Coral Reef Fish Larvae Research Articles

Coral reef fish larvae show no evidence for map-based navigation after physical displacement

Millions of minute, newly hatched coral reef fish larvae get carried into the open ocean by highly complex and variable currents. To survive, they must return to a suitable reef habitat within a species-specific time. Strikingly, previous studies have demonstrated that return to home reefs is much more frequent than would be expected by chance. It has been shown that magnetic and sun compass orientation can help cardinalfish maintain their innate swimming direction but do they also have a navigational map to cope with unexpected displacements? If displaced settling-stage cardinalfish Ostorhinchus doederleini use positional information during their pelagic dispersal, we would expect them to re-orient toward their home reef. However, after physical displacement by 180km, the fish showed a swimming direction indistinguishable from original directions near the capture site. This suggests that the tested fish rely on innate or learned compass directions and show no evidence for map-based navigation.

Rapid physiological and transcriptomic changes associated with oxygen delivery in larval anemonefish suggest a role in adaptation to life on hypoxic coral reefs.

Connectivity of coral reef fish populations relies on successful dispersal of a pelagic larval phase. Pelagic larvae must exhibit high swimming abilities to overcome ocean and reef currents, but once settling onto the reef, larvae transition to endure habitats that become hypoxic at night. Therefore, coral reef fish larvae must rapidly and dramatically shift their physiology over a short period of time. Taking an integrative, physiological approach, using swimming respirometry, and examining hypoxia tolerance and transcriptomics, we show that larvae of cinnamon anemonefish (Amphiprion melanopus) rapidly transition between "physiological extremes" at the end of their larval phase. Daily measurements of swimming larval anemonefish over their entire early development show that they initially have very high mass-specific oxygen uptake rates. However, oxygen uptake rates decrease midway through the larval phase. This occurs in conjunction with a switch in haemoglobin gene expression and increased expression of myoglobin, cytoglobin, and neuroglobin, which may all contribute to the observed increase in hypoxia tolerance. Our findings indicate that critical ontogenetic changes in the gene expression of oxygen-binding proteins may underpin the physiological mechanisms needed for successful larval recruitment to reefs.

Ear Development in Select Coral Reef Fishes: Clues for the Role of Hearing in Larval Orientation Behavior?

The pelagic larvae of coral reef fishes can detect and discriminate among different types of sounds suggesting that they may be able to localize acoustic cues and thus identify suitable settlement sites on reefs. It has been suggested that a description of ear development may provide clues about the importance of sound in larval behavior. Here we provide the first description of the ontogeny of the ear in the pelagic larvae and juveniles of representatives of three important families of coral reef fishes: Gobiidae (Elacatinus lori, E. colini), Apogonidae (Cheilodipterus quinquelineatus), and Pomacentridae (Amblyglyphidodon leucogaster, Amphiprion polymnus). Histological analysis revealed that the ear of the larvae and juveniles of the five study species lack any of the morphological specializations known in percomorph fishes. However, it showed that the ears of the two gobies are quite similar with respect to size and shape of the three otolithic organs, but that ears of the cardinalfish and damselfishes are different with respect to the absolute and relative sizes and position of the otolithic organs, as well as the timing of the appearance of the lagena; however, the functional significance of these features are unknown. It is concluded that if hearing plays a role in orientation behavior in the pelagic larvae of coral reef fishes then their “unremarkable” ears are sufficient to allow these diminutive fishes to carry out the extraordinary feat of navigating the open ocean and successfully locating coral reefs on which to settle.

Energy use, growth and survival of coral reef snapper larvae reared at elevated temperatures

The success of individuals during the pelagic larval phase is critical to maintaining healthy and viable populations of coral reef fishes; however, it is also the most environmentally sensitive and energetically demanding life stage. Climate change is increasing the frequency and intensity of marine heatwaves, which could have significant effects on the development and survival of larval coral reef fishes. However, little is known about how the larvae of pelagic-spawning coral reef fishes will be affected due to the difficulty of spawning and rearing these species in captivity. In this study, we tested how elevated temperatures, similar to those occurring during a marine heatwave, affected the yolk utilization, growth, and survival of larval, Lutjanus carponotatus, a common mesopredatory fish on Indo-west Pacific coral reefs. Eggs and larvae were reared at a current-day average summer temperature (28.5 °C) and two elevated temperatures (30 °C and 31.5 °C) until 14 d post-hatch (dph). Larvae in the elevated temperatures depleted their yolk reserves 39% faster than at the control temperature. The standard length of larvae was 55% (30 °C) and 92% (31.5 °C) longer in the elevated temperature treatments than the control temperature at 14 dph. Conversely, survival of larvae was 54% (30 °C) and 68% (31.5 °C) lower at elevated temperatures compared with the control temperature. This study provides new insights as to how the early life stages of coral reef fishes could be affected by ocean warming and marine heatwaves, with implications for their population dynamics.

