Pilidiophora

Yurchenko, Olga V. & Chernyshev, Alexey V., 2022, Sperm morphology and some aspects of acrosomal complex development in four species of Heteronemertea (Pilidiophora, Nemertea), Zoologischer Anzeiger 299, pp. 38-48 : 42-44

4.1. Sperm overview of Pilidiophora

As a rule, sperm morphology is related with the fertilization type and/or oocyte size (Franz´en & Sencenbaugh 1988). Indeed, the long-headed spermatozoa of L. viridis during pseudo-copulation move within gelatinous mass and additional mucus layers of the oocyte before fertilization (Dohren ¨2015). The elongated spermatozoa of C. lacteus , despite the obvious external fertilization, fuse with the oocyte surrounded by a thick chorion or vitelline envelope ( Stricker et al. 2001). Despite these facts, we could not find the evidence of clear correlation between short-head spermatozoa and oocyte diameter in the available literature, which can be explained by the scarcity of morphometric data or lack of dedicated studies. All the studied Kulikovia species have spermatozoa of the ect-aquasperm type characteristic of invertebrates with external fertilization and share their major traits (the small acrosomal complex and compact nucleus located coaxially, the short midpiece with five mitochondria and flagellum) with most other pilidiophoran nemerteans ( Heteronemertea + Hubrechtiiformes ). A similar organization of spermatozoa has also been documented for Lineus (= Ramphogordius ) lacteus , L. pseudolacteus , L. sanguineus (D¨ohren et al. 2010), H. juliae and Sonnenemertes cantelli Chernyshev et al., 2015 ( Chernyshev et al., 2020). Spermatozoa of M. bella have an elongated and curved nucleus. Similar sperm heads were reported for spermatozoa of Cerebratulus lacteus ( Clark & Hinsch 1969; Stricker & Folsom 1998), a species that belongs to another phylogenetic lineage, J ( Kajihara et al. 2022). Of particular interest is that the other species— Cerebratulus sp. , M. fasciolata , and M. akkeshiensis —have spermatozoa of the ect-aquasperm type with compact nucleus and a coaxial position of acrosomal vesicle and nucleus ( Iwata 1958; Franz´en 1983a; Stricker & Folsom 1998). This finding indicates that sperm of a similar shape emerged independently in different phylogenetic lines during the heteronemerteans’ evolution. Furthermore, the significant difference in spermatozoa organization may confirm the distant relationship of species within the genera Cerebratulus and Micrura , which indicates the necessity for their revision.

The anterior and posterior ends of the nucleus in the present species, as well as in other Pilidiophora, have a similar organization: the anterior end does not exhibit any invaginations and is slightly convex, while the posterior end has a nuclear implantation fossa and invaginations for mitochondria. This character relates pilidiophores with palaeonemerteans, but distinguishes them from hoplonemerteans, which are characterized mostly by the presence of the anterior nuclear fossa (D¨ohren et al. 2010).

All the considered species have five mitochondria in the midpiece that are partially depressed at the posterior end of the nucleus and are located around the centriolar complex. Such organization is common for the previously described pilidiophores ( Clark & Hinsch 1969; Stricker & Folsom 1998; D¨ohren et al. 2010). Most of the studied species have five mitochondria, but species with four or three posteriorly located mitochondria were also found ( Table 1). The shape of mitochondria is often rounded in Pilidiophora, but in Maculaura alaskensis ( Stricker & Folsom 1998) and M. bella the mitochondria are slightly elongated in the anterior/posterior direction, while in Lineus viridis they are elongated significantly (Dohren ¨& Bartolomaeus 2006). The number of mitochondria is reduced by fusion during spermiogenesis (therefore, a small number of spermatozoa containing six mitochondria are present). In some of the species, it reaches 5, while in the others 4 and even 3, but never fewer and single ( Table 1). The possible evolutionary transformations of the mitochondrial part in the midpiece of Pilidiophora spermatozoa were discussed previously (Dohren ¨et al. 2010; Chernyshev et al. 2020; Yurchenko et al. 2021b). To date, the following hypothesis seems to be the most plausible: as in Palaeonemertea, the ancestors of Pilidiophora probably had a single ring-shaped mitochondria, but in Pilidiophora the mitochondria stopped merging at the spermatid stage, and three/four/five mitochondria remain non-fused in spermatozoa. A similar scenario reported for palaeonemerteans, in particular Callinera sp. ( Yurchenko et al. 2021b).