Shelter effects of coralline algal turfs: protection for Turbo cornutus juveniles from predation by a predatory gastropod and wrasse

Laboratory experiments were conducted to examine the shelter effects of turfs of articulated coralline algae (ACA) for the Japanese spiny turban snail Turbo cornutus. The muricid gastropod Ergalatax contractus and the cockerel wrasse Pteragogus flagellifer were used as predators. When the muricid snail was used as a predator, the existence of ACA fronds improved the survival rate of the turban snail juveniles compared with the experimental treatment without ACA fronds. Refuge provided by ACA fronds against predation by the muricid snail was more notable for smaller-sized turban snail juveniles (5.8 mm in shell height) than those of a larger size (8.6 mm in shell height). In the experiment using the wrasse as a predator, a shelter effect was demonstrated for turban snail juveniles of two size groups (4.3 and 10.2 mm in shell height). ACA turfs hindered foraging by the wrasse, so the survival rate of the juveniles was greatly increased. No clear difference was observed between the two size groups of the turban snail in terms of the shelter effect of ACA turf. The shelter effect of ACA turfs for turban snail juveniles is thought to derive from the physical characteristics of the algae, which form a visual and physical barrier.