, Volume 131, Issue 2, pp 259-267

Description and comparison of major foregut ossicles in hydrothermal vent crabs

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Major foregut (gastric mill) ossicles, including the dorsal median tooth, lateral teeth, accessory lateral teeth, and cardiopyloric valve, of hydrothermal vent crabs were dissected and examined during the summer of 1996 from specimens housed at the Natural History Museum of Los Angeles County. Ossicles are described for two species of hydrothermal vent crabs (family Bythograeidae Williams, 1980). The western Pacific Austinograea williamsi Hessler and Martin has an unusual dorsal median tooth. The surrounding cuticular flange is scalloped and bears spinulose setae at the tip of each of the protruding edges, a condition perhaps unique in the Brachyura. The lateral teeth are mostly unremarkable, bearing the typical large anterior denticles and deep serrations seen in other crab families, but with a higher number of serrations than is known for any species previously described. The accessory lateral teeth bear flattened, plate-like spines that are widest basally and that taper to a cylindrical tip. The basic armature of the foregut of Bythograea thermydron Williams, known only from vents in the eastern Pacific, is very similar. Scalloping of the median tooth borders is less pronounced, however, and the shape of the tooth itself and of the plate from which it arises is slightly different. The lateral teeth bear fewer and more widely spaced grooves, and the cardiopyloric valve entrance appears less setose at its extremity. Comparison with foregut ossicles in other crab families based on earlier studies, most of which have not employed SEM, reveals some similarities between bythograeids and some xanthids, but does not clarify the phylogenetic position of the bythograeids. Because of the paucity of other SEM studies of the brachyuran foregut, it is difficult to ascertain whether some of the many spine and setal types in the bythograeid foregut are unique or even unusual compared to those of other crab families. Nothing about the foregut of the vent crabs is indicative of their unusual habitat. Anecdotal observations of feeding in vent crabs indicate that they are opportunistic scavengers and omnivores, which is in keeping with the non-specialized nature of the foregut. The debate between adaptation vs phylogeny as determinants of the form of the gastric mill components is briefly discussed.

Received: 10 December 1996 / Accepted: 13 November 1997