Rapid response to changing light environments of the calanoid copepod Calanus sinicus

Calanus sinicus is a large calanoid copepod and a dominant species in the coastal waters of Japan. During a research cruise in Sagami Bay on 18 June 1996, we found C. sinicusperforming an "unusual" diel vertical migration (DVM), a behavior that has not been reported in previous studies on this species. This study examined the DVM of C. sinicus under different light environments and revealed the copepod's characteristic response to light. Field and laboratory results show that the DVM of C. sinicus is flexible and also confirmed its sensitivity and its rapid response to changing light environments. It is suggested that C. sinicus reacts to changes in absolute light intensity. This feature may be common in oceanic copepod species. The copepod's quick reaction to light variation provides decreased predation risks and increased feeding opportunities, which make them a dominant survivor in coastal water habitats.