Molecular and morphological systematics of neustonic nudibranchs (Mollusca : Gastropoda : Glaucidae : Glaucus), with descriptions of three new cryptic species

A recent molecular phylogenetic study on Glaucus, a genus of neustonic aeolid nudibranchs, revealed undescribed cryptic diversity. Glaucus atlanticus is sister to the traditional species Glaucus marginatus, which is a complex of four genetically distinct cryptic species (Informal clade ‘Marginatus’). The present paper revises the systematics of Glaucus and provides formal descriptions for three new species in the informal clade ‘Marginatus’ substantiated by species delimitation analyses. Molecular and morphological evidence confirms that the type species of Glaucus, Glaucus atlanticus, has a cosmopolitan subtropical distribution and is characterised by having a uniseriate ceratal arrangement, a penial spine and a longitudinal, medial silver stripe on the sole of the foot. Examination of type material indicates that the name G. marginatus should be retained for the most widespread of these species, found in the Indian and Pacific Oceans. This species is characterised by molecular diagnostic characters as well as the presence of a bursa copulatrix. Glaucus marginatus is sister to the undescribed species Glaucus bennettae, sp. nov., which is found in the South Pacific Ocean and lacks a bursa copulatrix. The other two undescribed species, Glaucus thompsoni, sp. nov. and Glaucus mcfarlanei, sp. nov. are only known from the North Pacific Ocean, and are characterised by molecular diagnostic characters as well as possessing and lacking a bursa copulatrix, respectively. Because sister species of Glaucus differ in their reproductive anatomy, we hypothesise that mating behaviour has played a role in cladogenesis in this group. ZooBank Publication code: http://zoobank.org/References/E352E264-A440-4AF1-8565-B57B7EEE25BC

Population density and group size effects on reproductive behavior in a simultaneous hermaphrodite

Background

Despite growing evidence that population dynamic processes can have substantial effects on mating system evolution, little is known about their effect on mating rates in simultaneous hermaphrodites. According to theory, mating rate is expected to increase with mate availability because mating activity is primarily controlled by the male sexual function. A different scenario appears plausible in the hermaphroditic opisthobranch Chelidonura sandrana. Here, field mating rates are close to the female fitness optimum, suggesting that mating activity remains unresponsive to variation in mate availability.

Results

Applying an experimental design that aims at independent experimental manipulation of density and social group size, we find substantial increases in mate encounter rate with both factors, but no statistically detectable effects on mating rate in C. sandrana. Instead, mating rate remained close to the earlier determined female fitness optimum.

Conclusions

We demonstrate that mating rate in C. sandrana is largely unresponsive to variation in mate availability and is maintained close to the female fitness optimum. These findings challenge the prevailing notion of male driven mating rates in simultaneous hermaphrodites and call for complementary investigations of mating rate effects on fitness through the male sexual function.

Evidence for costs of mating and self-fertilization in a simultaneous hermaphrodite with hypodermic insemination, the Opisthobranch Alderia willowi

Simultaneous hermaphrodites offer the chance to study antagonistic coevolution between the sexes when individuals function in both roles. Traumatic mating by hypodermic insemination has repeatedly evolved in hermaphroditic taxa, but evidence for the fitness costs of such male-advantage traits is lacking. When reared in isolation, specimens of the sea slug Alderia willowi (Opisthobranchia: Sacoglossa) initially laid clutches of unfertilized eggs but 4 days later began self-fertilizing; this is only the third report of selfing in an opisthobranch. Hypodermic insemination may allow selfing in Alderia if penetration of the body wall bypasses internal mechanisms that promote outcrossing. Selfing specimens and slugs reared in pairs had reduced fecundity compared to isolated slugs laying unfertilized clutches, suggesting that hypodermic insemination imposes a cost of mating. Egg production increased for field-caught slugs separated after mating compared to slugs held in pairs, a further indication that accessibility to mates imposes a fitness cost to the female function. Such antagonism can confer a competitive advantage to slugs mating in the male role but diminish reproduction in the female role among hermaphrodites capable of long-term sperm storage. Alderia willowi is also a rare case of poecilogony, with adults producing either planktotrophic or lecithotrophic larvae. Our rearing studies revealed that most slugs switched between expressed development modes at some point; such reproductive flexibility within individuals is unprecedented, even among poecilogonous species.

Multiple mating, sperm storage, and mating preference in Aplysia californica

Aplysia californica, a simultaneous hermaphrodite, has the ability to store and digest allosperm and frequently mates with multiple partners. Using controlled matings and behavioral observations, we characterized the mating behavior of multiply-mated A. californica. Regardless of their initial mating role, all animals mated a second time. Individuals acting as the initial sperm donor showed no preference for second mating role when paired with a "virgin" partner. However, individuals acting as the initial sperm recipient showed a highly significant preference for acting as the sperm donor in subsequent matings, when paired with a "virgin" partner. This preference disappeared when the initial sperm recipient spawned a fertilized egg mass prior to the second mating bout. On average, singly mated animals laid 2.85 +/- 0.39 (mean +/- SE, n = 13) egg masses before accepting sperm from a second donor. These results support the notion that depletion of stored allosperm plays a crucial role in the resolution of sexual conflict and the mating decisions of A. californica.

