Circatidal activity rhythms in the soldier crab Mictyris guinotae

Mictyris guinotae is endemic to the Ryukyu Islands, Japan. During low tide, the crabs emerge onto the tidal flat to feed, and then burrow into the sand before the incoming tide. They feed in droves during daytime, but separately at night. Under constant conditions without sand sediment, crabs exhibited a bimodal daily activity pattern, with a free-running period of ~12.8 h, comprising an active phase of ~11 h alternating with a resting phase of ~1 h, and a lag of ~3 h between the activity peak and low tide. Crabs were more active during the notional night-time than during the notional daytime. In crabs placed in an arena with sand sediment, a free-running period of ~12.8 h comprised a surface-active phase of ~3 h and a subsurface resting phase of ~9 h, with a lag of 1.5 h. In contrast to the non-sand condition, more crabs were active during daytime than during night-time. Thus, M. guinotae possesses circatidal and circadian locomotor rhythms that are modified by the sediment.