Austinograea williamsi, New Genus, New Species, a Hydrothermal Vent Crab (Decapoda: Bythograeidae) from the Mariana Back-Arc Basin, Western Pacific

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Abstract
AUSTINOGRAEA williamsi is described from hydrothermal vents in the Mariana Back-Arc Basin, a spreading center in the western North Pacific, at depths of 3,595-3,660 m. This is the first bythograeid crab described from the western Pacific, extending the range of the Bythograeidae approximately 12,500 km from the nearest previous collection sites along the East Pacific Rise. The species differs remarkably from all other brachyuran crabs in lacking eyes or moveable eyestalks; the possible remnant of the eyestalk is fused to the surrounding orbital region of the carapace and bears no cornea or pigment. The species is further distingushed by the coxa of the third maxilliped, which is nearly covered by the juxtaposition of the margin of the carapace with the sternum and chelipedal coxa, a character proposed herein to be of generic importance. Other characters serving to separate A. williamsi from species in the genera Bythograea and Cyanagraea are the presence of setose fields on the subhepatic surface and on the ventral dactylar surface of the chelae in both sexes, densely setose ventral margins on the merus and basi-ischium of all walking legs, chelipeds with fingers that meet along their entire occluding margins in both sexes (i.e., do not gape), and a relatively straight male first pleopod that bears small spinules and is longer than the second pleopod. The third maxillipeds and male pleopods of Cyanagraea, previously unknown, are described from a specimen taken at 13.degree.N on the East Pacific Rise; the pleopods are more similar to those of Austinograea than to those of Bythograea. A key to the genera of the Bythograeidae is provided.

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