Critical swimming speeds of late-stage coral reef fish larvae: variation within species,among species and between locations

The swimming abilities of larval fishes are important for their survival, potentially affecting their ability to avoid predators, obtain food and control dispersal patterns. Near settlement swimming abilities may also influence spatial and temporal patterns of recruitment. We examined Critical speed (U-crit) swimming ability in late stage larvae of 89 species of coral reef fishes from the Great Barrier Reef and the Caribbean. Coefficients of variation in U-crit calculated at the individual level were high (28.4%), and this was not explained by differences in size or condition factor of these same larvae. Among species U-crit ranged from 5.5 cm s⁻¹ to 100.8 cm s⁻¹ (mean=37.3 cm s⁻¹), with 95% of species able to swim faster than the average current speed around Lizard Island, suggesting that most species should be capable of influencing their spatial and temporal patterns of settlement. Inter-specific differences in swimming ability (at both the family and species levels) were significantly correlated with size and larval morphology. Correlations were found between swimming performance and propulsive area, fineness ratio and aspect ratio, and these morphological parameters may prove useful for predicting swimming ability in other taxa. Overall, the swimming speeds of larvae from the same families at the two locations were relatively similar, although the Lutjanidae and Acanthuridae from the Caribbean were significantly slower than those from the great barrier reef. Differences in swimming speed and body form among late stage larvae suggests that they will respond differently to factors influencing survival and transport during their pelagic phase, as well as habitat use following settlement.     

Behaviour that influences dispersal and connectivity in the small,young larvae of a reef fish

Determining the scale of larval dispersal and population connectivity in demersal fishes is a major challenge in marine ecology. Historically, considerations of larval dispersal have ignored the possible contributions of larval behaviour, but we show here that even young, small larvae have swimming, orientation and vertical positioning capabilities that can strongly influence dispersal outcomes. Using young (11–15 days), relatively poorly developed (8–10 mm), larvae of the pomacentrid damselfish, Amblyglyphidodon curacao (identified using mitochondrial DNA), we studied behaviour relevant to dispersal in the laboratory and sea on windward and leeward sides of Lizard Island, Great Barrier Reef. Behaviour varied little with size over the narrow size range examined. Critical speed was 27.5 ± 1.0 cm s⁻¹ (30.9 BL s⁻¹), and in situ speed was 13.6 ± 0.6 cm s⁻¹. Fastest individuals were 44.6 and 25.0 cm s⁻¹, for critical and in situ speeds, respectively. In situ speed was about 50% of critical speed and equalled mean current speed. Unfed larvae swam 172 ± 29 h at 8–10 cm s⁻¹ (52.0 ± 8.6 km), and lost 25% wet weight over that time. Vertical distribution differed between locations: modal depth was 2.5–5.0 and 10.0–12.5 m at leeward and windward sites, respectively. Over 80% of 71 larvae observed in situ had directional swimming trajectories. Larvae avoided NW bearings, with an overall mean SE swimming direction, regardless of the direction to nearest settlement habitat. Larvae made smaller changes between sequential bearings of swimming direction when swimming SE than in other directions, making it more likely they would continue to swim SE. When swimming NW, 62% of turns were left (more than in other directions), which would quickly result in swimming direction changing away from NW. This demonstrates the larvae knew the direction in which they were swimming and provides insight into how they achieved SE swimming direction. Although the cues used for orientation are unclear, some possibilities seemingly can be eliminated. Thus, A. curacao larvae near Lizard Island, on average swam into the average current at a speed equivalent to it, could do this for many hours, and chose different depths in different locations. These behaviours will strongly influence dispersal, and are similar to behaviour of other settlement-stage pomacentrid larvae that are older and larger.     

