Emergence rhythms of subtidal small invertebrates in the subtropical sea: Nocturnal patterns and variety in the synchrony with tidal and lunar cycles

Abstract

The subtidal zones near the shore are inhabited by many small invertebrates, including benthos and plankton. To characterize their emergence in the water column with regards to day/night, tidal, and lunar cycles, field investigations were carried out at the subtropical island (Iriomote-jima), Okinawa Prefecture. By use of two impeller pumps installed in both surface and bottom waters, invertebrates were sampled continuously for 23 days. Although most patterns were much the same between the surface and bottom waters, the abundance of animals was different between the two depths. A notable feature was that nocturnal patterns were very dominant. More than half of these patterns were not affected by the tidal cycle at all. In contrast, the pattern of Ericthonius sp. (Amphipoda) showed a clear synchrony with the nocturnal tide. Other patterns were weakly modified by the nocturnal tide (e.g. Propallene longiceps; Pantopoda). A pattern coincided with the lunar phase was only seen in Vargula hilgendorfii (Myodocopida). Most arthropods would hide in the bottom substrate, or would swarm under or near the lower pump in the daytime, and they would disperse in the water column at night. A variety in the synchrony with nocturnal tides strongly supports a notion that the tidal rhythm is only a variation of the day/night rhythm, rather than the hypothesis that both rhythms are present simultaneously in an animal. Statistical methods (autocorrelogram and periodogram) are used to demonstrate the tide-correlated component of the activity. However, these methods are not sufficient for this purpose; visual inspection of the pattern is very important.