Settlement behaviour of larvae of the Stripey Snapper, <Emphasis Type="Italic">Lutjanus carponotatus</Emphasis> (Teleostei: Lutjanidae)

Larval behaviour is important to dispersal and settlement, but is seldom quantified. Behavioural capabilities of larval Lutjanus carponotatus in both offshore pelagic and reef environments at Lizard Island, Great Barrier Reef were observed in situ to determine if they were sufficient to influence dispersal. Offshore, larvae swam with higher directional precision and faster on the windward side of the island (28 cm.s⁻¹) than on the leeward side (16 cm s⁻¹). Most larvae swam directionally. Mean swimming directions were southerly in the windward area and northerly in the leeward area. Larvae avoided the surface and remained mostly between 3–15 m. Larvae released near reefs were 2–3 times faster swimming away from reefs (19 cm s⁻¹) than swimming toward or over them (6–8 cm s⁻¹). Speed swimming away was similar to that offshore. Of 41 larvae released near reefs, 73% reached the reef, 59% settled, and 13% of those reaching the reef were eaten. Larvae settled onto hard and soft coral (58%), topographic reef features (29%) and sand and rubble (13%). Settlement depth averaged 5.5 m (2–8 m). Before settling larvae spent up to 800 s over the reef (mean 231 s) and swam up to 53 m (mean 14 m). About half of the larvae interacted with reef residents including predatory attacks and aggressive approaches by residents and aggressive approaches by settling larvae. Settlement behaviour of L. carponotatus was more similar to a serranid than to pomacentrids. Settlement-stage larvae of L. carponotatus are behaviourally capable, and have a complex settlement behaviour.     

In situ observation of settlement behaviour in larvae of coral reef fishes at night

The swimming behaviour of 534 coral reef fish larvae from 27 species was explored at Moorea Island (French Polynesia) while they searched for a suitable settlement habitat, on the first night of their lagoon life. Most larvae swam actively (74%) and avoided the bottom (77%). A significant relationship was highlighted between the vertical position of larvae in the water column and the distance they travelled from lagoon entrance to settlement habitat: larvae swimming close to the surface settled farther away on the reef than bottom-dwelling larvae.     

Pelagic to demersal transition in a coral‐reef fish,the orbicular batfish Platax orbicularis

Behavioural and ecological observations were made on young, reared Platax orbicularis in Opunohu Bay, Moorea, French Polynesia, during their transition from the pelagic, dispersive stage to the reef‐orientated demersal stage. Seventy‐two young P. orbicularis (17–75 mm standard length, L_S) were released in the pelagic zone and 20 (40–70 mm L_S) adjacent to the reefs. Swimming speed was slow (mean 5·2 cm s^?1) and independent of size. An ontogenetic descent was observed: the smallest P. orbicularis swam at the surface, medium‐sized P. orbicularis swam in midwater (mean 5–13 m) and the largest P. orbicularis swam to the bottom, where many lay on their sides. Platax orbicularis swam southerly on average, away from the ocean and into the bay. Smaller P. orbicularis were more likely to swim directionally than larger individuals. Young P. orbicularis released near reef edges swam at similar, but more variable speeds (mean 6·6 cm s^?1). About half of those released near reefs swam away, but fewer swam away from an inshore fringing reef than from a patch reef near the bay mouth. Many P. orbicularis swam up the slope onto the reef top, but the little settlement observed was near the reef base. Average, near‐reef swimming direction was also southerly. Some reef residents, in particular the triggerfish Balistapus undulatus, harassed young P. orbicularis.     

