A New Species of Gigantidas (Bivalvia: Mytihdae) from a Vent Site on the Kaikata Seamount Southwest of the Ogasawara (Bonin) Islands, Southern Japan

Abstract

A new species of mytilid mussel, Gigantidas horikoshii n. sp., is described. This new species was found burrowing in a sandy bottom from which warm water was seeping out, on the submarine volcano Kaikata Seamount, located southwest of the Ogasawara (Bonin) Islands in Southern Japan. G. horikoshii n. sp. is characterized by the absence of a welldeveloped valvular siphonal membrane, the absence of a continuation of the inner mantle fold across the anterior adductor muscle and by the attachment point of the anterior retractor on the valve being situated out of the umbonal cavity in the same manner as in G. gladius, the only other known species of the genus Gigantidas. The present species is distinguishable from G. gladius by the large swollen umbonal cavity, the markedly concave ventral margin and shell proportions such as shell height and shell width relative to shell length.