Behaviour of pelagic larvae of four coral-reef fish species in the ocean and an atoll lagoon

In-situ behaviour of settlement-stage larvae (10-30 mm) of four coral-reef fishes - Acanthurus triostegus, Chromis viridis, Neoniphon argenteus and Ptereleotris sp. - differed between lagoon and ocean at Rangiroa Atoll, Tuamotu Islands, French Polynesia. Divers released 130 larvae individually in midwater, and recorded larval swimming speed, depth and direction. All species swam faster than average currents, and C. viridis swam faster in the lagoon than in the ocean. Vertical distribution behaviour of all species differed between ocean and lagoon, generally by larvae swimming deeper in the ocean. Nearly all individual larvae swam directionally. Within a species, distribution of average bearings of individual larvae was not directional, nor did it differ between ocean and lagoon. Larvae detected predators 3-6 m away, and stopped or changed depth or direction to avoid them. We therefore reject the 'simplifying assumptions' that reef-fish larvae are passive or that their behaviour is independent of location. Behavioural flexibility of settlement-stage reef-fish larvae has implications for dispersal, retention and population connectivity. This constitutes the first report of larval reef-fish behaviour in the open ocean. However, in the ocean, many larvae descended rapidly below safe diving depth, and adult remoras interfered, making in-situ study of larval behaviour difficult.