Ultrastructural Study of Spermiogenesis in the ChitonAcanthopleura haddonifrom the Jordan Gulf of Aqaba (Red Sea)

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Abstract
Ultrastructural study of spermiogenesis in the chiton Acanthopleura haddoni (Mollusca, Polyplacophora) reveals that early spermatids have round to oval nuclei with granular chromatin. A few mitochondria, a Golgi body, two centrioles, and elements of endoplasmic reticulum are present. Middle spermatids have nuclei with filamentous chromatin material, and the nuclear envelope exhibits a posterior implantation fossa to which a basal plate is aligned. The proximal and distal centrioles are at right angles, and pericentriolar bodies are observed lateral to the proximal centriole. A primordium of the tail appears early in spermiogenesis. No proacrosomal vesicles were observed, and the Golgi body was always located posterior to the nucleus. In late spermatids, the nucleus is bullet-shaped, and chromatin is sheet-like. In fully developed spermatids, the nucleus is extremely dense, and has an anterior extension. No acrosome is present. Three mitochondria constitute a very short middle piece, and an eccentric tail is established. Throughout spermiogenesis, spermatids were connected by cytoplasmic bridges, and no microtubules were observed. The sperm of this species of chiton fits into the modified primitive sperm model.