The effect of a lunar eclipse on the vertical migration behaviour of Meganyctiphanes norvegica (Crustacea : Euphausiacea) in the Ligurian Sea

The vertical migration of a zooplankton community dominated by the euphausi id Meganyctiphanes norvegica was monitored between 16 and 23 September 1997 with a 153 kHz Acoustic Doppler Current Profiler (ADCP) and a MOCNESS net. The sampling period covered a phase in the lunar cycle when the rise of th e moon (full moon) coincided initially with sunset and then became progress ively later. On 16 September 1997, a lunar eclipse occurred 45 min after su nset, lasting for similar to 2 h. At dusk, the ADCP observed the upward ver tical migration of two principal backscattering bands similar to 10 min apa rt with vertical velocities of up to 7 cm s(-1). After a period at the surf ace, a more diffuse band subsequently sank at a slower rate (1-2 cm s(-1)) to a depth of 75-100 m. Net samples showed that the earlier band consisted mainly of the pteropod Cavolinia inflexa, whilst the later band was mostly euphausiids, predominantly M.norvegica. This species was also the major con stituent of the band that sank. The timing of upward migration was relative ly constant over the sampling period, but there was an increasing delay of the secondary sinking until 21 September. This showed as a strong correlati on between the onset of sinking and the time of moonrise. The lunar eclipse on 16 September perturbed this pattern, such that animals did not sink soo n after their arrival at the surface, as occurred on 17 September, but rema ined at the surface until the end of the umbra This suggests that M.norvegi ca can perceive moonlight and that this influences vertical migration. Evid ence that the behaviour is not solely mediated by this exogenous factor, ho wever, is seen in the pattern that emerged after 21 September, when midnigh t sinking occurred at a relatively constant time after sunset and before mo onrise. These observations support the hypothesis that moonlight is a Zeitg eber for an endogenous rhythm that synchronizes secondary sinking behaviour with the lunar cycle.