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

The subtidal zones near the shore are inhabited by many small invertebrates , including benthos and plankton. To characterize their emergence in the wa ter column with regards to day/night, tidal, and lunar cycles, field invest igations were carried out at the subtropical island (Iriomote-jima), Okinaw a Prefecture. By use of two impeller pumps installed in both surface and bo ttom 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 featu re 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 patt ern of Ericthonius sp. (Amphipoda) showed a clear synchrony with the noctur nal tide. Other patterns were weakly modified by the nocturnal tide (e.g. P ropallene longiceps; Pantopoda). A pattern coincided with the lunar phase w as only seen in Vargula hilgendorfii (Myodocopida). Most arthropods would h ide 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 varie ty in the synchrony with nocturnal tides strongly supports a notion that th e tidal rhythm is only a variation of the day/night rhythm, rather than the hypothesis that both rhythms are present simultaneously in an animal. Stat istical methods (autocorrelogram and periodogram) are used to demonstrate t he tide-correlated component of the activity. However, these methods are no t sufficient for this purpose; visual inspection of the pattern is very imp ortant.