Comparative spermatology of 11 species of Polyplacophora (Mollusca) from the suborders Lepidopleurina, Chitonina and Acanthochitonina

Abstract

Transmission electron microscopy of the spermatozoa and spermatogenesis of 11 species (in three suborders Chitonina, Acanthochitonina, Lepidopleurina) of chiton has shown that each species has a sperm with a unique morphology indicating that spermatozoa can be used as a taxonomic character. Although structure is species-specific, similarities between species within suborders and subfamilies can be recognized. The spermatozoa of species from the suborders Chitonina and Acanthochitonina have a head comprising nuclear material only, the anterior portion of which is in the form of a long thin (approximately 80 nm diameter) filament. In many species the centrioles and mitochondria of the mid-piece are lateral in position, the mitochondria often being sited anteriorly alongside the nucleus. By contrast, Leptochiton asellus , a member of the more ancient suborder Lepidopleurina, has a sperm with a head comprising a nucleus and an acrosome. The mid-piece is also more conven­tional in structure with a ring of five or six spherical mitochondria (sited behind the nucleus) that surround the centrioles. The presence of the acrosome in L. asellus suggests that in the more recent chitons the acro­some has been secondarily lost. It is proposed that loss of the acrosome is correlated to a modification in egg-coat thickness. A preliminary examination of the structure of the eggs of three species has shown that those of L. asellus are surrounded by a very thick chorion (14-30 μm) whereas in Acanthochitona crinitus and Dinoplax gigas there are regions of the chorion that are less than 2 μm thick. The morphological changes that occur during spermatogenesis are very similar in the Chitonina and Acanthochitonina. During spermiogenesis the nucleus elongates to develop a long anterior filament. Chro­matin condensation within the nucleus involves the formation of fibrils that become orientated along its long axis. Closely associated with the elongating nucleus is a manchette. In L . asellus a spherical proacrosomal vesicle appears in the spermatocytes. This vesicle becomes compressed as it matures and simultaneously it migrates to the presumptive anterior end of the spermatid where it invaginates and elongates. Although the pattern of chromatin condensation in the nucleus is similar to that described above, a manchette has not been observed.

Sperm Ultrastructure of the Protobranchia: Comparison with Other Bivalve Mollusks and Potential Taxonomic and Phylogenetic Significance

“…Interestingly, the same structure has been noted in a few cerithioidean gas¬ tropods (Healy, 1983) and certain Annelida (see Franzen, 1975;Jamieson & Rouse, 1989). A similar puzzle is presented by the "teardrop-shaped" nucleus of S. velesiana', such a shape has not previously been observed in any other examined bivalve species but is reminiscent of that seen in Polyplacophora, with the closest comparison occurring in the primitive Lepidopleurina (Hodgson et al, 1988;Healy et al, 1995;Pashchenko & Drozdov, 1998;Healy, 2000). Once again, only further study will reveal the frequency of this nuclear shape and its possi¬ ble function compared to the other types seen within the Solemyida.…”

Sperm morphology in the marine gastropodArchitectonica perspectiva (Mollusca): Unique features and systematic relevance

“…In all other investigated molluscs, the anterior extremity of the axoneme (or centriole if present) lies close to or abuts the base of the sperm nucleus (Popham 1979, Dufresne-Dube et al 1983, Kohnert and Storch 1984, Koike 1985, Healy 1988a, Hodgson et al 1988, Buckland-Nicks and Chia 1989. The fact that the column is continuous with the centriolar-axonemal complex and shows transverse banding suggests it is some form of centriolar derivative, perhaps even a highly modified rootlet.…”

Ultrastructural study of spermatogenesis in Phoronopsis harmeri (Lophophorata, Phoronida)

“…Thus, early sperm development in P. harmeri is an example of 'flagellate spermatogenesis' (see Reunov 2001) which is typical of Metazoa with external or external/internal fertilization and 'primitive' (sensu FranzØn 1956) spermatozoa. This type of sperm development has been described for representatives of Porifera (Paulus 1989), Cnidaria (Larkman 1984), Priapulida (Adrianov et al 1992), Nemertini (Reunov and Klepal 1997), Polychaeta (Eckelbarger 1984), Aplacophora (Hodgson et al 1988), Bivalvia Reunov et al 1999), Echinodermata (Au et al 1998), Brachiopoda , Tunicata (Villa and Tripepi 1983) and Cephalochordata (Welsch and Fang 1996).…”