Nighttime emergence patterns of planktonic and benthic crustaceans in a shallow subtidal environment

A number of planktonic and benthic crustaceans, including their larvae, inhabit the shallow subtidal zones along seacoasts. They are attracted to light at night. Using this characteristic, we collected such crustaceans at the seacoast of the Inland Sea of Japan in spring and autumn. The specimens collected were of 14 orders. About one-third were amphipods (23 species), but the most common were copepods and the larvae of decapod crustaceans (i.e., mysis, zoeas, and megalopae). Emergence of benthic animals (e.g., Philomedes japonica, Nannastacus sp., Pontogeneia rostraca, and Paradexamine barnardi) coincided with the day-night cycle, and showed a 24-h period on the correlogram. In contrast, the period of the emergence patterns in most planktonic animals (e.g., copepods, Lucifer hanseni, and larvae of decapod crustaceans) was 24.5-25 h, suggesting a tidal influence. Planktonic animals may respond to changes of hydrostatic pressure or the drift of a current, which would have exogenously resulted in the tide-related patterns. The tides seem to have affected the pattern more strongly in spring than in autumn. Relations between the emergence pattern and the times of high tide were clearly different between zoeas and megalopae; i.e., while a number of zoeas emerged during the receding tides, megalopa larvae swarmed around the time of high tide. The different timings of emergence might be related to the dispersal to the open sea (zoeas) and the return to the adult habitat (megalopae). The number of specimens collected every night fluctuated markedly, but did not show the rhythmicity coinciding with the lunar phase in most species.