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Late pelagic stages of coral reef fishes captured with light-traps were individually released during daylight by SCUBA divers in open water, 20-35 m deep, in the Great Barrier Reef Lagoon at three sites > 1 km from the reefs of Lizard Island. Observations in situ on 111 individuals of 11 families, but primarily Apogonidae, Chaetodontidae and Pomacentridae, constitute the first data of their kind. Most fish showed no overt reaction to the divers. Some individuals of some taxa of three families settled quickly to the bottom. Acceptable observations on swimming were made on 66 larvae. Individuals selected a wide range of depths, but when grouped by family, mean depths chosen by individuals were: apogonids, 6.5 (� 1.5, 95% CI) m; pomacentrids, 7.7 (� 1.5) m; and chaetodontids, 9.3 (� 1.3) m. Rough estimates of speed of up to 30 cm s-1 varied among taxa. Swimming directions of 59 of the 66 larvae were non-random. Mean directions differed among sites and were offshore at all of them. Most larvae swam offshore regardless of the side of the island where they were released. The late pelagic stages of coral reef fishes are strong swimmers capable of active horizontal and vertical movement. They swim directionally, can apparently detect reefs >1 km away, and orientate relative to those reefs. A taxonomic component is evident in many of these behaviours.

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Nearly all demersal teleost marine fishes have pelagic larval stages lasting from several days to several weeks, during which time they are subject to dispersal. Fish larvae have considerable swimming abilities, and swim in an oriented manner in the sea. Thus, they can influence their dispersal and thereby, the connectivity of their populations. However, the sensory cues marine fish larvae use for orientation in the pelagic environment remain unclear. We review current understanding of these cues and how sensory abilities of larvae develop and are used to achieve orientation with particular emphasis on coral-reef fishes. The use of sound is best understood; it travels well underwater with little attenuation, and is current-independent but location-dependent, so species that primarily utilize sound for orientation will have location-dependent orientation. Larvae of many species and families can hear over a range of ~100-1000 Hz, and can distinguish among sounds. They can localize sources of sounds, but the means by which they do so is unclear. Larvae can hear during much of their pelagic larval phase, and ontogenetically, hearing sensitivity, and frequency range improve dramatically. Species differ in sensitivity to sound and in the rate of improvement in hearing during ontogeny. Due to large differences among-species within families, no significant differences in hearing sensitivity among families have been identified. Thus, distances over which larvae can detect a given sound vary among species and greatly increase ontogenetically. Olfactory cues are current-dependent and location-dependent, so species that primarily utilize olfactory cues will have location-dependent orientation, but must be able to swim upstream to locate sources of odor. Larvae can detect odors (e.g., predators, conspecifics), during most of their pelagic phase, and at least on small scales, can localize sources of odors in shallow water, although whether they can do this in pelagic environments is unknown. Little is known of the ontogeny of olfactory ability or the range over which larvae can localize sources of odors. Imprinting on an odor has been shown in one species of reef-fish. Celestial cues are current- and location-independent, so species that primarily utilize them will have location-independent orientation that can apply over broad scales. Use of sun compass or polarized light for orientation by fish larvae is implied by some behaviors, but has not been proven. Use of neither magnetic fields nor direction of waves for orientation has been shown in marine fish larvae. We highlight research priorities in this area.

Climate change is leading to shifts in species geographical distributions, but populations are also probably adapting to environmental change at different rates across their range. Owing to a lack of natural and empirical data on the influence of phenotypic adaptation on range shifts of marine species, we provide a general conceptual model for understanding population responses to climate change that incorporates plasticity and adaptation to environmental change in marine ecosystems. We use this conceptual model to help inform where within the geographical range each mechanism will probably operate most strongly and explore the supporting evidence in species. We then expand the discussion from a single-species perspective to community-level responses and use the conceptual model to visualize and guide research into the important yet poorly understood processes of plasticity and adaptation. This article is part of the theme issue ‘The role of plasticity in phenotypic adaptation to rapid environmental change’.