Diversity in the patterns of daily emergence/dispersal behavior and timing systems in subtidal small invertebrates

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Many small invertebrates inhabiting shallow subtidal zones emerge at times into the water column. Daily timing of emergence/dispersal behavior shows a wide variety of synchrony with day/night and tidal cycles, depending on the species. To detect any regional difference in their patterns, the sampling was carried out at four regions : the north temperate sea (Akkeshi), the middle temperate sea (Sugashima), The Inland Sea (Ushimado), and the subtropical sea (Iriomote-jima). The pattern of major taxa was examined by visual inspection and by two statistical methods. The composition of the taxa, mostly crustaceans, was similar in each region. The daily fluctuation in number of 'taxa' showed a nocturnal pattern in each region. Nocturnal patterns were conspicuous at Iriomote-jima. With regard to the synchrony with the tidal cycle, the patterns with a double-tidal interval occupied the majority. Many patterns were slightly modified by the tide. Variation of synchrony with the tide indicates that by definition the tidal rhythm can only be distinguished from the day/night rhythm. In the new definition, daily activity patterns synchronized with the day/night cycle alone can be called a day/night rhythm, and those modified by the tidal cycle as well as synchrony with day/night cycle, even if very weakly, can be called a tidal rhythm. Since the tidal rhythm is only a variation of the circadian rhythm, it is entrained by at least two kinds of zeitgeber. Factors correlated with the tidal cycle, e.g., hydrostatic pressure or turbulence of water is one zeitgeber. Moonlight cycle could also be the tidal zeitgeber. On the other hand, we have demonstrated that 24-hour light/dark cycles are much more reliable zeitgeber of the tidal rhythm than tide-correlated factors. Such a characteristic of the tidal rhythm is explainable in terms of coupled two oscillators.