MORPHOLOGICAL OBSERVATIONS ON THE SPERMATOZOA OF ECHINOTHURID SEA URCHINS*

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Abstract
The morphology of the spermatozoa of 3 spp. of echinothurid sea urchins, Asthenosoma ijimai, Araeosoma owstoni and Hapalosoma gemmiferum, was investigated using transmission and scanning electron microscopy. The spermatozoa of these echinothurid sea urchins have similar fine structure, but they differ in several features from the more familiar regular sea urchins. The external anatomy of the head region of the echinothrurid spermatozoon is diagnostic in that it has a highly elongated head. The spermatozoon of echinothurid sea urchins has a very long slender nucleus, protruding on its proximal end, so that the shape of the nucleus resembles a spearhead. The acrosomal granule in the acrosomal vesicle of the echinothurid spermatozoon is not a mass of homogenous particulate material but an electron opaque rod condensed in the central part of the acrosomal vesicle. Scanning electron microscopic examination revealed that echinothurid spermatozoa from acrosomal processes similar to those of other regular sea urchins. The basal body is situated just beneath the middle of the posterior protrusion of the nucleus. The distal centriole is located beside the basal body almost in contact with it. The axis of the distal centriole is nearly parallel to that of the basal body. A satellite complex can be recognized around the posterior part of the proximal centriole.