The relative influence of environmental cues on reproductive allocation of a highly iteroparous coral reef fish

Pelagic eggs and larvae of many coral reef fishes will encounter a dynamic and risky environment as they disperse between the reef and offshore habitats. Life-history theory predicts that spawning adults should synchronize their reproductive effort with specific environmental conditions that facilitate offspring survival. Favourable conditions for reproduction may be determined by local environmental conditions at the spawning site, or signalled by larger-scale environmental cues, such as the lunar cycle. Multiple cues may interact in complex ways to cause additional variation in spawning intensity. We evaluated a set of environmental variables that potentially determine temporal and spatial variation in spawning patterns of a highly iteroparous fish, the sixbar wrasse (Thalassoma hardwicke). Specifically, we monitored focal territories of terminal-phase males over a 5-month period, quantified spawning activities, and evaluated a hierarchical set of predictive models using a model selection approach (AICc). Temporal variation in spawning (and population densities at the spawning site) was most strongly associated with the lunar cycle and maximal around the new moon. Local hydrodynamic conditions and other environmental variables observable at the spawning site were less strongly correlated with temporal variation in spawning. Territory proximity to the reef edge was a strong predictor of spatial variation in spawning intensity; territories closest to the reef edge experienced more spawning. These observations suggest that females make predictable decisions about where and when they spawn. Females appear to recognize strong, persistent spatial gradients in spawning habitat quality, and primarily vary their spawning effort in accordance with a large-scale environmental cue (the lunar cycle).

Travel with your kin ship! Insights from genetic sibship among settlers of a coral damselfish.

Coral reef fish larvae are tiny, exceedingly numerous, and hard to track. They are also highly capable, equipped with swimming and sensory abilities that may influence their dispersal trajectories. Despite the importance of larval input to the dynamics of a population, we remain reliant on indirect insights to the processes influencing larval behavior and transport. Here, we used genetic data (300 independent single nucleotide polymorphisms) derived from a light trap sample of a single recruitment event of Dascyllus abudafur in the Red Sea (N = 168 settlers). We analyzed the genetic composition of the larvae and assessed whether kinship among these was significantly different from random as evidence for cohesive dispersal during the larval phase. We used Monte Carlo simulations of similar‐sized recruitment cohorts to compare the expected kinship composition relative to our empirical data. The high number of siblings within the empirical cohort strongly suggests cohesive dispersal among larvae. This work highlights the utility of kinship analysis as a means of inferring dynamics during the pelagic larval phase.

Hydrodynamic and biological constraints on group cohesion in plankton

The dynamics of plankton in the ocean are determined by biophysical interactions. Although physics and biotic behaviors are known to influence the observed patchiness of planktonic populations, it is still unclear how much, and if, group behavior contributes to this biophysical interaction. Here, we demonstrate how simple rules of behavior can enhance or inhibit active group cohesion in plankton in a turbulent environment. In this study, we used coral-reef fish larvae as a model to investigate the interaction between microscale turbulence and planktonic organisms. We synthesized available information on the swimming speeds and sizes of reef fish larvae, and developed a set of equations to investigate the effects of viscosity and turbulence on larvae dispersion. We then calculated the critical dispersion rates for three different swimming strategies – cruise, random-walk, and pause-travel – to determine which strategies could facilitate group cohesion during dispersal. Our results indicate that swimming strategies and migration to low-turbulence regions are the key to maintaining group cohesion, suggesting that many reef fish species have the potential to remain together, from hatching to settlement. In addition, larvae might change their swimming strategies to maintain group cohesion, depending on environmental conditions and/or their ontogenic stage. This study provides a better understanding of the hydrodynamic and biological constraints on group formation and cohesion in planktonic organisms, and reveals a wide range of conditions under which group formation may occur.

Environmental conditions and paternal care determine hatching synchronicity of coral reef fish larvae

For all fishes, hatching is a short but crucial event, and the conditions under which it occurs considerably influence the success of the larvae. For coral reef fish, hatching is even more important because it marks the beginning of the dispersal phase. The timing of hatching dictates the conditions that the larvae will encounter, potentially influencing their survival and dispersal. Despite this importance, very few studies have looked at hatching of marine fishes in the natural environment because of important technical constrains. In this study, we directly examined the temporal pattern of hatching during the night. Using remote night vision cameras and infrared lights to avoid disturbances, we successfully documented in situ hatching events of three coral reef fish species, all benthic brooders (Abudefduf saxatilis, Stegastes partitus, and Elacatinus lori). Hatching timing, rate, and duration were computed from the observations, and highlight different hatching strategies used by the fish species. The analysis of the fish behaviors shows that the males display parental care beyond the incubation period of the eggs and increase activity during the hatching events. With this study, we can relate the hatching events to the environmental context, giving us a better understanding of the factors influencing the beginning of the larval phase. These observations highlight the benefit of in situ studies to better understand the characteristics and potential consequences of hatching.

