Recent studies have revealed that reef fish larvae have excellent sustained swimming capabilities and considerable potential for modifying their dispersal patterns by active swimming. However, these studies concentrate solely on the late pelagic phase. We examined the development of swimming abilities from hatching through to settlement in 3 reef fish species (Pomacentrus amboinensis, Sphaeramia nematoptera, Amphiprion melanopus). Larval rearing provided larvae at all stages of development. Experiments were conducted in flow chambers designed for measuring the critical and sustained swimming capability of young larvae. In all 3 species, critical swimming ability increased steadily with age, size, relative propulsive area and developmental stage of the larvae. In contrast, sustained swimming ability showed a marked inflection during development. Differences among species throughout development appear to reflect variations in the developmental patterns of the 3 species. Propulsive area was highly correlated with swimming ability and may prove useful for estimating swimming capabilities among species. The results suggest that some species have the potential to actively modify their dispersal patterns from an early age.

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Larval fishes exhibit marked vertical zonation patterns that are important in planktonic predator-prey interactions. This zonation has been related to both proximate physical (e.g., light intensity) and ultimate biological factors (e.g., predator risk, prey availability). We hypothesize that changes in visual sensitivity and feeding performance in larval fishes provide a constraint on depth distributions and that this constraint may represent the major determinant of vertical distribution patterns. To test the generality of the relationship between larval age and visual sensitivity, larvae were obtained from captive breeding adults of seven species (three Apogonidae and four Pomacentridae). The lowest light intensity at which feeding behavior occurred was measured using infrared video techniques with natural prey items. The visual sensitivity of the larvae, measured as initiation of feeding, increased by about 3 orders-of-magnitude during the larval phase. The abilities of the larvae were broadly similar within families, but there were marked differences between families. Pomacentrids were about an order-of-magnitude less light-sensitive than apogonids. Larval apogonids are potentially capable of feeding at a depth of about 100 m in clear tropical waters at 4 mm standard length (SL) and to 200 m at 12 mm SL. The patterns of ontogenetic and phylogenetic differences in light sensitivity are consistent with field observations on the vertical distributions of larvae in these taxa.

Ontogenetic change in the visual acuity of Premnas biaculeatus larvae was determined both behaviourally and anatomically. Visual acuity improved substantially between early feeding (day 3 post-hatch) and the pre-settlement (day 10 post-hatch) larvae but, at both ages, the anatomically-measured visual acuity was greater than that determined behaviourally. It appears that estimated anatomical visual acuity values substantially over-estimate the functional visual acuity realized under normal conditions. The distribution of the reactive angles indicated that most frequently prey within 0 to 9 of the longitudinal larval axis elicited a feeding response at both larval ages. This suggests that stereoscopic vision is used extensively during feeding in this species. The prey capture success with rotifers ranged from 96% at 3 days post-hatch to 100% at 10 days post-hatch. These values differ markedly from previous studies on temperate species and highlight the well developed abilities of larval P. biaculeatus at a given size or age. 1996 The Fisheries Society of the British Isles

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