Factors influencing coexistence of two brachyuran crabs, helice tridens and Parasesarma plicatum, in an estuarine salt marsh, Japan

Abstract

The spatial distributions of two grapsoid crabs, Helice tridens and Parasesarma plicatum, overlap broadly in salt marsh habitats in the Yoshino River Estuary, Japan. In order to clarify factors influencing the coexistence of the two species, resource utilization was compared between the two species in space, time, and food, and intraspecific and interspecific predation was evaluated. The two species showed similar distributional patterns along the length of estuary with respect to elevation and sediment, reed, and debris conditions. Most crabs of both species dwelled solitarily in burrows. Parasesarma plicatum was most active during daytime low tide, whereas the activity of H. tridens did not differ between daytime low tide and other periods. Although both crab species had broad omnivorous diets (e.g., feeding on plants, brachyurans, insects, and detritus), H. tridens tended to consume more animal food items than P. plicatum. Laboratory experiments revealed that adults of both species preyed on subadults of the same species and different species. Adults of P. plicatum preyed on more subadults of H. tridens than conspecific subadults, whereas adults of H. tridens preyed on subadults of conspecifics and P. plicatum in similar frequency. Differences in resource utilization in conjunction with effects of predation and cannibalism, which would tend to reduce resource limitation, likely contribute to the coexistence to these two ecologically similar brachyuran crab species.