Diel colour changes in male Sapphirina nigromaculata (Cyclopoida, Copepoda).

Males of the pelagic copepod genus Sapphirina are known to exhibit brilliant iridescence, presumably to attract mates. However, the males of some species have been described as not being able to produce iridescence. To resolve this apparent contradiction, we conducted detailed observations of colouration patterns in Sapphirina nigromaculata, which has been previously described as both iridescent and non-iridescent. The colouration of male S. nigromaculata ranges from intensely iridescent to transparent with pigmented spots. These two phases appear to have a circadian rhythm, with males turning iridescent at midnight and remaining in that state until several hours after sunrise, then becoming transparent again towards the afternoon. Adult S. nigromaculata occurrence in surface waters shows temporal changes, in that they occasionally occur in higher densities than their average in the water column, particularly from midnight to around noon. The synchronization of iridescence in concert with upward migration is considered to be an adaptation for increasing the likelihood of encountering a mate. Observations using a video plankton recorder revealed that the transparent phase is an adaptation for decreasing predation risk by making the copepods more difficult to distinguish while they are associated with doliolids. [ABSTRACT FROM AUTHOR]