Potential depensatory mechanisms operating on reproductive output in gonochoristic molluscs, with particular reference to strombid gastropods

Molluscs are typically sessile or slow moving, yet successful reproduction requires close proximity to potential mates. Three mechanisms are identified whereby reproductive potential of a population can be limited under conditions of low density. The first is the reduction in numbers of spawners as abundance decreases. The second reflects the difficulty in finding mates, and takes the form of either (1) search time for slow but motile species, or (2) wasted spawning (or non-spawning) for sessile species, where gametes are not fertilized. The third mechanism is a breakdown of a positive feedback loop between contact with males (either direct or through chemical cues) and rate of gametogenesis and spawning in females, i.e., sexual facilitation. The second and third mechanisms are functions of local density, rather than overall abundance. Literature review and present studies on strombid gastropods indicate the potential for these mechanisms to occur. Many species are characterized by behaviours, such as aggregative settlement, that serve to overcome this problem. Intensive fishing practices may invoke these depensatory mechanisms as local density and abundance are reduced, thereby increasing the chance of recruitment failure.