The influence of swimming speed on sustained swimming performance of late-stage reef fish larvae

Replicate clutches of larvae were swum in a swimming flume at 4, 7, 10, 13, and 16 cm s^-1 and the time swum until exhaustion recorded. There was a significant relationship between sustained swimming time and swimming speed for both maximum (R²=0.77; P<0.05) and mean sustained swimming times (R²=0.78; P<0.05), with fish swimming at slower speeds swimming longer and covering greater total distances. The relationship observed agrees with theoretical principles relating increasing swimming speed with increasing drag. We used our data for Amphiprion melanopus, combined with published information, to predict the swimming speeds that other reef fish taxa should be able to maintain for significant lengths of time (12-48 h) using three different models. The results agree well with field estimates (R² values from 0.45 to 0.84), suggesting that there may be underlying factors influencing swimming ability in reef fish larvae that can be used to predict swimming abilities of different taxa. These models suggest that sustained swimming behaviour by reef fish larvae could have a much greater impact on modifying larval dispersal than previously thought.     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

Larval bloom of the oviparous sponge Cliona viridis: coupling of larval abundance and adult distribution

 We performed an intensive year-round sampling with the aim of studying the abundance of sponge larvae in four Mediterranean benthic communities: photophilic algae, sciaphilous algae, semi-obscure (i.e. low light-intensity) caves and sandy bottoms. We record here for the first time, a larval bloom of Cliona viridis (Schmidt 1862), the most common excavating sponge in the Mediterranean, which took place simultaneously in several rocky communities of the Blanes sub-littoral (NE Spain), and discuss the role of restricted larval dispersal in the distribution of adult sponges. In the communities studied, C. viridis larvae bloomed synchronously once, in June. Spawning and consequent embryo development presumably occurred in May, when water temperature was 16 °C. The free larva is a small, evenly ciliated, weakly swimming parenchymella with low dispersal capabilities. The number of larvae m⁻³ and sponge abundance (as percent cover and biomass) were significantly higher in the community of sciaphilous algae than in the other communities studied. Because of limited larval dispersal, larval and adult abundance in the communities were positively correlated. Larvae developed into juvenile sponges 10 to 15 d after settlement. Settlers displayed distinctive features: a peripheral cuticle, vacuolar etching-like cells at the sponge base, absence of oscular chimneys, and the presence of zooxanthellae, which were presumably transmitted during oocyte maturation. Received: 24 January 2000 / Accepted: 4 July 2000     

Energetic costs of swarming behavior for the copepod Dioithona oculata

The cyclopoid copepod Dioithona oculata forms dense swarms within shafts of sunlight that penetrate the mangrove prop-root habitat of islands off the coast of Belize. Previous studies, based on in situ video recordings and laboratory studies, have shown that D. oculata is capable of maintaining fixed-position swarms in spite of currents of up to 2 cm s⁻¹. The purpose of this study was to examine the energetic costs of maintaining these swarms, in terms of increased metabolic costs of maintaining position in currents and in terms of reduced feeding rates in densely packed swarms during the day. Using a sealed, variable-speed flow-through chamber, the respiration rates of D. oculata were measured while swarms maintained position in different current speeds. The results indicate that active metabolism (swimming at maximum speed to maintain the swarm in a current) is approximately three times greater than routine metabolism (normal swimming speeds in the absence of currents), indicating a significant metabolic cost of maintaining swarms in the presence of currents. In addition, gut-pigment analysis indicated that feeding rates of these copepods were often reduced in swarms during the day compared to when the copepods were dispersed at night. Given the high “cost” of swarming, the adaptive value of swarming in terms of reduced predation, increased opportunities for mating, and reduced dispersal, must be substantial. Received: 4 June 1997 / Accepted: 18 September 1997     

In situ ontogeny of behaviour in pelagic larvae of three temperate, marine, demersal 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, and changed direction towards NE (parallel to 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.     

Swimming ability and its rapid decrease at settlement in wrasse larvae (Teleostei: Labridae)

Wrasses are abundant reef fishes and the second most speciose marine fish family, yet little is known of their larval swimming abilities. In August 2010 at Moorea, Society Islands, we measured swimming ability (critical speed, Ucrit) of 80 settlement-stage larvae (11–17 mm) of 5 labrid species (Thalassoma quinquevittatum [n = 67], Novaculichthys taeniourus [n = 6], Coris aygula [n = 5], Halichoeres trimaculatus [n = 1] and H. hortulanus [n = 1]) and 33 new recruits of T. quinquevittatum. Median (mdn) larval Ucrit was 7.6–12.5 cm s⁻¹. In T. quinquevittatum (n = 67), larvae of 12.5–14.5 mm swam faster (mdn 16.9 cm s⁻¹) than smaller or larger larvae (mdn 3.9 and 3.2 cm s⁻¹, respectively). Labrid larvae Ucrit is similar to that of other similar-sized tropical larvae, so labrids and species with comparable settlement sizes should have similar abilities to influence dispersal. Ucrit of T. quinquevittatum recruits decreased to 47–56% of larval Ucrit in 2 days, implying rapid physiological changes at settlement.     

Temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid corals

Acroporid corals are the main reef-building corals that provide three-dimensional habitats for other reef organisms, but are decreasing on many reefs worldwide due to natural and anthropogenic disturbances. In this study, temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid coral species, Acropora muricata and A. valida, were examined through laboratory rearing experiments to better understand the potential for larval dispersal of this important coral group. Many larvae were attached (but not metamorphosed) to settlement tiles on the first examination 3–4 days after spawning (AS). The first permanent larval settlement (i.e. metamorphosed and permanently settled juvenile polyps) occurred at 5–6 days AS, and most larval settlement (85–97% of total) occurred within 9–10 days AS. Larval survivorship decreased substantially to around 50% by the first week of the experiment and to approximately 10% by the second to third week. The rates of larval attachment, settlement, and the initial drop in survivorship of larvae suggest that effective dispersal of some acroporid species may largely be completed within the first few weeks AS.     

In situ swimming and settlement behaviour of larvae of an Indo-Pacific coral-reef fish, the coral trout Plectropomus leopardus (Pisces: Serranidae)

Late larvae of the serranid coral trout Plectropomus leopardus (Lacepède), captured in light traps, were released during the day both in open water and adjacent to two reefs, and their behaviour was observed by divers at Lizard Island, northern Great Barrier Reef. Coral trout larvae (n = 110) were present in light-trap catches from 18 November to 3 December 1997, including new moon (30 November). The swimming speed of larvae in open water or when swimming away from reefs was significantly greater (mean 17.9 cm s⁻¹) than the speed of larvae swimming towards or over reefs (mean 7.2 cm s⁻¹). Near reefs, larvae swam at average depths of 2.7 to 4.2 m, avoiding 0 to 2 m. In open water, swimming depth varied with location: larvae >1 km east of Lizard Island swam steeply downward to >20 m in 2 to 4 min; larvae >1 km west oscillated between 2.6 and 13 m; larvae 100 to 200 m east of Lizard Island oscillated between 0.8 and 15 m. Nearly all larvae swam directionally in open water and near reefs. In open water, the average swimming direction of all larvae was towards the island, and 80% (4 of 5) swam directionally (p < 0.05, Rayleigh's test). Larvae swam directionally over the reef while looking for settlement sites. The frequency of behaviours by larvae differed between two reefs of different exposure and morphology. Depending on site, 26 to 32% of larvae released adjacent to reefs swam to open water: of these, some initially swam towards or over the reef before swimming offshore. In some cases, offshore-swimming seemed to be due to the presence of predators, but usually no obvious cause was observed. Depending on the reef, 49 to 64% of the larvae settled. Non-predatory reef residents aggressively approached 19% of settlers. Between 5 and 17% of the larvae were eaten while approaching the reef or attempting to settle, primarily by lizardfishes but also by wrasses, groupers and snappers. A higher percentage of larvae settled in the second week of our study than in the first. Average time to settlement was short (138 s ± 33 SE), but some larvae took up to 15 min to settle. Average settlement depth was 7.5 to 9.9 m, and differed between locations. No settlement took place on reef flats or at depths <4.2 m. Larvae did not appear to be selective about settlement substrate, but settled most frequently on live and dead hard coral. Late-stage larvae of coral trout are capable swimmers with considerable control over speed, depth and direction. Habitat selection, avoidance of predators and settlement seem to rely on vision. Received: 7 July 1998 / Accepted: 26 January 1999     

Population structure of the Spanish sardine Sardinella aurita: natural morphological variation in a genetically homogeneous population

The importance of larval dispersal in determining the distribution and abundance of benthic marine organisms is well recognized; however, the contribution of post-larval dispersal has not been measured. I compared the dispersal of swimming larvae with that of rafting colonies in a population of compound ascidians, Botrylloides sp., living attached to leaves of the eelgrass Zostera marina in Tomales Bay, California, USA in 1990–1992. Colonies rafting on broken eelgrass traveled over 200 times farther and had comparable recruitment success relative to swimming larvae. The recruitment of rafting colonies into new habitats was facilitated by the ability of thesecolonial animals to grow asexually onto surrounding substrata. Rafting colonies brooded larvae that were subsequently relased after the colony settled into a new habitat. These results suggest that colonization of new habitats can occur by post-larvae as well as larvae, and that long-range dispersal by species with short-lived larvae may occur by post-larval rafting.     

