Larvae have been difficult to study because their small size limits our ability to understand their behavior and the conditions they experience. Questions about larval transport focus largely on (a) where they go [dispersal] and (b) where they come from [connectivity]. Mechanisms of transport have been intensively studied in recent decades. As our ability to identify larval sources develops, the consequences of connectivity are garnering more consideration. Attention to transport and connectivity issues has increased dramatically in the past decade, fueled by changing motivations that now include management of fisheries resources, understanding of the spread of invasive species, conservation through the design of marine reserves, and prediction of climate-change effects. Current progress involves both technological advances and the integration of disciplines and approaches. This review focuses on insights gained from physical modeling, chemical tracking, and genetic approaches. I consider how new findings are motivating paradigm shifts concerning (1) life-history consequences; (2) the openness of marine populations, self-recruitment, and population connectivity; (3) the role of behavior; and (4) the significance of variability in space and time. A challenge for the future will be to integrate methods that address dispersal on short (intragenerational) timescales such as elemental fingerprinting and numerical simulations with those that reflect longer timescales such as gene flow estimates and demographic modeling. Recognition and treatment of the continuum between ecological and evolutionary timescales will be necessary to advance the mechanistic understanding of larval and population dynamics.

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Recent progress in understanding larval dispersal: new directions and digressions

Coral reef fish larvae settle close to home

The Ecology of Fishes on Coral Reefs

A pelagic larval stage is found in nearly all demersal marine teleost fishes, and it is during this pelagic stage that the geographic scale of dispersal is determined. Marine biologists have long made a simplifying assumption that behaviour of larvae--with the possible exception of vertical distribution--has negligible influence on larval dispersal. Because advection by currents can take place over huge scales during a pelagic larval stage that typically lasts for several days to several weeks, this simplifying assumption leads to the conclusion that populations of marine demersal fishes operate over, and are connected over, similar huge scales. This conclusion has major implications for our perception of how marine fish populations operate and for our management of them. Recent (and some older) behavioural research-reviewed here-reveals that for a substantial portion of the pelagic larval stage of perciform fishes, the simplifying assumption is invalid. Near settlement, and for a considerable portion of the pelagic stage prior to that, larvae of many fish species are capable of swimming at speeds faster than mean ambient currents over long periods, travelling tens of kilometres. Only the smallest larvae of perciform fishes swim in an energetically costly viscous hydrodynamic environment (i.e., low Reynolds number). Vertical distribution is under strong behavioural control from the time of hatching, if not before, and can have a decisive, if indirect, influence on dispersal trajectories. Larvae of some species avoid currents by occupying the epibenthic boundary layer. Larvae are able to swim directionally in the pelagic environment, with some species apparently orientating relative to the sun and others to settlement sites. These abilities develop relatively early, and ontogenetic changes in orientation are seemingly common. Larvae of some species can use sound to navigate, and others can use odour to find settlement habitat, at least over small scales. Other senses may also be important to orientation. Larvae are highly aware of their environment and of potential predators, and some school during the pelagic larval stage. Larvae are selective about where they settle at both meso and micro scales, and settlement is strongly influenced by interactions with resident fishes. Most of these behaviours are flexible; for example, swimming speeds and depth may vary among locations, and speed may vary with swimming direction. In direct tests, these behaviours result in dispersal different from that predicted by currents alone. Work with both tropical and temperate species shows that these behaviours begin to be significant relatively early in larval development, but much more needs to be learned about the ontogeny of behaviour and sensory abilities in larvae of marine fishes. As a preliminary rule of thumb, behaviour must be taken into account in considerations of dispersal after the preflexion stage, and vertical distribution behaviour can influence dispersal from hatching. Larvae of perciform fishes are close to being planktonic at the start of the pelagic period and are clearly nektonic at its end, and for a substantial period prior to that. All these things differ among species. Larvae of clupeiform, gadiform and pleuronectiform fishes may be less capable behaviourally than perciform fishes, but this remains to be confirmed. Clearly, these behaviours, along with hydrography, must be included in modelling dispersal and retention and may provide explanations for recent demonstrations of self-recruitment in marine fish populations. Current work is directed at understanding the ontogeny of the gradual transition from planktonic to nektonic behaviour. Although it is clear that larvae of perciform fishes have the ability to strongly influence their dispersal trajectories, it is less clear whether or how these abilities are applied.

