The circa-tidal rhythm of larval release in the incubating crabSesarma

Abstract

1. Time of day of larval release by ovigerous females of the terrestrial crabSesarma haematocheir collected from the Kasaoka population (Okayama Prefecture, Japan) was examined using a method which detected the decrease in transmission of an infrared beam due to the release of zoea larvae. 2. Recordings under non-tidal and continuously dark (DD) conditions demonstrated a freerunning tidal rhythm of larval release. The timing of release approximately coincided with the times of nocturnal high water occurring in the field during the first 10 days. Then followed a ‘phase jump’, about 8 h where release time advanced to the conjugated high water. Correlation of release time with this high-water curve was reduced for the next 10 days, accompanied by desynchronization among individuals. 3. Artificial 24-h light-dark cycles (LD) caused a delay or advance phase-shift of this free-running rhythm after a series of transients. Under these conditions individuals did not become strongly desynchronized, and the new release timing was related to the phase difference between the natural and artificial light regimes, closely dependent on the time lag between sunset and light-off. This indicates that the onset of darkness is one of the environmental Zeitgebers primarily concerned in controlling the phase of theSesarma tidal rhythm. 4. An artificial tidal cycle simulating ebb and flood of tides in the habitat of crabs could not entrain the circa-tidal rhythm ofSesarma. Simulated sounds of the surf also failed to shift the phase of the ongoing rhythm. 5. An experiment with short photoperiods (LD 3∶21) gave no indication of a bimodal tidal component (interval of about 12.4 h) in theSesarma circa-tidal rhythm; the underlying component was clearly ‘unimodal’ (period of about 24.8 h). 6. Possible explanations are given for the circatidal rhythm ofSesarma which free-runs through the night at semimonthly intervals, and for its phase-shift caused by the 24-h light-dark cycles.