Complex behaviour by coral-reef fish larvae in open-water and near-reef pelagic environments

We present the first in situ observations of the pelagic larvae of coral-reef fishes feeding, schooling and being preyed upon. In addition, we report on their behavioural interactions with adult and juvenile fishes. Observations on over 500 larvae of over 50 species (mostly from four families) near the end of their pelagic interval were made in both open water (> 1 km offshore) and near-reef environments. Nearly 10% of larvae were seen to feed in open water, but < 1% fed near the reef. Presettlement schooling was observed in five species of four families. We observed no predation upon larvae in open water except near the bottom. Near the reef, 8.5% of larvae were eaten. The main predators near and on the reef were a species of wrasse and lizardfishes. Rates of predation seem to differ among genera of pomacentrids, perhaps related to differences in behaviour when settling. When confronted with adult fishes, which happened largely near the reef, larvae reacted with a limited range of behaviours, including sheltering near the observer, swimming to the surface, slowing or stopping, or swimming offshore. The frequency of these behaviours differed among larvae of three pomacentrid genera. Interactions with reef residents, particularly pomacentrids, were common, and usually involved aggression by the resident toward settling larvae. This may act to discourage settlement during the day when such residents are active. These data show that behaviour of late larvae of coral-reef fishes is complex and can greatly influence survival and recruitment. Further, behaviour differs among taxa, showing that not only are larvae not passive, but also that a generalised behaviour of larvae does not exist.     

Nocturnal orientation to reefs by late pelagic stage coral reef fishes

 The nocturnal orientation behaviour of the late pelagic stages of two reef fish families (Apogonidae and Pomacentridae) was examined using behavioural cages deployed in the field. The behavioural cages enabled the fish to choose between swimming towards or away from the reef in response to natural cues. Overall, 55% of fish displayed a choice in the experiments, however, the proportion varied between the two families, with 67% of pomacentrids and 27% of apogonids displaying a choice. In both families, of the fish which displayed a choice, the proportion of fish swimming towards the reef was significantly greater than 50%, as random movement would predict (64% of pomacentrids and 67% of apogonids swam towards the reef). This proportion did not vary significantly among four field sites with different current regimes and geographic locations. The results suggest that the late pelagic stages of reef fish display nocturnal orientation behaviour, possibly in response to sound, which may aid in their settlement on reefs. Accepted: 18 August 1997     

Depth distributions of coral reef fishes: the influence of microhabitat structure,settlement, and post-settlement processes

Many coral reef fishes have restricted depth ranges that are established at settlement or soon after, but the factors limiting these distributions are largely unknown. This study examines whether the availability of microhabitats (reef substrata) explains depth limits, and evaluates whether juvenile growth and survival are lower beyond these limits. Depth-stratified surveys of reef fishes at Kimbe Bay (Papua New Guinea) showed that the abundance of new settlers and the cover of coral substrata differed significantly among depths. A field experiment investigated whether settling coral reef fishes preferred particular depths, and whether these depth preferences were dependent on microhabitat. Small patch reefs composed of identical coral substrata were set up at five depths (3, 6, 10, 15 and 20 m), and settlement patterns were compared to those on unmanipulated reef habitat at the same five depths. For all species, settlement on patch reefs differed significantly among depths despite uniform substratum composition. For four of the six species tested, depth-related settlement patterns on unmanipulated habitat and on patch reefs did not differ, while for the other two, depth ranges were greater on the patch reefs than on unmanipulated habitat. A second experiment examined whether depth preferences reflected variation in growth and survival when microhabitat was similar. Newly settled individuals of Chrysiptera parasema and Dascyllus melanurus were placed, separately, on patch reefs at five depths (as above) and their survival and growth monitored. D. melanurus, which is restricted to shallow depths, had highest survival and growth at the shallowest depth. Depth did not affect either survival or growth of C. parasema, which has a broader depth range than D. melanurus (between 6 and 15 m). This suggests that the fitness costs potentially incurred by settling outside a preferred depth range may depend on the strength of the depth preference.     

