Apparent differential phenotypic responses by kelp forest grazers to disease-driven removal of sea star predators.

The potential for aquatic gastropods to display phenotypic plasticity in response to predator cues is well documented. However, long-term phenotypic responses to predator exposure are difficult to evaluate at large scales in the field. Thus, the extent to which comparatively dilute predator cues experienced by natural snail populations influence morphometric development and whether energetic costs associated with defensive morphology have allometric impacts on other life-history characteristics is unclear. The 2013 sea star wasting disease outbreak in central California, USA provided a unique framework for a large-scale natural predator removal experiment, comparing the shell morphometrics and gonadosomatic index of subtidal turban snail populations at kelp forest sites where local predatory sea stars were completely absent or nearly so (SS-), with paired sites maintaining low predator densities (SS+). All three snail species displayed higher proportional allocation to shell mass at SS+ locations and concomitantly higher reproductive allocation with predators absent (SS-). Dietary stable isotope analysis suggests this may be partially an energetic consequence of behavioural grazing shifts displayed by snails following predator release. Interestingly, morphometric shifts in shell structure differed among the three species and appeared closely related to species-specific predator avoidance strategies.