Abstract
Use by a symbiotic crab Sestrostoma toriumii of burrows of a mud shrimp Upogebia yokoyai was investigated in laboratory experiments. Crabs used shrimp burrows in the presence of the host when the mud surface was covered with water. Crabs entered shrimp burrows soon after introduction to the tank (taking a mean of 656 s to enter), where they spent 61–81% of their total time in the experimental tanks. Half the crabs left the burrow at least once on the first day, whereas only a few crabs left on the second day. Although the crabs were expelled by the host shrimps in some cases, it is not known whether exiting is always due to host aggressive behavior. Two individuals that did not enter a shrimp burrow buried themselves in the sediment most of the time. Burying in the surface sediment may also be an adaptive behavior of S. toriumii when the crab has left the burrow for some reason. By living in the shrimp burrow, S. toriumii would benefit through predator avoidance.