Auditory sensitivity in settlement-stage larvae of coral reef fishes

The larval phase of most species of coral reef fishes is spent away from the reef in the pelagic environment. At the time of settlement, these larvae need to locate a reef, and recent research indicates that sound emanating from reefs may act as a cue to guide them. Here, the auditory abilities of settlement-stage larvae of four species of coral reef fishes (families Pomacentridae, Lutjanidae and Serranidae) and similar-sized individuals of two pelagic species (Carangidae) were tested using an electrophysiological technique, auditory brainstem response (ABR). Five of the six species heard frequencies in the 100–2,000 Hz range, whilst one carangid species did not detect frequencies higher than 800 Hz. The audiograms of the six species were of similar shape, with best hearing at lower frequencies between 100 and 300 Hz. Strong within-species differences were found in hearing sensitivity both among the coral reef species and among the pelagic species. Larvae of the coral reef species had significantly more sensitive hearing than the larvae of the pelagic species. The results suggest that settlement-stage larval reef fishes may be able to detect reef sounds at distances of a few 100 m. If true hearing thresholds are lower than ABR estimates, as indicated in some comparisons of ABR and behavioural methods, the detection distances would be much larger.     

Predators reduce abundance and species richness of coral reef fish recruits via non-selective predation

Predators have important effects on coral reef fish populations, but their effects on community structure have only recently been investigated and are not yet well understood. Here, the effect of predation on the diversity and abundance of young coral reef fishes was experimentally examined in Moorea, French Polynesia. Effects of predators were quantified by monitoring recruitment of fishes onto standardized patch reefs in predator-exclosure cages or uncaged reefs. At the end of the 54-day experiment, recruits were 74% less abundant on reefs exposed to predators than on caged ones, and species richness was 42% lower on reefs exposed to predators. Effects of predators varied somewhat among families, however, rarefaction analysis indicated that predators foraged non-selectively among species. These results indicate that predation can alter diversity of reef fish communities by indiscriminately reducing the abundance of fishes soon after settlement, thereby reducing the number of species present on reefs.     

Trawl fishery resources in the “southern waters” of Japan

Settlement-stage larvae of the coral reef fishes Ostorhinchus doederleini (Apogonidae) and Pomacentrus coelestis (Pomacentridae) prefer the odor of their settlement reef to that of other nearby reefs. It was unknown whether these olfactory preferences are temporally stable or the result of recent olfactory experience. Ostorhinchus doederleini and P. coelestis larvae were held in aquaria and exposed to water from either their settlement reef or a neighboring reef for 5–9 days and their olfactory preference was tested. We show that exposure to water from another reef did not influence olfactory preference. Ostorhinchus doederleini olfactory preference declined slightly over time whereas P. coelestis preference was gradually lost after 2–3 days in captivity. Neither species switched their preference to the new reef odor. While we cannot determine conclusively the time window of odor learning, imprinting at or shortly after birth is logical and has been demonstrated in other fish species.     

Coral Settlement on a Highly Disturbed Equatorial Reef System

Processes occurring early in the life stages of corals can greatly influence the demography of coral populations, and successful settlement of coral larvae that leads to recruitment is a critical life history stage for coral reef ecosystems. Although corals in Singapore persist in one the world’s most anthropogenically impacted reef systems, our understanding of the role of coral settlement in the persistence of coral communities in Singapore remains limited. Spatial and temporal patterns of coral settlement were examined at 7 sites in the southern islands of Singapore, using settlement tiles deployed and collected every 3 months from 2011 to 2013. Settlement occurred year round, but varied significantly across time and space. Annual coral settlement was low (~54.72 spat m^-2 yr^-1) relative to other equatorial regions, but there was evidence of temporal variation in settlement rates. Peak settlement occurred between March–May and September–November, coinciding with annual coral spawning periods (March–April and October), while the lowest settlement occurred from December–February during the northeast monsoon. A period of high settlement was also observed between June and August in the first year (2011/12), possibly due to some species spawning outside predicted spawning periods, larvae settling from other locations or extended larval settlement competency periods. Settlement rates varied significantly among sites, but spatial variation was relatively consistent between years, suggesting the strong effects of local coral assemblages or environmental conditions. Pocilloporidae were the most abundant coral spat (83.6%), while Poritidae comprised only 6% of the spat, and Acroporidae <1%. Other, unidentifiable families represented 10% of the coral spat. These results indicate that current settlement patterns are reinforcing the local adult assemblage structure (‘others’; i.e. sediment-tolerant coral taxa) in Singapore, but that the replenishment capacity of Singapore’s reefs appears relatively constrained, which could lead to less resilient reefs.     

