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Aplacophoran species are worm- or tubelike, from 1 mm to 300 mm in length, without a shell but covered by a coat of adpressed or upright aragonite sclerites. Each species has a characteristic body shape and sclerite morphology (see Treece 1979). There are a large number of unexamined aplacophoran species, both Chaetodermomorpha (= Caudofoveata), to which the Prochaetodermatidae covered here belongs, and Neomeniomorpha (= Solenogastres ) in the collections from the Gulf of Mexico. There is an unusually rich prochaetodermatid fauna in the Gulf of Mexico, with 5 genera, all with worldwide distributions, and with species ranging between the continental shelf at depths less than100 m to hadal depths greater than 8000 m. A total of 1046 Prochaetodermatidae, including 7 new species, were collected from the Gulf of Mexico in the years 1983, 1984, 1985, and 2000 at depths between 334 m and 3000 m. This prochaetodermatid fauna can be contrasted to only 3 known genera and 3 species in the Western Atlantic, north of Cape Hatteras. Prochaetodermatidae burrow through the surface substrate down to more than 10 cm, feeding on organic detritus and foraminifera. Like most molluscs, most Aplacophora have a radula, but only in the Prochaetodermatidae is it a rasping organ. Little is known about the life cycle of Prochaetodermatidae . The sexes are separate, and they presumably have lecithotrophic larvae, for they have settled into screened experimental boxes placed at ~1500 m on the sea floor (Scheltema 1987). The only aplacophorans previously reported from the Gulf of Mexico were collected during a Bureau of Land Management study of the continental shelf (Treece 1979). There were 134 individuals of unnamed species in 4 putative genera—?Prochaetoderma, Scutopus, ?Falcidens, and ?Pruvotina—certain from the illustrations only for Scutopus . The ?Prochaetoderma, ?Falcidens, and ?Pruvotina are certain only at the level of family. All the species reported here are therefore new, and because they are still in manuscript, they are referred to by genus name and the first letter of their to-be-published species names. Five of the seven species (Spathoderma b, Claviderma a, Claviderma m, Prochaetoderma g, and Niteomica c) have geographic distributions extending into the Atlantic south to the Guiana Basin. Spathoderma b and Niteomica c were also found in the very small and few samples available to us from off the coast of North Carolina and the south565 31 Aplacophora (Mollusca) of the Gulf of Mexico Dmitry L. Ivanov and Amélie H. Scheltema  Aplacophora. After Treece 1979, modified by F. Moretzsohn. 566 ~ Aplacophora (Mollusca) Abbreviations used under the heading “GMx range”: ne = northeast; nw = northwest. Acknowledgments This contribution was funded by the Mary Sears Endowed Visitor Program of the Woods Hole Oceanographic Institution and by MARB of Texas A&M University, Galveston . References 1. Blake, J. A., J. A. Muramoto, B. Hilbig, and I. P. Williams. 1992. Biological and sedimentological investigation of the sea floor at the proposed U. S. Navy ocean disposal site. July 1991 survey (R.V. WECOMA). Benthic biology and sedimentation characterization. Report for PRC Environmental Management, Inc., by Science Applications International Corporation. iii + 130 pp. 2. Grassle, J. F., and N. J. Maciolek. 1992. Deep-sea species richness: regional and local diversity estimates from quantitative bottom samples. American Naturalist 139: 313–341. 3. Ivanov, D. L., and A. H. Scheltema. In press. Western Atlantic Prochaetodermatidae from 35°N south to the Argentine Basin including the Gulf of Mexico (Mollusca: Aplacophora). Zootaxa. 4. Scheltema, A. H. 1987. Reproduction and rapid growth in a deep-sea aplacophoran mollusc, Prochaetoderma yongei. Marine Ecology Progress Series 37: 171–180. 5. Treece, G. D. 1979. Four new records of aplacophorous mollusks from the Gulf of Mexico. Bulletin of Marine Science 29(3): 344–364. east coast of Florida, with Claviderma a also found off the southeast coast of Florida. One could consider that these 5 species probably extend from Cape Hatteras into the Gulf of Mexico and south to at least the Guiana Basin. Although large collections have been made in the Argentine Basin, the Gulf of Mexico species did not occur there. We had no collections to examine between the Guiana and Argentine basins. Two of the species may be endemic to the Gulf of Mexico, Chevroderma c and Spathoderma q. The prochaetodermatids of the Gulf of Mexico belong primarily to the upper slope, that is, at depths less than 2000 m. Only Spathoderma b was collected below 2000 m. Prochaetodermatids are usually in deep-sea collections , and of all aplacophoran families, members of the Prochaetodermatidae are sometimes numerically dominant in...

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