Dwarf males in the teredinidae (bivalvia, pholadacea).

Extreme sexual dimorphism in the Bivalvia is rare. The occurrence of dwarf males in Zachsia appears to be the first case in the Teredinidae and the first outside the Leptonacea. Female Zachsia release straight-hinge larvae that develop in the plankton and settle on living rhizomes of Phyllospadix. Larvae entering mantle pouches of females become males. Evolution of this life history pattern is tied to problems of living in a fragile, patchy habitat-that is, the rhizomes of Phyllospadix.