Characterization and isolation of DNA microsatellite primers in the cardinalfish (Apogon doederleini)

Proper management of reef areas depends greatly on understanding the degree of dispersal of each species involved. The larvae of most reef fishes disperse from the natal reef before or soon after hatching and return to the reef environment after a pre‐settlement stage of several days to weeks. We characterized eight polymorphic microsatellite loci for the cardinal fish Apogon doederleini to study the spatial scale of connectivity of populations of different reefs of the Great Barrier Reef, Australia.     

Vertical distribution behaviour and its spatial variation in late-stage larvae of coral-reef fishes during the day

Daytime vertical distribution behaviour of settlement-stage reef-fish larvae in the upper 18?m was documented by diver observations of 497 pelagic larvae of 7 species 100–1000?m offshore of Lizard Island, Great Barrier Reef. Four species were studied on 2 sides of the island. Depth amplitude and depth frequency differed among species, locations and times. Four pomacentrids had modal depths in the upper 30–50% of the water column. A lutjanid and 2 chaetodontids had modal depths 0–2?m deeper than the deepest-swimming pomacentrid. On the leeward side, 6 of 7 species swam deeper and/or more variably offshore. On the windward side, 1 of 4 species swam deeper or more variably offshore. No larvae swam deeper than 18?m on the leeward side, but 31% of larvae of 3 species did so on the windward side. Three of 4 species swam deeper and/or more variably on the windward than leeward side. Vertical distributions in relatively shallow water are apparently strongly influenced by water-column depth and bottom type.     

Ecological structure of assemblages of coral reef fishes on isolated patch reefs

A 9-year study of the structure of assemblages of fish on 20 coral patch reefs, based on 20 non-manipulative censuses, revealed a total of 141 species from 34 families, although 40 species accounted for over 95% of sightings of fish. The average patch reef was 8.5 m² in surface area, and supported 125 fish of 20 species at a census. All reefs showed at least a two-fold variation among censuses in total numbers of fish present, and 12 showed ten-fold variations. There was also substantial variation in the composition and relative abundances of species present on each patch reef, such that censuses of a single patch reef were on average about 50% different from each other in percent similarity of species composition (Czekanowski's index). Species differed substantially in the degree to which their numbers varied from census to census, and in the degree to which their dispersion among patch reefs was modified from census to census. We characterize the 40 most common species with respect to these attributes. The variations in assemblage structure cannot be attributed to responses of fish to a changing physical structure of patch reefs, nor to the comings and goings of numerous rare species. Our results support and extend earlier reports on this study, which have stressed the lack of persistant structure for assemblages on these patch reefs. While reef fishes clearly have microhabitat preferences which are expressed at settlement, the variations in microhabitat offered by the patch reefs are insufficient to segregate many species of fish by patch reef. Instead, at the scale of single patch reefs, and, to a degree, at the larger scale of the 20 patch reefs, most of the 141 species of fish are distributed without regard to differences in habitat structure among reefs, and patterns of distribution change over time. Implications for general understanding of assemblage dynamics for fish over more extensive patches of reef habitat are considered.     

Competition for shelter in a high-diversity system: structure use by large reef fishes

Competition among large reef fishes for shelter beneath tabular structures provides a rare opportunity to study competition in a species-rich environment. The system permits a detailed study of localised competition with major implications for coral reefs with respect to human impacts including climate change. Using underwater video cameras, this study examined competition among 30 species of large reef fishes (from nine families) for access to shelter provided by 26 tabular structures, which may be the highest reported diversity of vertebrates competing for a single resource. Mean concentrations of fishes under tabular structures were also among the highest biomass recorded on reefs (4.71 kg m⁻²). A generated dominance hierarchy for the occupation of shelter appeared to be primarily driven by the size of fishes. In contrast to previous studies, fishes higher in the hierarchy tended to exhibit the lowest levels of aggression. However, size difference between fishes was found to be strongly negatively correlated with the proportion of aggressive interactions (R ² = 0.971, P < 0.0001). The strong competition for the shade provided by these corals highlights concerns about future shifts in the structure of large reef fish communities as corals are lost. This is particularly concerning given the critical functional roles played by certain species of large reef fishes that utilise tabular structure for shelter and which occupy the lower ranks of the dominance hierarchy.