Larvae of a colonial ascidian use a non-contact mode of substratum selection on a coral reef

The rate at which larvae successfully recruit into communities of marine benthic invertebrates is partially dependent upon how well larvae avoid benthic predators and settle on appropriate substrata. Therefore, to be able to predict recruitment success, information is needed on how larvae search for settlement sites, whether larvae preferentially settle on certain substrata, and the extent to which there are adequate cues for larvae to find these substrata. This article describes how larvae of the colonial ascidian Diplosoma similis find settlement sites on a coral reef. Direct field observations of larval settlement were made on a fringing reef in Kaneohe Bay, Oahu, Hawaii, between September 1985 and April 1986. A comparison of the substrata that larvae contacted prior to settlement relative to the percentage cover of these substrata on the study reef suggests that larvae are using a non-contact mode of substratum identification to locate suitable settlement sites. This mode of substratum identification allowed 74% of larvae to evade predation by benthic organisms who would otherwise have eaten larvae if they had been contacted. Of those larvae that evaded predation, 88% subsequently settled on the same two substrata upon which most adults are found (dead coral or the green alga Dictyosphaeria cavernosa). This pattern of settlement was probably a result of active selection, since the two substrata cover only 14.4% of the reef's surface and currents had little effect on the direction in which larvae swam. An important contributing factor to the high success rate of larval settlement on suitable substrata was the lack of any temporal decay in substratum preference. It is concluded that for Diplosoma similis larval supply is a sufficient predictor of larval settlement rate. However, for marine invertebrates whose larvae are passively dispersed and exhibit a greater temporal decay in substratum preference, larval settlement should generally have a greater dependency on spatial variation in the abundance of benthic predators and suitable substrata.     

Settlement patterns and presettlement behavior of the naked goby,Gobiosoma bosci,a temperate oyster reef fish

Dispersion, distribution, development and feeding incidence of larvae of the naked goby,Gobiosoma bosci (Lacepéde), were examined for linkages between larval behavior while near the reef surface and later patterns of settlement and recruitment. Field sampling and experiments were conducted during the summers of 1988 and 1989 in the Flag Pond oyster reef along the western shore of the Chesapeake Bay near Camp Conoy, Maryland, USA. Results indicated that prior to settlement most demersal larvae aggregate in shoals and exhibit distinct microhabitat preferences on the reef. In a field experiment, larvae settled both during the day and at night. Dispersion at settlement was aggregated, suggesting that demersal shoaling influences settlement patterns in this species. The distribution of demersal larvae also indicated that larval swimming behavior is sufficiently strong to permit active control of position on reefs. Large demersal larvae settled rapidly when brought to the laboratory, but small larvae in demersal shoals appeared to require additional growth and morphological development prior to settlement. Development of the pelvic fins, used by juveniles and adults for perching on the substrate, may be a good indicator of competence to settle in this species. The adaptive significance of demersal shoaling by small larvae of the naked goby, and the fate of these larvae, remains perplexing because the low feeding rates found for larvae shoaling near the reef surface should slow or prevent the growth and development required prior to settlement. Observations made by other authors indicate that demersal shoaling and the use of water directly overlying reefs may be common behaviors of temperate and tropical reef fishes.     

Laboratory study of predation by Aurelia aurita on larvae of cod,flounder, plaice and herring: development and vulnerability to capture

Capture success of the medusa Aurelia aurita preying on various developmental stages of fish larvae was measured together with larval reactivity and escape speed after being stung. These experiments were conducted in the spring of 1983 with A. aurita medusae collected from Loch Etive, Scotland and laboratory-reared larvae of Gadus morhua L., Platichthys flesus L., Pleuronectes platessa L. and Clupea harengus L. Capture success of the medusae increased with medusa size, but decreased with advancing larval development. Smaller species of larvae were more vulnerable to capture. Larval reactivity to encounters with medusae increased with advancing development, and larger species of larvae were more reactive to encounters. Larval escape swimming speeds also increased with advancing larval development and size. These results indicate that earlier stages of larvae within a species and smaller species of larvae at a given stage are more vulnerable to predation by medusae since they are less reactive to encounters. Apparently they are more susceptible to the effects of neurotoxins. Predation rates on different developmental stages of herring larvae are documented and compared with rates predicted by a predation model. Predictions fell within the range of observed predation rates, but tended to overestimate rates by larger medusae feeding on larger herring larvae. This indicates the possibility of predator satiation and/or behavioural avoidance.     

