from intertidal waters to abyssal depths. Varying in size from approximately 2 mm to 400 mm, sipunculans occupy a variety of habitats. Some species burrow into sandy or muddy sediments, whereas others occur under rocks, in crevices, in algal mats, or entwined among the roots of seagrasses or the byssal threads of mussels. In the tropics they are commonly associated with coral reefs where they may inhabit coral rubble or other calcareous substrates, often in burrows or borings of their own formation. First considered as a separate phylum in 1898 (Sedgwick 1898), the group is distinguished by a unique combination of characters, such as a spacious, unsegmented body cavity; unpaired, unsegmented ventral nerve cord; anterior protrusible “introvert” bearing a terminal mouth; and, internally, an elongate esophagus and recurved intestine with anus opening dorsally, usually at the base of the introvert near its junction with the posterior body or trunk. Over the past 50 years, since the last treatment of the fauna of the Gulf of Mexico (GMx) (Hedgpeth 1954), the phylum Sipuncula has been subjected to major taxonomic revisions, at both higher and specific taxonomic levels. In 1965 the term Sipuncula was proposed as a designation for the phylum (Stephen 1965), thus replacing the previously used terms of Sipunculoidea (Sedgwick 1898) and Sipunculida (Hyman 1959). Subsequently, the term sipunculan was proposed as the common name for an animal belonging to the phylum (Stephen and Edmonds 1972). These changes avoid the use of the ending –oidea, which, accordingtorecommendation29aofthe1961InternationalCode of Zoological Nomenclature, should be used to indicate superfamilies, and the ending –id, which is used in reference to a member of the family Sipunculidae. The terms Sipuncula and sipunculans have been adopted by all those currently engaged in sipunculan systematics as well as by authors of recent textbooks in invertebrate zoology. The most recent and extensive taxonomic revision of the phylum Sipuncula and the one recognized in this chapter is that proposed by E. B. Cutler in 1994. In this review of the biology of sipunculans, Cutler summarizes taxonomic revisions, published by Cutler and colleagues over a 20-year period (Cutler 1994). Prior to Cutler’s revisions , the accepted classification of the phylum was that of Stephen and Edmonds (1972), who were the first to designate families within the phylum; they recognized 4 families , 17 genera, and 321 species. Cutler’s 1994 treatise recognizes 6 families, 4 orders, and 2 classes. Several genera have been redefined and the number of species reduced from 321 to 150. In accordance with these revisions, 5 of the 11 species listed in the 1954 Bulletin 89 have undergone taxonomic 803 41 Sipuncula of the Gulf of Mexico Mary E. Rice  Sipuncula. After Brusca 1980, modified by F. Moretzsohn. 804 ~ Sipuncula collections have been transferred to the National Museum of Natural History, and their USNM catalog numbers are listed in the endnotes of the appended checklist. The endnotes are also utilized to report synonyms of names found in the literature or in museum collections for the species indicated. The overall geographic range, as reported in the appended checklist, is taken from Cutler’s 1994 review of the phylum. It should be noted that none of the species listed is endemic or nonindigenous in the Gulf of Mexico (see taxon summary). This chapter summarizes the current state of knowledge of the sipunculan fauna in the Gulf of Mexico. However , it is focused almost entirely on the southeast and northeast areas of the Gulf from which identified collections are available. There are vast areas of the Gulf that have yet to be explored and there are large existing collections that have yet to be examined or identified. For example , in the Smithsonian’s National Museum of Natural History, there is a collection of sipunculans from Louisiana ’s Lease Areas in the Gulf of Mexico that numbers over a thousand specimens of unconfirmed identification . Another region of the Gulf insufficiently reported or explored is the more tropical southern areas along the Mexican coast. Abbreviations These are the abbreviations used in the checklist: ben = benthic; bur = burrower or borer; crr = coral reef; dps = deep sea; hcs = hydrocarbon seep; hsb = hard substrate; inf = infaunal; int = intertidal; orf = oyster reef; rbl = rubble; sft = soft substrate (mud, sand, clays); sub = subtidal . Abbreviations in the “GMx range” column are for quadrants shown in chapter 1, fig. 1. In the “Overall geographic range” column, E = East; N = North; NE = northeast...

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