     

Rapid behavioral responses of an invertebrate larva to dissolved settlement cue

Larvae of the nudibranch Phestilla sibogae were used to study whether a natural dissolved settlement cue (from their prey, Porites compressa, an abundant coral on Hawaiian reefs) induces behavioral responses that can affect larval transport to suitable settlement sites. As cue and larvae are mixed in the turbulent flow over a reef, cue is distributed in fine-scale filaments that the larva experiences as rapid (seconds) on/off encounters. To examine larval responses in this setting, individual larvae were tethered in a small flume with flow simulating water velocity relative to a freely swimming larva, and their responses to realistic temporal patterns of cue encounter were videotaped. Competent larvae quickly ceased swimming in cue filaments and resumed swimming after exiting filaments. The threshold cue concentration eliciting a response was 3%-17% of concentrations within heads of P. compressa in nature. When moving freely in filtered seawater, competent larvae swam along straight paths in all directions at approximately 0.2 cm s(-1), whereas in water conditioned by P. compressa, most ceased swimming and sank at approximately 0.1 cm s(-1). The ability of larvae to rapidly respond (by sinking) to brief encounters with dissolved settlement cues can enhance their rapid transport to the substratum, even in wave-driven turbulent flow.     

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.     

Temperate macroalgae impacts tropical fish recruitment at forefronts of range expansion

Warming waters and changing ocean currents are increasing the supply of tropical fish larvae to temperature regions where they are exposed to novel habitats, namely temperate macroalgae and barren reefs. Here, we use underwater surveys on the temperate reefs of south-eastern (SE) Australia and western Japan (~33.5°N and S, respectively) to investigate how temperate macroalgal and non-macroalgal habitats influence recruitment success of a range of tropical fishes. We show that temperate macroalgae strongly affected recruitment of many tropical fish species in both regions and across three recruitment seasons in SE Australia. Densities and richness of recruiting tropical fishes, primarily planktivores and herbivores, were over seven times greater in non-macroalgal than macroalgal reef habitat. Species and trophic diversity (K-dominance) were also greater in non-macroalgal habitat. Temperate macroalgal cover was a stronger predictor of tropical fish assemblages than temperate fish assemblages, reef rugosities or wave exposure. Tropical fish richness, diversity and density were greater on barren reef than on reef dominated by turfing algae. One common species, the neon damselfish (Pomacentrus coelestis), chose non-macroalgal habitat over temperate macroalgae for settlement in an aquarium experiment. This study highlights that temperate macroalgae may partly account for spatial variation in recruitment success of many tropical fishes into higher latitudes. Hence, habitat composition of temperate reefs may need to be considered to accurately predict the geographic responses of many tropical fishes to climate change.

     

Experimental gill-netting of reef fish: Species-specific responses modify capture probability across mesh sizes

