SPERM OF THE ASCOTHORACICAN PARASITEDENDROGASTER, THE MOST PRIMITIVE FOUND IN CRUSTACEA

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Abstract
A description of the sperm of an Antarctic Dendrogaster species, an ascothoracican parasite of starfish, based on light microscopy and scanning and transmission electron microscopy, is presented. The sperm heads are 6–7 μm, long, with a laterally-placed acrosomal vesicle at the tip. The midpiece is as long as the head, and tapers to a flagellum 45–50 μm long. The basal body of the axoneme is embedded in the posterior part of the nucleus, and the axoneme has the usual ring of nine microtubule bundles, though interior bundles were indistinguishable. Present knowledge of other ascothoracican sperm is reviewed, and the present sperm are shown to be the most primitive yet discovered in Crustacea. The hypothesis for the neotenic derivation of the Maxillopoda (Copepoda, Cirripedia and Ascothoracica, Mystacocarida, and Branchiura) from a eumalacostracan form is examined in light of this new information, and is found wanting. Furthermore, it is suggested that similarities among the spermatozoa of several maxillopodan groups may be synapomorphies, requiring a reconsideration of the classification of Ascothoracica as an order within the Cirripedia.