Nocturnal occurrence, and synchrony with tidal and lunar cycles, in the invertebrate assemblage of a subtropical estuary

Abstract

A number of small invertebrates occur in the water column of the subtropical estuary. The occurrence patterns of estuarine animals may be more strongly synchronized with the tidal cycle than those of subtidal animals. Sampling was carried out in the estuary at Iriomote-jima (24.5°N and 123.7°E) in Japan. An impeller pump was fixed 30 cm above the riverbed at the outlet to the sea, and the pump ran continuously for 20 d from 17 June to 8 July in 1999, to precisely assess the influence of the tidal cycle. Collection of small invertebrates was done with a nylon net (mesh size of 300-500 μm). At least 153 taxa were collected, and patterns of numerically dominant 24 taxa (all crustaceans) were inspected. A notable feature was very clear nocturnalism (levels N2 and N3). In addition, there was a wide variety in synchrony with the tidal cycle from very clear patterns (level T3) to no clear patterns (level T0). Most patterns (80%) showed well-demarcated synchrony with the tides (T2 and T3). A variety of synchrony was also observed on the tidal phase. The zoeas of some animals occurred at the receding tide, and those of other animals occurred at around high tide. The timing of their occurrence may be reflected by the site and time of larval release. The water temperature of the tidal flat would highly increase at low water by day, and the seawater is highly transparent in subtropical sea, though the estuarine water is turbid. Planktonic animals would be transported to the sea with the receding tide at night. Nocturnal occurrence may be adaptive to avoid high temperatures and vulnerability to visual predators in the sea during the transport of larvae. All megalopae occurred at the rising tide. They would go upstream using the rising tide. The occurrence patterns of most invertebrates were characterized by the semilunar rhythm. The adaptive function of this rhythm may be related to mating or fertilization rather than survival of the larvae.