Life on the edge: do biomechanical and behavioral adaptations to wave-exposure correlate with habitat partitioning in predatory whelks?

The rocky littoral is a highly stressed but species rich environment in which adaptive trade-offs underlie the specialization that permits habitat partitioning by closely-related species. Habitat partitioning by prey species is generally attributed to differential tolerances to physiological stresses and competition for resources. Partitioning by predators is, however, less understood and also mainly related to thermal tolerance. We test the hypothesis that differential biomechanical and behavioural adaptations to wave action can result in habitat partitioning of mobile predators using 2 Mediterranean whelks. Stramonita haemastoma is found mainly at wave-exposed habitats on lower midlittoral or shallow (1 to 3 m) subtidal rocks. By contrast, Hexaplex trunculus is found only subtidally in more quiescent littoral environments (shallow bays and lagoons or deeper in wave-exposed sites). Morphologically, S. haemastoma is squatter than H. trunculus and the ratio of its foot surface area to the maximum projected surface area of its shell is 35% higher, making S. haemastoma less prone to dislodgment by waves. Mechanically, at least a 2-fold higher force (12 N) is required to dislodge S. haemastoma from the substrate, probably due to its larger foot, because tenacity per foot area was similar between species. The feeding technique and foraging behaviour of S. haemastoma is also adapted to life in high-energy environments. Higher feeding rates and much shorter prey handling time compared to H. trunculus allow this species shorter foraging bouts that reduce the risk of dislodgment and increase predation success in wave-exposed areas. H. trunculus thrives in low-flow habitats where food density is low compared to shallow subtidal rocks. This is compensated for by a broader diet than S, haemastoma, feeding on any available food item, including conspecifics, and by reducing adhesion when not feeding, thus saving energy. S, haemastoma is more adapted to survive desiccation conditions than H. trunculus, which may exclude the latter from intertidal habitats. Our findings suggest that habitat partitioning between the whelks is maintained mainly by a 'distinct preferences' mechanism that might be relevant for other benthic predatory species.