Are larvae of demersal fishes plankton or nekton

Understanding connectivity remains a fundamental challenge to marine ecology due to technical limitations of tracking larval dispersal. Marine population genetic analyses are often used to make inferences about the scale of population connectivity. For species with a larval phase, pelagic larval duration (PLD) is assumed to influence the scale of connectivity. If PLD and genetic metrics are reliable proxies of connectivity, the 2 should be well correlated. Previous tests report conflicting results, with many reports that global FST (Wright's fixation index) correlates poorly with PLD, and one very high correlation of isolation-by-distance (IBD) slope, which is derived from FST, with PLD. First we clarify the expectations for the performance of these different proxies in light of the latest understanding of larval dispersal dynamics. We then test the hypothesis that IBD slope may be a more robust correlate with dispersal scale than global FST with a new dataset of recent marine genetic studies. Re-evaluation of previously published and new datasets revealed a consistent, moderate fit (R 2 ~0.30) between genetic and PLD proxies of dispersal (using either IBD slope or global FST), with significant improvement for small-scale (<650 km) studies (R 2 = 0.50), and important effects of marker type. Significant effects of number of individuals and number of populations sampled on the genetic metrics in our dataset suggest a common need for more robust sampling designs. These results syn- chronize previous studies on this topic and provide validation that PLD and genetic metrics typically reflect scales of dispersal, as intended, at least when sampling design is robust.

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Marine connectivity: a new look at pelagic larval duration and genetic metrics of dispersal

At Lizard Island, Great Barrier Reef, catches of fish larvae by light traps that broadcast nocturnal reef sounds (noisy traps) were compared with catches by quiet traps over two 2.5 week new-moon periods in November (XI) 2000 and January (I) 2001. Three areas were sampled: near-reef (NR, 500 m from the shore) in I, middle (M, 650 m) in I and XI and offshore (O, >1000 m) in XI. The most abundant taxa captured were Apogonidae, Blenniidae, Chaetodontidae, Lethrinidae, Mullidae and Pomacentridae. Significant differences in catch were found between areas, and a position effect was found at the O and M areas. At the NR and M areas, no taxa had significantly greater catches in quiet traps, but larvae of five taxa had significantly greater catches in noisy traps. These were (areas and times of greater catches): Apogonidae (NR; M XI), Mullidae (M I & XI), Pomacentridae (NR; M I & XI), Serranidae (M I) and Sphyraenidae (NR). At the offshore area, five taxa (Apogonidae, Blenniidae, Chaetodontidae, Mullidae and Pomacentridae) had significantly greater catches in quiet traps and only Lethrinidae had significantly greater catches in noisy traps. Thus some taxa (particularly apogonids and pomacentrids which had catches up to 155% greater in noisy traps, but also lethrinids and mullids, and perhaps others), were attracted to reef sounds at night, but this apparently varied with location and time. The sound-enhanced catches imply a radius of attraction of the sound 1.02-1.6 times that of the light. More than 65 m from the speaker,the broadcast sound levels at frequencies typical of fish hearing were equivalent to background levels, providing a maximum radius of sound attraction in this experiment.

Coral‐reef sounds enable nocturnal navigation by some reef‐fish larvae in some places and at some times

Prerequisites for understanding dispersal in pelagic larvae of demersal fishes are data on when swimming abilities of larvae are sufficiently developed to be able to alter passive dispersal trajectories. In laboratory swimming chambers, the development of critical speed and endurance swimming was measured in reared larvae of 4 species of warm-temperate marine and estuarine fishes that spawn pelagic eggs (Sciaenidae, Argyrosom us japonicus; Sparidae, Pagrus auratus, Acanthopagrus australis; Percichthyidae, Macquaria novemaculeata). Size was a better predictor of swimming ability than age. Increase in critical speed with growth was best portrayed by linear or 'flat' curvilinear relationships. Increase in endurance was best portrayed by strongly concave curvilinear relationships. The percichthyid larvae had the highest critical speed initially, but speed increased slowly with growth. The 2 sparids had the greatest increase in speed with growth, and the sciaenid the least. The greatest increase in endurance with growth was found in P. auratus, but performance of M. novemaculeata was only slightly less. The slowest increase in endurance with growth was found in A. japonicus, but, by settlement, its performance was similar to the other species. Until notochord flexion was complete, both speed and endurance were limited. Thereafter, swimming performance improved markedly at a species-specific rate. At settlement, larvae of these species could swim more than 10 km and at speeds of 15 to 20 cm s -1 (=12 to 20 BL s -1 ), which exceeded the average currents in their coastal environment. Following notochord flexion, all larvae swimming at critical speed were in an inertial environment, and this corresponded to when substantial endurance swimming developed. Whether these potential performances are actually realized in the field remains to be determined, but they provide the potential to strongly influence dispersal.

