Field and laboratory quantification of alternative use of host burrows by the varunid crab Sestrostoma toriumii (Takeda, 1974) (Brachyura: Varunidae)

Abstract

Sestrostoma toriumii (Takeda, 1974) (Varunidae) is a symbiotic brachyuran crab that uses the burrows of the mud shrimp Upogebia yokoyaiMakarov, 1938(Upogebiidae) in tidal flats along the coasts of Japan, Korea, and Hong Kong. We studied the host specificity of S. toriumii by quantifying the use of alternative hosts, namely, the callianassid shrimp Nihonotrypaea japonica (Ortmann, 1891) and the brachyuran crab Macrophthalmus banzai Wada & K. Sakai, 1989 (Macrophthalmidae). Sestrostoma toriumii was present at higher densities in tidal flats dominated by U. yokoyai and N. japonica, but was never found in tidal flats dominated by M. banzai. Laboratory experiments showed that S. toriumii used the burrows of N. japonica in almost the same way as described previously for their use of U. yokoyai burrows; however, S. toriumii used the burrows of M. banzai less frequently than those of N. japonica. Sestrostoma toriumii avoided hostile responses of the host species through escape and pass-under behavior to avoid agonistic behavior by the crustacean host when it encountered the crustaceans in artificial burrows built from vinyl tubes. These avoidance behaviors were observed more often in tubes containing U. yokoyai and N. japonica. Our results support earlier field studies that showed most burrow associates of upogebiid shrimps also use callianassid shrimps as hosts. Crabs that utilized M. banzai burrows were nevertheless able to remain in the burrow for a relatively long period. This behavioral plasticity might facilitate host switching by S. toriumii.