Modified Sperm Ultrastructure in Four Species of Soft-Bodied Echinoids (Echinodermata: Echinothuriidae) From the Bathyal Zone of the Deep Sea

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Abstract
Sperm of the bathyal echinothuriid echinoids Phormosoma placenta, Sperosoma antillense, Araeosoma fenestratum, and A. belli are similar to those of other echinoids, but have several unique morphological features involving the acrosomal vesicle, the nucleus, and the middlepiece. The acrosomal vesicle shows regional staining differences including a densely staining central region surrounded by an electron-opaque component. Sperm nuclei are highly elongated and abruptly taper posteriorly. With the exception of one species, the nuclei lack a distinct centriolar fossa. Intracellular droplets resembling lipid extend from the extreme posterior region of the middlepiece to form a collar around the proximal portion of the axoneme. The presence of lipid-like bodies in the middlepiece suggest that the sperm are long-lived and therefore require additional energy stores not found in most metazoan sperm. These findings are compared with a similar study of sperm ultrastructure in three shallow-water echinothuriid species, and their potential significance is discussed in relation to the present knowledge of echinothuriid reproductive biology.