Natural female mating rate maximizes hatchling size in a marine invertebrate

1. Males and females often differ in their optimal mating rates, resulting potentially in conflicts over remating. In species with separate sexes, females typically have a lower optimal mating rate than males, and can regulate contacts with males accordingly. The realized mating rate may therefore be closer to the female's optimum. In simultaneous hermaphrodites, however, it has been suggested that the intraindividual optimization between 'male' and 'female' interests generates more 'male'-driven mating rates. 2. In order to assess the consequences of variation in mating rate on 'female' reproductive output, we exposed the simultaneously hermaphroditic sea slug Chelidonura sandrana to four mating rate regimes and recorded the effects on a variety of fitness components. 3. In focal 'females', we found (i) a slight but significant linear decrease in fecundity with mating rate, whereas (ii) maternal investment in egg capsule volume peaked at an intermediate mating rate. 4. Combining the observed fecundity cost with the apparent benefits of larger offspring size suggests that total female fitness is maximized at an intermediate mating rate. With the latter being close to the natural mating rate of C. sandrana in the field, our findings challenge the assumption of 'male'-driven mating systems in simultaneous hermaphrodites. 5. Our study provides experimental evidence for various mathematical models in which female fitness is maximized at intermediate mating rates.

Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity

BACKGROUND

In simultaneous hermaphrodites with copulation and internal fertilization it is often unclear whether reciprocal sperm exchange results from the unconditional willingness of both partners to donate and receive sperm, or whether it follows from a more controlled process such as conditional reciprocal sperm exchange, i.e. sperm trading. While in some sea slugs mating is assumed to be based on sperm trading, it seems to be unconditional in others. Here, we describe the unusual mating behaviour of Elysia timida, a small sacoglossan, focussing on indications for conditional reciprocity.

RESULTS

E. timida shows an as yet unique combination of a long series of hypodermic transfers followed by a short phase with standard insemination into a female genital aperture. Hypodermic transfer takes place in the form of repeated small injections into the dorsal surface of the partner, interrupted by synchronised circling movements. In the final mating phase sperm is transferred into the female genital aperture in a short period. In both phases the two mating individuals show a high degree of transfer symmetry and synchrony. While total duration and number of transfers were balanced within pairs, they varied significantly between pairs. Furthermore, looking at individual hypodermic transfers within pairs, reciprocal transfers lasted longer than unilateral transfers. Final sperm transfers were always reciprocal except for two cases which also diverted from the usual pattern in ways that were suggestive of a conflict over reciprocity.

CONCLUSION

Our results suggest that individual mating decisions in E. timida depend on what the partner does, indicating conditional reciprocity. If hypodermic transfers also involve the transfer of sperm (which remains to be confirmed), this system represents an up to now unique transition stage between hypodermic and standard insemination, both of which are widespread in this group of sea slugs, but never have been observed to co-occur within the same species.

Evolutionary stability of egg trading and parceling in simultaneous hermaphrodites: The chalk bass revisited

Several species of simultaneously hermaphroditic seabasses living on coral reefs mate by alternating male and female roles with a partner. This is known as egg trading, one of the classic and most widely cited examples of social reciprocity among animals. Some of the egg-trading seabass species, including the chalk bass, Serranus tortugarum, switch mating roles repeatedly, having subdivided their clutch of eggs into parcels offered to the partner for fertilization. Here we attempt to understand these dynamics as a pair of evolutionary games, modifying some previous approaches to better reflect the biological system. We find that the trading of egg clutches is evolutionarily stable via byproduct mutualism and resistant to invasion by rare individuals that take the male role exclusively. We note why and how parceling may reflect sexual conflict between individuals in the mating pair. We estimate evolutionarily stable parcel numbers and show how they depend on parameter values. Typically, two or more sequential parcel numbers are evolutionarily stable, though the lowest of these yields the highest fitness. Assuming that parcel numbers are adjusted to local conditions, we predict that parcel numbers in nature are inversely related both to mating group density (except at low density) and predation risk.

Evolutionary Conflict: Sperm Wars, Phantom Inseminations

A new experimental study has provided the first definitive evidence for conditional punishment of 'cheats' in a sperm-trading simultaneous hermaphrodite: the sea slug Chelidonura hirundinina. This also provides a rare unequivocal example of conditional reciprocity averting a 'tragedy of the commons' in biology.