Gill-nets are highly selective in terms of the sizes of fish they catch, but often unselective in terms of the suite of fish species they capture. We investigated gill-net selectivity from the point of view of behavioural interactions between the fish and the gear. We observed interactions between fish and gill-nets of three mesh sizes (65 mm, 88 mm & 110 mm) set over rocky reefs in southern New Zealand. There were significant differences among eight species of mobile reef fish in their response to gill-nets and in their capture rates. Some species were more vulnerable because of their use of habitat, swimming motion or morphology. Species that occupied low visibility habitats (e.g., the herbivorous Odax pullus, which mostly swims beneath the algal canopy) were more susceptible to being caught because they had little time to detect and avoid the gill-nets. Species with carangiform or sub-carangiform swimming motion (e.g., Latridopsis ciliaris or O. pullus) were more susceptible to being caught because once in the gill-net, they could only attempt to force their way forwards becoming wedged further into the mesh. Species whose morphology makes tangling in the mesh more likely (e.g., large or protruding spines (Aplodactylus arctidens), fins (L. ciliaris) or opercula) are also more susceptible to being caught. Some species, particularly the common labrid Notolabrus celidotus, were less susceptible than other species to being caught. Fewer than 1% of 538 N. celidotus observed within one metre of the gill-nets were caught. Most N. celidotus altered their swimming direction near the gill-nets and did not hit the mesh. N. celidotus that swam through the nets were smaller than those that swam over the gill-nets or turned away. The fact that different size classes had different responses suggests that interactions with the gill-net are actively controlled. To divers, it appeared that this species could readily detect the gill-nets and treated them as part of the seascape. Furthermore, their labriform swimming motion allowed them to swim backwards out of gill-nets to avoid becoming caught. The species-specific responses of reef fish near the gill-nets and behavioural differences may explain the low numbers of some common reef fish that are caught in gill-nets and the disproportionately high numbers of others. The potentially great ancillary effects from by-catch of important species of untargeted reef fish, birds and marine mammals make gill-nets a somewhat blunderbuss method of catching fish on coastal reefs.     

Habitat differentiation in the early life stages of simultaneously mass-spawning corals

The settlement process of coral larvae following simultaneous mass-spawning remains poorly understood, particularly in terms of population and community parameters. Here, the larval settlement patterns of Acropora corals, which are the most diverse genera of scleractinian corals at the species (haplotype) level, were investigated within a single subtropical reef. Across a 4-year period (2007–2010), the mitochondrial and nuclear molecular markers of 1,073 larval settlers were analyzed. Of the 11 dominant haplotypes of recruited populations, nine exhibited non-random patterns of settlement distribution. This result suggests that the actual habitat segregation starts during the early swimming larval stages of their life history, rather than by natural selection after random settlement. In addition, the presence of a depth-related settlement pattern supports that species-specific vertical zonation of coral larvae may play a role in the establishment of habitat segregation. Moreover, in some species that showed a preference toward the shoreward area of the bay, the settlement pattern was consistent with that of the adult distribution. This result indicates that the gametes were not mixed between fore and back reefs in the period from fertilization to settlement during the mass-spawning event, even within a single small reef. Another compatible hypothesis of this pattern is that the larvae are able to recognize various types of environmental information, facilitating the selection of optimal micro-habitats. Overall, Acropora coral larvae that are produced from a simultaneous mass-spawning event may have adapted to complex reef topography by means of multi-step habitat selection at settlement, corresponding to different spatial scales.     

Maximum sustainable swimming speeds of late-stage larvae of nine species of reef fishes

We examined the maximum sustainable swimming speed of late-stage larvae of nine species of tropical reef fishes from around Lizard Island, Great Barrier Reef, Australia. Larvae were captured in light traps and were swum in flumes at different experimental swimming speeds (of 5 cm s⁻¹ intervals) continuously for 24 h. Logistic regression was used to determine the speed at which 90% of larvae were able to maintain swimming, and this was used to indicate the maximum sustainable swimming speed for each species. Maximum sustainable swimming speeds varied among the species examined, with the lethrinid maintaining the fastest sustainable swimming speed (24 cm s⁻¹), followed by the Pomacentridae (10-20 cm s⁻¹) and the Apogonidae (8-12 cm s⁻¹). U-crit (maximum speed) explained 64% of the variation in sustainable speed among species, whereas total length only explained 33% of the variation in sustained swimming. A regression fitted across species suggests that 50% U-crit is a good approximation of the speed able to be maintained by these larvae for 24 h. A model based on a cubic relationship between sustained swimming time and speed was found to be more successful than either length or U-crit as a method of estimating sustainable swimming speed for most of the species examined. Overall, we found that swimming speed is an important factor when considering the potential for active swimming behaviour to influence dispersal patterns, recruitment success and levels of self-recruitment in reef fish larvae and needs to be carefully considered in models of larval dispersal.