Ultrastructure of the spermatozoon of Potamonautes perlatus sidneyii (Heterotremata, Brachyura, Crustacea)

Features of the spermatozoon of Potamonautes which are general for heterotremes and endorse its inclusion in the Heterotremata are: extension of the subacrosomal chamber almost to the anterior apex of the sperm; presence of an acrosome ray zone; and presence of a thickened ring (though here vestigial) where the capsule surrounds the base of the subacrosomal chamber. A feature shared with 'higher' heterotremes is the restriction of cytoplasm to the periacrosomal region, the arms being nuclear only; and loss of a posterior median process, containing chromatin, which is present in raninids and majids. Peculiarities of Potamonautes relative to other heterotremes (and paguroid anomurans) are (i) the indistinct substructure of the acrosomal ray zone; (ii) absence of a recognizable inner acrosomal zone differentiated from this, and (iii) reduction of the thickened ring, the latter trend being taken to completion in grapsid Thoracotremata. Unknown in other Brachyura are (iv) a rim-like electron pale expansion around the operculum and (v) the elongate condition of at least one of the two centrioles. These five unusual features are regarded as apomorphies of Potamonautes.