Sperm ultrastructure of Microhedyle remanei, an interstitial acochlidian gastropod with dermal fertilization

Abstract

The ultrastructure of sperm of an acochlidian opisthobranch is described for the first time in detail, in the tiny, gonochoristic Microhedyle remanei (Microhedylidae) from Bermuda. Transmission electron microscopy (TEM) shows the spermatozoa of M. remanei sharing many plesiomorphic features with opisthobranch gastropods, such as having an apical spiral nucleus with a basal invagination filled with a bell-shaped centriolar derivative; there is a single glycogen helix embedded into matrix and paracrystalline material. Sperm of M. remanei are characterized by possessing a strongly helically coiled nucleus with up to five keels terminating into one very prominent and intertwined keel. The sperm midpiece shows the glycogen helix which is very densely arranged. This clearly differs from Hedylopsis ballantinei, a member of the related acochlidian family Hedylopsidae, where three glycogen helices with different lengths and a probably much shorter nucleus are present. This variation among acochlidian sperm may be phylogenetically relevant and may be related to special acochlidian reproductive features such as sperm transfer via hypodermic injection or dermal fertilization via spermatophores.

The Antarctic holothurian genus Echinopsolus Gutt, 1990 (Dendrochirotida, Cucumariidae): brood pouches, spermatozoa, spermatozeugmata and taxonomic implications

“…In addition there is ultrastructural indication for poor swimming endurance of single spermatozoa: the small mid-piece volume argues for limited energy reserves and the restricted area of contact between the mitochondrial derivatives and the flagellum for a limited flow of metabolites (contrary e.g. to sperm cells, that have to drill actively through the female body wall as in the acochlidian gastropods; Neusser et al 2007;Jörger et al 2009). The big and prominent acrosomal vesicle on the other hand should mediate a fast penetration of the egg integument.…”

Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)

“…For acochlidian species, all of the available original or secondary literature was considered (e.g. Bergh, 1895; Kowalevsky, 1901; Bücking, 1933; Küthe, 1935; Odhner, 1937, 1938, 1952; Marcus, 1953; Marcus & Marcus, 1954, 1955; Bayer & Fehlmann, 1960; Challis, 1968, 1970; Swedmark, 1968, 1971; Kirsteuer, 1973; Doe, 1974; Wawra, 1974, 1978, 1979, 1980, 1986, 1987, 1988a, c, 1989, 1992; Westheide & Wawra, 1974; Morse, 1976, 1994; Kudinskaya & Minichev, 1978; Haynes & Kenchington, 1991; Huber, 1993; Haase & Wawra, 1996; Fahrner & Haszprunar, 2002; Sommerfeldt & Schrödl, 2005; Neusser et al. , 2006, 2007a, b, 2008; Neusser & Schrödl, 2007; Jörger et al.…”

“…Thus, there seems to be considerable variation amongst asperspinid and microhedylid species, which should be studied in detail. Sperm ultrastructure. The sperm of H. ballantinei is very different from sperm of M. remanei , A. murmanica , and P. milaschewitchii on the ultrastructural level regarding head and midpiece structure (Sommerfeldt & Schrödl, 2005; Neusser et al. , 2007a, 2008; Jörger et al.…”

“…reduced to 12–15 dagger‐like rhachidian teeth. Acochlidian species have a variety of aberrant reproductive features (Swedmark, 1968; Wawra, 1992; Morse, 1994), such as sperm transfer by hypodermic injection, via a hollow penial stylet in Hedylopsis spiculifera (Kowalevsky, 1901) (see Wawra, 1989), and the use of spermatophores, at least in Asperspinidae and Microhedylidae (see Sommerfeldt & Schrödl, 2005; Neusser et al. , 2006, 2007a, Neusser, Martynov & Schrödl, 2008).…”

Sperm movement and morphology in a simultaneous hermaphroditic sea slug

“…Ultra-structural observations using electron microscopy have revealed that sperm flagella in such species are quite specific regarding flagellum structure compared to external fertilizers (e.g., Sommerfeldt & Schrödl, 2005;Neusser et al, 2007), which shed their gametes into seawater. Sperm of external fertilizers typically have the ability to move vigorously and are referred to as "aqua sperm" (Jamieson, 1987;Rouse & Jamieson, 1987), their flagella are short and simple structure that conserves typical motility apparatus, 9+2 structure.…”