Habitat partitioning between prey soldier crab Mictyris brevidactylus and predator fiddler crab Uca perplexa

Abstract

Interaction and habitat partition between the soldier crab Mictyris brevidactylus (prey) and the fiddler crab Uca perplexa (predator) were examined at a sandy tidal flat on Okinawa Island, Japan, where they co-occur. Both live in dense colonies. When the soldier crabs were released in the densely populated habitat of the fiddler crab, male fiddler crabs, which maintain permanent burrows in hard sediment, preyed on small soldier crabs and repelled large ones. Thus, the fiddler crabs prevented the soldier crabs from trespassing. It was also observed whether soldier crabs burrowed successfully when they were released 1) where soldier crab burrows just under the sand were abundant, 2) in a transition area between the two species, 3) an area without either species, and 4) where artificial tunnels simulated soldier crabs' feeding tunnels were made by piling up sand in the area lacking either species. In contrast to the non-habitat area, many soldier crabs burrowed in the sediment near the release point in the tunnel, transition and artificial tunnel areas. This indicates that the feeding tunnels on the surface attracted other crabs after emergence. When the large male fiddler crabs were transplanted into the artificial burrows made in soft sediment of the soldier crab habitat, all left their artificial burrows by 2days. In the fiddler crab habitat, however, about one-third of the transplanted male fiddler crabs remained in the artificial burrows after 3days. The soldier crabs regularly disturb the sediment by the up and down movement of their burrow (small air chamber) between tides. This disturbance probably prevents the fiddler crab from making and occupying permanent burrows. Thus, it appears that these crabs divide the sandy intertidal zone by sediment hardness and exclude each other by different means.