Hitchhiking on drifting seaweed reduces predation risk in juveniles of the swimming crab Portunus tritberculatus

Early juveniles of the swimming crab Portunus trituberculatus are known to use drifting seaweed before transitioning to benthic habitats in a behaviour that is thought to be a strategy to avoid shallow water predators. However, little is known about whether the predation risk of P. trituberculatus juveniles is reduced in such seaweed. To assess the nursery function of drifting seaweed for P. trituberculatus as a refuge from predation, we conducted field experiments to compare the relative survival of first to fifth instar (i.e. small to large body size) juveniles tethered to floating macroalgae that mimicked drifting seaweed and to unstructured sand habitats. The survival of tethered juveniles was higher in floating algae than in sand. This was particularly evident among first- to third-instar juveniles, which exhibited less burying behaviour. Thus, first- to third-instar juveniles with small body size and less burying habits are likely to be most vulnerable to predation, and drifting seaweed may function as a nursery vehicle, providing refuge from predators.