Dispersal mechanisms in amphipods: a case study of <Emphasis Type="Italic">Jassa herdmani</Emphasis> (Crustacea,Amphipoda) in the North Sea

Jassa herdmani (Walker 1893), a tube-building amphipod typical of hard substrates, was found in large densities on shipwrecks from the Belgian part of the North Sea, in association with the hydrozoan Tubularia indivisa. In this area, shipwrecks only represent the source of hard substrates in an environment dominated by soft sediments. Nevertheless, the long-distance dispersal of Jassa species has never been investigated. Therefore, we tested the hypothesis of dispersal with currents, by investigating the behaviour of J. herdmani in the laboratory. Size distribution revealed that newly released juveniles (<1 mm) predominated throughout the year and the lower frequency of the size class 1–3 mm indicates a high mortality or a dispersal at this life stage. Individuals of J. herdmani may initiate actively the transport by tidal or surface currents by swimming to the surface of the water or by floating at the surface, as suggested by behaviours noted in the laboratory. The reaction of the amphipods to a current was investigated in the laboratory. We could not detect any sexual function associated with drifting and there was also no preponderance of a size class in the drifting individuals. When testing different substrates, we observed a significant influence of the substrate type on the frequency at which drifting occurred: J. herdmani showed a better adherence on T. indivisa compared to other substrates. Finally, the amphipod showed also a preference for its host compared to other substrates, which suggests a possible detection mechanism.     

Availability of dissolved organic matter offsets metabolic costs of a protracted larval period for <Emphasis Type="Italic">Bugula neritina</Emphasis> (Bryozoa)

For nearly a century researchers have investigated the uptake and utilization of dissolved organic matter (DOM) by marine invertebrates, but its contribution to their growth, reproduction, and survival remains unclear. Here, the benefit of DOM uptake was assessed for the marine bryozoan Bugula neritina (Linnaeus 1758) through performance comparisons of individuals in the presence and absence of DOM. The experiments were performed using B. neritina collected from floating docks in Beaufort, NC, USA from July to September 2004. Seawater was subjected to ultraviolet irradiation to reduce naturally occurring DOM, and then enriched with either 1 μM of palmitic acid or a mixture containing 1 μM each of glucose, alanine, aspartic acid and glycine. Larvae in DOM-enriched and DOM-reduced treatments were sampled and induced to metamorphose following 1, 6, 12, and 24 h of continuous swimming at 25°C. Sampled larvae were assessed for initiation of metamorphosis, completion of metamorphosis, and ancestrular lophophore size to determine the extent to which energy acquired from DOM uptake could offset the metabolic costs of prolonged larval swimming. DOM treatment had no significant effect on initiation of metamorphosis, but did have a significant effect on completion of metamorphosis and lophophore size. Larvae swimming in DOM-enriched treatments for 24 h experienced a 20% increase in metamorphic completion rate, compared to larvae swimming for 24 h in the DOM-reduced treatment. In addition, larvae in the amino acid and sugar mixture for 24 h had a significantly larger lophophore surface area and volume (23 and 31%, respectively), compared to larvae in DOM-depleted seawater. To ensure that the increases in performance found in larvae with access to DOM were not due to a decrease in metabolic activity, the respiration rates for these larvae were compared to those of larvae in DOM-depleted seawater. There were no significant differences between these treatments, indicating that the increases in performance were due to the energy acquired from DOM. These results clearly show that for B. neritina, DOM uptake results in increased metamorphic success and in the size of the feeding apparatus following an extended larval swimming duration.     

