SPERMATOPHORE FORMATION IN THE SIMULTANEOUSLY HERMAPHRODITIC (: : )

by

HELMUT BAMINGER1 and MARTIN HAASE2

(Conservation Biology Group, Department qf Integrative Biology, University of Basel, St. Johanns-Vorstadt 10, CH-4056 Basel, Switzerland)

ABSTRACT

In helicid gastropods including Arianta arbustorum (LINNAEUS,1758) sperm are recip- rocally exchanged during copulation through spermatophores. A. arbustorum has been ex- tensively investigated with respect to sexual selection and a considerable amount of data on sperm and spermatophore transfer are available. However, nothing is known about sper- matophore formation and its possible influence on sperm exchange. As a contribution to the very limited knowledge on spermatophore formation in stylommatophoran gastropods in general and in order to complement the existing behavioural studies from a functional- anatomical point of view, we investigated spermatophore formation in A. arbustorum. Spermatophores were collected at certain intervals from 2 to 90 min after the onset of copulation either through forceful separation of the pairs (after which spermatophores were expelled by the snails) or through dissection of electrocuted and fixed . We found that spermatophore formation was initiated more or less synchronously in mating partners a few minutes after the onset of copulation and continued until shortly before transfer, which occurred after more than 60 min. Growth of the sperm container, which is probably mediated through the incoming ejaculate dilating the viscous, workable material secreted by the epiphallus, followed a saturation curve where the final length was reached slightly earlier than the final volume. However, growth and final size were not adjusted among partners confirming earlier findings that there is no sperm trading in A. arbustorum. Spermatophores collected from separated snails were larger than those from fixed individ- uals indicating that spermatophore formation is not immediately stopped after separation. This means that the former protocol is less precise for the investigation of spermatophore formation than a treatment involving rapid fixation. In A. arbustorum, spermatophore for- mation differs largely from that in the only other investigated helicid snail, Helix pomatia Linnaeus 1758, in which the container is assembled and the spermatophore transferred in less than 10 min. KEY WORDS:Arianta arbustorum, mating, Pulmonata, reciprocity, spermatophore.

Present address: Museum of Natural History of Vienna, Burgring 7, A-1014 Wien, Austria. '` Author to whom correspondence should be sent, e-mail: [email protected] 348

INTRODUCTION

Sperm transfer and number of sperm transferred play a central role in sexual selection (e.g., PARKER, 1998). Among internally fertilizing hermaphrodites, gastropods are probably the best studied examples in this respect (BAUR, 1998; MICHIELS, 1998). Gastropods with internal fertilization transfer sperm in form of free sperm or the sperm are either aggregated into conglomerates (spermatozeugmata) or encapsulated into spermatophores (MANN, 1984). Spermatophores occur in more than 40 families of prosobranchs, opisthobranchs and pulmonates (MANN, 1984; TOMPA, 1984; ROBERTSON, 1989). Pulmonate spermatophores appear not to be homologous with those of prosobranchs and opisthobranchs, because they are formed by the cpiphallus and flagellum, organs not present in the other two subclasses (ROBERTSON, 1989). In general, snails form a single spermatophore during each mating (but see LIKHAREV & WIKTOR, 1980). In terrestrial pulmonates, the spermatophore often has a species-specific morphology with taxonomic significance (e.g., LIND, 1973; SCHIKOW, 1978; WIKTOR, 1987; CASTILLEJO, 1992; NAGGS, 1994; GARRIDO et al., 1995). The adaptive significance of the spermatophore in stylommatophoran pulmonates with well-developed copulatory organs (penes) and internal - - fertilization is at first glance unclear. One might expect internally fertilizing gastropods to have either penes or spermatophores, not both. LINO (1973) assumed that transferring sperm via spermatophores ensures that some spermatozoa, perhaps the most active ones, can escape the digesting bursa copulatrix and reach the spermatheca, the sperm storing part of the fertilization pouch. Thus, the spermatophore probably serves as a defense for sperm against the "genitodigestive" tract of a receiver (cf VREYS et al., 1997; BAUR, 1998). According to LIND (1973), a spermatophore function like that in Helix pmnatia is probably found in some other Stylommatophora, at any rate within the Helicidae. Recently, the helicid Arianta arbustorwn has been studied extensively with respect to sexual selection, especially sperm competition (e.g., BAUR, 1994; HAASE & BAUR, 1995; BAUR et al., 1998; LOCHER & BAUR, 1999, 2000; BAMINGER & HAASE, 2000; BAMINGER et al., 2000) and interesting data on sperm transfer have become available. The number of spermatozoa transferred in a spcrmatophore varies consider- ably : sperm number ranged from 0.8 to 5.8 Mio in 66 snails from the Swiss Prealps (LOCHER & BAUR, 1999; see also BAMINGER et al., 2000). Sperm number is positively correlated with the size of the sperm container (LOCHER & BAUR, 1997; BAUR et al., 1998; LOCHER & BAUR, 1999). BAUR, et al. (1998) and LOCHER & BAUR (1999) found a