Morphology of gametes in five species of far-eastern chitons

Abstract

Summary Sperm development, structure of the gametes and some aspects of fertilization in the far-eastern chitons Leptochiton assimilis, Deschayesiella curvata (fam. Leptochitonidae), Tonicella undocaerulea (fam. Tonicellidae), Mopalia retifera (fam. Mopaliidae) and Ischnochiton hakodadensis (fam. Ischnochitonidae) are described by transmission electron microscopy. In the testis spermatogenetic cells are closely associated with auxiliary cells, which are structurally identical to those of other mollusks and probably have a nutritive function. Spermatogenesis in all species did not differ from that of chiton species already studied and it was confirmed that an acrosome is formed from the Golgi apparatus during spermiogenesis. The spermatozoa of all studied species have a structure that is typical for chitons. The sperm of Deschayesiella curvata (Order Lepidopleurida) have a head consisting of a large acrosome and nucleus with an (3.5–4.2μm) asymmetrically positioned anterior extension. The centrioles are surrounded by five spherical mitochondria. The spermatozoa in Tonicella undocaerulea, Mopalia retifera and Ischnochiton hakodadensis (Order Chitonida) have a small acrosome and a nucleus with long thin anterior extension; mitochondria are lateral to the nucleus. There are considerable interspecific differences in the structure of egg envelopes. Our results, along with those of other workers, show that gamete structure of chitons is specific to each family. A cortical reaction which results in formation of a fertilization envelope takes place in the egg of chitons after fertilization.