Isolation by distance in the scleractinian coral <Emphasis Type="Italic">Seriatopora hystrix</Emphasis> from the Red Sea

Pelagic dispersal of larvae in sessile marine invertebrates could in principle lead to a homogeneous gene pool over vast distances, yet there is increasing evidence of surprisingly high levels of genetic differentiation on small spatial scale. To evaluate whether larval dispersal is spatially limited and correlated with distance, we conducted a study on the widely distributed, viviparous reef coral Seriatopora hystrix from the Red Sea where we investigated ten populations separated between ~0.150 km and ~610 km. We addressed these questions with newly developed, highly variable microsatellite markers. We detected moderate genetic differentiation among populations based on both F _ST and R _ST (0.089 vs. 0.136, respectively) as well as considerable heterozygote deficits. Mantel tests revealed isolation by distance effects on a small geographic scale (≤20 km), indicating limited dispersal of larvae. Our data did not reveal any evidence against strictly sexual reproduction among the studied populations.     

Effect of light intensity on activity and food-searching of larval herring,Clupea harengus: a laboratory study

Larvae of Clupea harengus were reared from spawning herring caught in March 1982 and 1983 in the Firth of Clyde, Scotland. An infra0red observation technique was used to record the behaviour of larval herring both in shallow dishes using a top view and in a tank 2 m deep using a side view. The amount of time larvae spent swimming, which was minimum in complete darkness, increased with increasing light intensity and as the larvae grew. Maximum swimming speeds of feeding larvae were recorded at light intensities between 10 and 100 lux. The presence of food organisms (Artemia sp., Brazilian strain) at light intensities below the feeding threshold (0.1 lux) caused an increase in the proportion of time spent active, but light intensities above the threshold had different effects, depending on developmental stage: larvae of 12 mm increased swimming speed, but 21 mm larvae decreased speed. In the 2 m deep tank in darkness, larvae displayed inactive periods wherein they sank head first, interspersed with periods of upward swimming. As light intensity increased, vertical swimming was replaced by horizontal swimming. These results are discussed with reference to food searching and vertical migration of larval herring in the sea.     

The swimming kinematics of larval Atlantic cod, Gadus morhua L., are resilient to elevated seawater pCO2

Kinematics of swimming behavior of larval Atlantic cod, aged 12 and 27 days post-hatch (dph) and cultured under three pCO₂ conditions (control-370, medium-1800, and high-4200 μatm) from March to May 2010, were extracted from swim path recordings obtained using silhouette video photography. The swim paths were analyzed for swim duration, distance and speed, stop duration, and horizontal and vertical turn angles to determine whether elevated seawater pCO₂—at beyond near-future ocean acidification levels—affects the swimming kinematics of Atlantic cod larvae. There were no significant differences in most of the variables tested: the swimming kinematics of Atlantic cod larvae at 12 and 27 dph were highly resilient to extremely elevated pCO₂ levels. Nonetheless, cod larvae cultured at the highest pCO₂ concentration displayed vertical turn angles that were more restricted (median turn angle, 15°) than larvae in the control (19°) and medium (19°) treatments at 12 dph (but not at 27 dph). Significant reduction in the stop duration of cod larvae from the high treatment (median stop duration, 0.28 s) was also observed compared to the larvae from the control group (0.32 s) at 27 dph (but not at 12 dph). The functional and ecological significance of these subtle differences are unclear and, therefore, require further investigation in order to determine whether they are ecologically relevant or spurious.     

Gene flow at three spatial scales in a coral reef fish, the three-spot dascyllus, Dascyllus trimaculatus

Dispersal in coral reef fishes occurs predominantly during the larval planktonic stage of their life cycle. With relatively brief larval stages, damselfishes (Pomacentridae) are likely to exhibit limited dispersal. This study evaluates gene flow at three spatial scales in one species of coral reef damselfish, Dascyllus trimaculatus. Samples were collected at seven locations at Moorea, Society Islands, French Polynesia. Phylogenetic relationships and gene flow based on mitochondrial control region DNA sequences between these locations were evaluated (first spatial scale). Although spatial structure was not found, molecular markers showed clear temporal structure, which may be because pulses of settling larvae have distinct genetic composition. Moorea samples were then compared with individuals from a distant island (750 km), Rangiroa, Tuamotu Archipelago, French Polynesia (second spatial scale). Post-recruitment events (selection) and gene flow were probably responsible for the lack of structure observed between populations from Moorea and Rangiroa. Finally, samples from six Indo-West Pacific locations, Zanzibar, Indonesia, Japan, Christmas Island, Hawaii, and French Polynesia were compared (third spatial scale). Strong population structure was observed between Indo-West Pacific populations. Received: 26 May 2000 / Accepted: 10 October 2000