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

Abstract

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 detect possible regional differences in their emergence patterns, sampling was carried out at four locations in Japan: a boreal sea (Akkeshi), a temperate sea (Sugashima), an inland sea (Ushimado) and a subtropical sea (Iriomote‐jima). The emergence patterns of major taxa were examined by visual inspection 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 Iriomote‐jima and least clear at Sugashima. The daily fluctuation in the number of individuals in each taxon varied widely, from very clearly nocturnal to weakly diurnal patterns. In Iriomote‐jima, the major taxa all showed well‐demarcated nocturnal patterns, so these patterns were classified as either level N2 or N3 with regard to the degree of synchrony with the day/night cycle. With regard to the synchrony with the tide, the majority of patterns in all locations showed a ‘double‐tidal interval’. Many patterns were slightly modified by the tidal cycle. These patterns were classified as level T1 or T2 with regard to the degree of synchrony with the tidal cycle. The synchrony with the tide was comparatively strong at Ushimado. The synchrony with day/night and tidal cycles varied even within the same species or closely related species. In benthic invertebrates, hiding or resting in the bottom substrates and swimming in the water column would occur alternatively. In planktonic animals, aggregation near the bottom and dispersal in the water column would occur alternatively. The daily timing of such activities may be synchronized with the day/night and tidal cycles to various degrees among species or populations, resulting in a wide variety of emergence patterns in subtidal small invertebrates. This type of behavior is not ‘daily (diel) vertical migration’; it should rather be called ‘daily emergence/dispersal’. Strong winds, rough waves and unknown seasonal factors would also affect emergence patterns. Furthermore, the transparency of the seawater may also strongly affect these patterns. Nocturnal patterns may be an adaptation to avoid vulnerability to sighted predators. Variation of synchrony with the tide indicates that by definition, the tidal rhythm can only be distinguished from the day/night rhythm. Hence, the daily patterns that are weakly modified by the tides (levels T1 and T2) should be called the tidal rhythms. As the period of such rhythms cannot be determined exactly by using statistical methods, lengthy field investigations and visual inspection of each pattern is essential to assess the influence of tides.

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