Laboratory quantification of burrow utilization by the symbiotic varunid crab <i>Sestrostoma toriumii</i>

Abstract

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.

Behavioural strategy of the ectosymbiotic crab (Sestrostomasp.) during ecdysis of the crab and its upogebiid shrimp host

“…moults with its body clinging onto the ventral abdominal segment of the host. During moulting, it is crucial for the symbiotic crab not to be detected by the host, because the host shrimp always cleans the burrow and often expels symbiotic animals (Itani, 2001; Henmi & Itani, 2014 b ; Henmi et al ., 2017). The abdomen of the host may be the safest space for the crab, because the host chelipeds and the cleaning legs (fifth legs) do not touch the ventral side of the first abdominal segment (Itani, 2001).…”

“…The crab genus Sestrostoma includes both burrow symbionts and an ectosymbiont. A congeneric crab ( S. toriumii ) is a co-habitant in the upogebiid shrimp burrows, where the crab is often expelled from the burrow by the shrimp (Itani, 2001; Henmi & Itani, 2014 b ). This crab has developed a ‘pass-under’ (ventral evasion) behaviour to escape the hostile response of the host species (Henmi et al ., 2017).…”