https://www.int-res.com/articles/meps2005/292/m292p287.pdf

Swimming ontogeny of larvae of four temperate marine fishes

The ontogeny of behaviour relevant to dispersal was studied in situ with reared pelagic larvae of three warm temperate, marine, demersal fishes: Argyrosomus japonicus (Sciaenidae), Acanthopagrus australis and Pagrus auratus (both Sparidae). Larvae of 5–14 mm SL were released in the sea, and their swimming speed, depth and direction were observed by divers. Behaviour differed among species, and to some extent, among locations. Swimming speed increased linearly at 0.4–2.0 cm s−1 per mm size, depending on species. The sciaenid was slower than the sparids by 2–6 cm s−1 at any size, but uniquely, it swam faster in a sheltered bay than in the ocean. Mean speeds were 4–10 body lengths s−1. At settlement size, mean speed was 5–10 cm s−1, and the best performing individuals swam up to twice the mean speed. In situ swimming speed was linearly correlated (R
2=0.72) with a laboratory measure of swimming speed (critical speed): the slope of the relationship was 0.32, but due to a non-zero intercept, overall, in situ speed was 25% of critical speed. Ontogenetic vertical migrations of several metres were found in all three species: the sciaenid and one sparid descended, whereas the other sparid ascended to the surface. Overall, 74–84% of individual larvae swam in a non-random way, and the frequency of directional individuals did not change ontogenetically. Indications of ontogenetic change in orientated swimming (i.e. the direction of non-random swimming) were found in all three species, with orientated swimming having developed in the sparids by about 8 mm. One sparid swam W (towards shore) when 10 mm. These results are consistent with limited in situ observations of settlement-stage wild larvae of the two sparids. In situ, larvae of these three species have swimming, depth determination and orientation behaviour sufficiently well developed to substantially influence dispersal trajectories for most of their pelagic period.

In situ ontogeny of behaviour in pelagic larvae of three temperate, marine, demersal fishes

Behaviour during the pelagic larval stage of coral-reef fishes can strongly influence dispersal, yet little is known of behavioural ontogeny. Speed, orientation and vertical distribution of larvae of 4 coral-reef fishes (Platax teira, Ephippidae; Lutjanus malabaricus, Lutjanidae; Epinephelus coioides, E. fuscoguttatus, Serranidae; 6 to 23 mm) were measured in situ off Taiwan. In E. coioides and E. fuscoguttatus, speed was 2 to 30 cm s -1 (4 to 19 body lengths s -1 , BL s -1 ), and increased at 1.4 to 2.3 cm s -1 mm -1 . In P. teira and L. malabaricus, speed was 11.2 to 16.6 cm s -1 (4 to 20 BL s -1 ) across the size range. All but the smallest, slowest larvae had Reynolds numbers >1000, and so swam in an inertial environment. In situ speeds were 39 to 87% of critical speeds, and smaller larvae swam nearer to critical speed than larger larvae. Of the larvae 71 to 90% swam directionally, but neither percentage of directional individuals nor orientation precision increased with size. P. teira swam toward the southwest (offshore). Epinephelus species undertook ontogenetic changes in orientation. Neither orientation nor ontogenetic changes were found in L. malabaricus. Horizontal swimming can influence dispersal directly. Vertical distribution, which differed among species, can influence dis- persal indirectly. P. teira became surface orientated, ascending 0.8 m per mm increase in length. L. malabaricus descended 0.5 m per mm increase in length. E. coioides ascended 0.4 m per mm increase in length. E. fuscoguttatus preferred greater depths, and lacked ontogenetic changes. The behaviours and their development show these larval reef fishes can influence dispersal in species- specific ways.

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Ontogeny of in situ behaviours relevant to dispersal and population connectivity in larvae of coral-reef fishes

Behavior of young (8−18 mm SL) giant trevally (Caranx ignobilis), a large coral-reef−associated predator, was observed in the laboratory and the ocean. Size was a better predictor of swimming speed and endurance than was age. Critical speed increased with size from 12 to 40 cm/s at 2.7 cm/s for each mm increase in size. Mean scaled critical speed was 19 body lengths/s and was not size related. Swimming speed in the ocean was 4 to 20 cm/s (about half of critical speed) and varied among areas, but within each area, it increased at 2 cm/s for each mm increase in size. Swimming endurance in the laboratory increased from 5 to 40 km at 5 km for each mm increase in size. Vertical distribution changed ontogenetically: larvae swam shallower, but more variably, and then deeper with growth. Two-thirds of individuals swam directionally with no ontogenetic increase in orientation precision. Larvae swam offshore off open coasts, but not in a bay. In situ observations of C. ignobilis feeding, interacting with pelagic animals, and reacting to reefs are reported.
Manusc

Amanda C. Hay

Papers

Coral‐reef sounds enable nocturnal navigation by some reef‐fish larvae in some places and at some times

Swimming ontogeny of larvae of four temperate marine fishes

In situ ontogeny of behaviour in pelagic larvae of three temperate, marine, demersal fishes

Ontogeny of in situ behaviours relevant to dispersal and population connectivity in larvae of coral-reef fishes

Behavioral ontogeny in larvae and early juveniles of the giant trevally, Caranx ignobilis (Pisces: Carangidae)