Littorines have separate sexes and internal fertilisation.  Observations on Littorina plena in British Columbia reveal that a pair initiates copulation by the male crawling on top of the female and inserting its penis into the right-hand side of the female’s mantle cavity to deposit sperm in her bursa.  The stacked nature of the copulatory position creates a larger search image and thus may increase the vulnerability of the pair to potential predators.  Does elevated risk of predation also affect the propensity of snails to form mating pairs?  These ideas are tested in field and laboratory experiments by researchers at the Bamfield Marine Sciences Centre, British Columbia.  First, snails are tethered in the field for 2d as “singles” or as simulated “mating pairs”, and their susceptibility to predators measured. Then, in the laboratory, snails are exposed to differing levels of “alarm-substance” scent, consisting of seawater flowing over crushed conspecific littorines from upstream tanks, and monitored for changing incidence of copulatory pairings. 

Results for the first part show, indeed, that simulated mating pairs in the field are more frequently killed than individual snails (see histogram upper Left). The uppermost snail is most vulnerable, and its death, most likely from crabs and, to a lesser extent, from shell-crushing pile perches, is inferred from the presence of shell fragments still attached to the glue. However, inactuality, attacks on pairs of snails lead to both members of the pair being killed in 89% of the time. Note in the histogram that size has no significant effect on susceptibility. 

In the laboratory, the littorines are significantly less likely to copulate when the simulated risk of predation is high, as when stimulated by the presence of the scent of crushed conspecific snails in the water (see histogram lower Left). Given that the major predators of L. plena when immersed are crabs and possibly fishes, the question arises as to whether the snails prefer to copulate during tidal air exposure when predation risk is lower.  The authors remark that this does happen, but whether it happens more in areas of greater predator densities is not known.  The authors conclude by saying that mating “decisions” in L. plena may represent a trade-off between risks of predation and desiccation, and also between present and future reproductive opportunities. Koch et al. 2007 Invert Biol 126: 257.

NOTE  mating pairs are created by glueing one snail’s shell to another in an orientation that simulates a natural mating position

NOTE  the authors do not explain why they used only crushed conspecific snails rather than, or in addition to, actual presence of predatory crabs or fishes

Studies on mate selection in Littorina subrotundata at the Bamfield Marine Sciences Centre, British Columbia show that it is the males, not the females, who are the choosy ones.  Larger males physically out-compete smaller males for access to females, and these larger males not only prefer to copulate with larger females, but favour virgin females rather than females that have recently copulated. The first part of the selection will maximise the male’s sperm investment, because larger females produce larger egg masses and often produce multiple clutches using the same batch of sperm. The second part of the selection minimises the potential for sperm competition.  The authors suggest that males can detect the presence of sperm from rival males within a female’s reproductive tract.  The paper is an interesting reversal on the commonly accepted thesis that sexual selection is primarily through choice by the female.  Zahradnik et al. 2008 J Moll Stud 74: 245. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  the authors do not test this directly; rather, they infer it from work on other non-west coast littorinid species

Littorina subrotundata