Daily rhythms of emergence of small invertebrates inhabiting shallow subtidal zones: A comparative investigation at four locations in Japan

Many small invertebrates inhabit the shallow subtidal zone and some of them emerge at times into the water column. The daily timing of their emergence is affected by the day/night and tidal cycles, and shows various patterns of synchrony with these cyclical factors, depending on the species.' To det ect possible regional differences in their emergence patterns, sampling was carried out: at four locations in Japan: a boreal sea (Akkeshi), a tempera te sea (Sugashima), an inland sea (Ushimado) and a subtropical sea (Iriomot e-jima). The emergence patterns of major taxa were examined by visual inspe ction and by two statistical methods (periodogram and autocorrelogram). The composition of the taxa collected by the pump system, mostly crustaceans, was similar in each location. The number of 'taxa' that emerged revealed a day/night rhythm in every location. This characteristic was clearest at Iri omote-jima and least clear at Sugashima. The daily fluctuation in the numbe r of individuals in each taxon varied widely, from very clearly nocturnal t o weakly diurnal patterns. In Iriomote-jima, the major taxa all showed well -demarcated nocturnal patterns, so these patterns were classified as either level Nz or Nj with regard to the degree of synchrony with the day/night c ycle. With regard to the synchrony with the tide, the majority of patterns in all locations showed a 'double-tidal interval'. Many patterns were sligh tly modified by the tidal cycle. These patterns were classified as level T- 1 or T-2 with regard to the degree of synchrony with the tidal cycle. The s ynchrony with the tide was comparatively strong at Ushimado. The synchrony with day/night: and tidal cycles varied even within the same species or clo sely related species. In benthic invertebrates, hiding or resting in the bo ttom substrates and swimming in the water column would occur alternatively. In planktonic animals, aggregation near the bottom and dispersal in the wa ter column would occur alternatively. The daily timing of such activities m ay be synchronized with the day/night and tidal cycles to various degrees a mong species or populations, resulting in a wide variety of emergence patte rns in subtidal small invertebrates. This type of behavior is not 'daily (d iel) vertical migration'; it should rather be called 'daily emergence/dispe rsal'. Strong winds, rough waves and unknown seasonal factors would also af fect emergence patterns. Furthermore, the transparency of the seawater may also strongly affect these patterns. Nocturnal patterns may be an adaptatio n to avoid vulnerability to sighted predators. Variation of synchrony with the tide indicates that by definition, the tidal rhythm can only be disting uished from the day/night rhythm. Hence, the daily patterns that are weakly modified by the tides (levels T-1 and Ttl should be called the tidal rhyth ms. As the period of such rhythms cannot be determined exactly by using sta tistical methods, lengthy field investigations and visual inspection of eac h pattern is essential to assess the influence of tides.