Circatidal rhythm of an intertidal crab, Hemigrapsus sanguineus : synchrony with unequal tide height and involvement of a light-response mechanism

Abstract

The circatidal rhythm of intertidal animals may reflect the inequality of the tides. In addition, a light-sensitive mechanism may be involved in their internal timing systems. To test these hypotheses, the larval release activity of the intertidal crab Hemigrapsus sanguineus was monitored under different light conditions in the laboratory. Under a 24-h light–dark (LD) cycle with the phase similar to the field, the activity coincided with the times of high tide in the field and showed a tidal rhythm. This rhythm free-ran in constant, dim-light conditions, suggesting that the timing is controlled by an endogenous clock. When the population was exposed to a 24-h LD cycle with the phase changed from that in the field, the tidal rhythm was phase-shifted; while the light cycle advanced in phase from the field caused a phase-advance of the rhythm, that delayed in phase induced a phase-delay of the rhythm. Thus, a light-response mechanism is definitely involved in circatidal timing systems. But the population rhythm showed a large variability among individuals, associated with the phase-shift, and the magnitude of the phase-shift did not accurately correspond to that of the light cycle. These results suggest that the light-response system can control the phase of the rhythm less stronger than that in estuarine crabs. Most releases occurred at higher high tides, but the release of some females obviously occurred at lower high tides. The larval release pattern thus could not be accounted for by a simple synchrony with higher high tides. Hatching of H. sanguineus occurred after a “hatching program” of 49.5 to 52.5 h. This program is initiated by some factor (hatching-program inducing stumuli: HPIS) transmitted from the female to the embryos. We speculated that this factor is effectively transmitted to the embryos when the habitat is exposed to air, i.e., at lower low-tide periods, and that once each embryo is stimulated, hatching occurs synchronously 2 d later during high tide. The release of HPIS is probably controlled by the circatidal clock of the female, and the 24-h LD cycle may participate in shifting this timing to the opposite low tide.