Orientation and navigation in coastal and estuarine zooplankton

Abstract

The axiom that zooplankton species exhibit upwards migration behaviour at night is challenged by recent findings. Such behaviour is not universal, may vary during ontogeny, and is occasionally reversed. Moreover, in some estuarine and coastal zooplankton species vertical migration rhythms are of tidal, not diel, periodicity. There is evidence for several species that vertical migrations are endogenously controlled, occurring in constant conditions in the laboratory, suggesting that they have arisen under considerable selection pressure. They appear to play a significant role in orientation and navigation of coastal and estuarine zooplankton, predicated on the selective advantage of closure of life cycles. Vertical migrations between water masses moving in different directions at tidal, diel or longer timescales permit dispersal and recruitment, or retention, of planktonic larvae and adults in favourable ecological locations. Exogenous factors serving as cues for, or directly controlling, vertical migration rhythms include light, hydrostatic pressure, salinity, temperature, water movements and gravity responses, besides biological factors such as the timing of larval release, duration of larval life and predator/prey interactions. Behavioural characteristics should be quantified and factored into dispersal models which assume that zooplankton adults and larvae are advected as passive particles.

Keywords:

• Plankton
• orientation
• migration
• tidal
• diel
• endogenous rhythms
• exogenous factors