Parasitic mites influence intra- and interpopulational variation in sperm length in a simultaneous hermaphrodite land snail (Gastropoda: Helicidae)

Transmission of parasitic mites (Riccardoella oudemansi) between limacid slug hosts: the role of parasite and host behaviour

Transmission between hosts is crucial to the growth, development and reproduction of many parasites. As a consequence, parasites are under selection to maximise transmission success and exhibit many behavioural and morphological adaptations that allow detection of, and movement between, hosts. However, transmission success is not determined by parasites alone, but is also shaped by host behaviours. Often, host behaviours function to minimise the risk of exposure to parasites; in some cases, however, host behaviours may be manipulated by parasites to increase transmission success. In this study, we investigated transmission of the parasitic mite Riccardoella oudemansi between slug (Limacus maculatus) hosts, considering the role of both host and parasite behaviour in determining transmission success. Host-host transmission occurred when slugs were in physical contact, but mites were also capable of moving across the substrate to locate new hosts, a process facilitated by mucus trails. We found no strong evidence that slugs avoid parasitised conspecifics, or that mites manipulate slug behaviour to increase transmission. Finally, mites showed a preference for the mucus of parasitised slugs, but did not discriminate between mucus from their own host and another parasitised slug. A general preference for mucus from parasitised slugs is likely to be important in encouraging mites to remain in close contact with their host and may also facilitate host-switching and outbreeding. We encourage further study of parasitism by Riccardoella in limacid slugs, where cross-species variation in host social behaviour may drive differences in the rate and success of parasite transmission across slug species.

Breeding system, shell size and age at sexual maturity affect sperm length in stylommatophoran gastropods

BACKGROUND

Sperm size and quality are key factors for fertilization success. There is increasing empirical evidence demonstrating that sperm form and function are influenced by selective pressures. Theoretical models predict that sperm competition could favour the evolution of longer sperm. In hermaphrodites, self-fertilizing species are expected to have shorter sperm than cross-fertilizing species, which use sperm stored from several mating partners for the fertilization of their eggs and thus are exposed to intense sperm competition. We tested this hypothesis by comparing original data on sperm length in 57 species of simultaneously hermaphroditic stylommatophoran gastropods from Europe and South America with respect to the species' breeding system. We used 28S rRNA nuclear and COI mitochondrial sequence data to construct a molecular phylogeny. Phylogenetic generalized linear models were applied to examine the potential influence of morphological and life-history characters.

RESULTS

The best-fit model revealed that the breeding system and age at sexual maturity influence sperm length in gastropods. In general, species with predominant cross-fertilization had longer sperm than species with predominant self-fertilization or a mixed breeding system. Across species with shells (snails), sperm length also increased with shell size.

CONCLUSIONS

Our study provides evidence that sperm length in stylommatophoran gastropods is influenced by the risk of sperm competition, as well as by age at sexual maturity and shell size. This finding extends present knowledge of sperm evolution to a group of so far poorly studied simultaneous hermaphrodites.