Dwarfism in the coastal decapod assemblage of Robinson Crusoe Island, Chile: a predator-driven pattern?

Abstract

Brachyuran decapod crustaceans (true crabs) are ecologically successful and usually an economically important group in many coastal marine ecosystems. Cobble habitat represents a common nursery and refuge for early benthic stages, as is true for many small infauna. In the shallow subtidal of Robinson Crusoe Island (RCI) off Chile, the cobble-dwelling decapod assemblage is similar in diversity, but conspicuously smaller in body size than their counterparts on the Chilean continental coast (CCC), 700 km away. Three of these species are shared between locations, yet the Island populations mature at a smaller size and do not attain as great a maximum size as their mainland counterparts. We explore the hypothesis that the difference in body size is related to contrasting levels of predation pressure between RCI and CCC. RCI has a much greater diversity and abundance of predatory fish than CCC. Cryptic coloration in island species, such as Eurypanopeus crenatus, suggests another adaptation to high levels of fish predation. Such contrasts of insular to continental systems provide an opportunity to test hypotheses on the ecological factors driving the evolution of body size in this group. Funding: FONDECYT 1080009, NatGeo/Waitt and NSF OCE-1039822.