Invasive freshwater snails provide resource for native marine hermit crabs

Invasive species can have significant direct and indirect impacts on native biota. Here, we conducted a survey of two invasive freshwater snail species (Melanoides tuberculata and Tarebia granifera) in Tobago and assessed the effects on shell use by native marine hermit crab populations (Clibanarius tricolor and C. vittatus). Abundant in freshwater, numbers of empty shells at the mouth of a river in Charlotteville (Northeast Tobago) increased from 75 to 800 m-2 after heavy rains in the wet-season of June 2012. Subsequent sampling of a nearby rocky shore revealed that the freshwater shells were occupied by 70.2% of hermit crabs. Mimicking a flood event, the experimental planting of marked alien freshwater shells showed that 20.3% of hermit crabs exchanged their marine shell for a freshwater shell. Similarly, a laboratory shell-choice experiment showed that C. tricolor hermit crabs changed shells repeatedly, and that 22.2% occupied a freshwater shell rather than a marine shell. Overall, due to the numeric abundance of freshwater shells and the compulsive shell changing behaviour of Clibanarius spp., the ongoing invasion of freshwater gastropods in the Caribbean may change the shell use patterns and population dynamics of the native hermit crabs.