Pylochelids, or the «symmetrical» hermit crabs are characterized by paired abdominal appendages (pleopods) on somites 2-6 of the calcified abdomen, in contrast to the asymmetrical pleopods, on an uncalcified abdomen, of the majority of hermit crabs in the superfamily Paguroidea. The spermatozoa of Pylocheles (Bathycheles) sp. are large cells (c. 14 µm wide) with a conspicuous depressed acrosome vesicle in the shape of a thick disk. A perforate (ring-like) operculum is positioned apically on the acrosome vesicle, and a wide, but shallow, perforatorial chamber penetrates the posterior end. Up to three short arms emanate from the cytoplasm of the sperm cell, but it is uncertain if these arms are microtubular in nature. The spermatozoal morphology provides no evidence for close affinities with any extant hermit crab family and indicates an isolated position for the Pylochelidae, though not incompatible with an origin from an ancestor shared with the Anomura.
Crustacea, Decapoda, Anomura, Paguroidea, Pylochelidae, Pylocheles, Spermatozoa, ultrastructure, symmetrical hermit crabs.