Live fast, die young: Behavioural and physiological impacts of light pollution on a marine fish during larval recruitment

Artificial light at night (ALAN) is a recently acknowledged form of anthropogenic pollution of growing concern to the biology and ecology of exposed organisms. Though ALAN can have detrimental effects on physiology and behaviour, we have little understanding of how marine organisms in coastal areas may be impacted. Here, we investigated the effects of ALAN exposure on coral reef fish larvae during the critical recruitment stage, encompassing settlement, metamorphosis, and post-settlement survival. We found that larvae avoided illuminated settlement habitats, however those living under ALAN conditions for 10 days post-settlement experienced changes in swimming behaviour and higher susceptibility to nocturnal predation. Although ALAN-exposed fish grew faster and heavier than control fish, they also experienced significantly higher mortality rates by the end of the experimental period. This is the first study on the ecological impacts of ALAN during the early life history of marine fish.

Investigations on larvae of commercial fish from Hurghada, Red Sea with notes on the spawning seasons and grounds of some species

The present work deals with the study of the fish larvae of coral reef fishes in the Red Sea (Hurghada). It aimed to study the spatial and temporal variations of fish larvae for the management purposes. Sampling stations were Marina, Sheraton, Magawish and Arabia. The samples were taken monthly throughout the period of study from August 2014 to July 2015 using plankton net with different mesh size (150, 350, and 500 micron). The highest density of larvae was observed in July where841 larvae /1000m3 were collected whereas, the lowest density was recorded in March and October with11/1000 m3. The highest number of species was recorded in Sheraton with 28 species followed by Magawish with 27 species recorded followed by Marina with 26 species recorded while the lowest was recorded in Arabia with 25 species. Concentration of larvae was significantly different between months and sites. There was a significant difference in the number of species between months but not between stations. Larvae of mullid fish were abundant in summer while larvae of clupeids were taken all the year round.

Parasites of coral reef fish larvae: its role in the pelagic larval stage

The pelagic larval stage is a critical component of the life cycle of most coral reef fishes, but the adaptive significance of this stage remains controversial. One hypothesis is that migrating through the pelagic environment reduces the risk a larval fish has of being parasitised. Most organisms interact with parasites, often with significant, detrimental consequences for the hosts. However, little is known about the parasites that larval fish have upon settlement, and the factors that affect the levels of parasitism. At settlement, coral reef fishes vary greatly in size and age (pelagic larval duration), which may influence the degree of parasitism. We identified and quantified the parasites of pre-settlement larvae from 44 species of coral reef fishes from the Great Barrier Reef and explored their relationship with host size and age at settlement, and phylogeny. Overall, less than 50% of the larval fishes were infected with parasites, and over 99% of these were endoparasites. A Bayesian phylogenetic regression was used to analyse host-parasite (presence and intensity) associations. The analysis showed parasite presence was not significantly related to fish size, and parasite intensity was not significantly related to fish age. A phylogenetic signal was detected for both parasite presence and intensity, indicating that, overall, closely related fish species were likely to have more similar susceptibility to parasites and similar levels of parasitism when compared to more distantly related species. The low prevalence of infection with any parasite type and the striking rarity of ectoparasites is consistent with the ‘parasite avoidance hypothesis’, which proposes that the pelagic phase of coral reef fishes results in reduced levels of parasitism.

High temporal resolution sampling reveals reef fish settlement is highly clustered

Coral reef fish larvae settle on reefs predominantly at night around the new-moon phase, after an early developmental period spent in the pelagic environment. Most sampling is conducted across whole nights, and any studies that have examined the frequency of arrival within nights have typically been limited to coarse sampling time scales of 1–5 h. Here, we present results for arrival numbers of fish caught between dusk and midnight from light traps sampled every 15 min at an Indonesian coral reef, providing the finest temporal resolution for this type of study to date. A spatial analysis by distance indices analysis, adapted to temporal data, revealed clustering of reef arrival times for many species, with an increase in catches immediately after dusk dropping off towards midnight. Importantly, the timing of clusters differed among species, indicating that different factors determine the timing of arrival among taxa. Our results support the hypothesis that larval behaviour influences the timing of arrival at a coral reef for different fish species.

Dimethyl Sulfide is a Chemical Attractant for Reef Fish Larvae

Transport of coral reef fish larvae is driven by advection in ocean currents and larval swimming. However, for swimming to be advantageous, larvae must use external stimuli as guides. One potential stimulus is “odor” emanating from settlement sites (e.g., coral reefs), signaling the upstream location of desirable settlement habitat. However, specific chemicals used by fish larvae have not been identified. Dimethyl sulfide (DMS) is produced in large quantities at coral reefs and may be important in larval orientation. In this study, a choice-chamber (shuttle box) was used to assess preference of 28 pre-settlement stage larvae from reef fish species for seawater with DMS. Swimming behavior was examined by video-tracking of larval swimming patterns in control and DMS seawater. We found common responses to DMS across reef fish taxa - a preference for water with DMS and change in swimming behavior - reflecting a switch to “exploratory behavior”. An open water species displayed no response to DMS. Affinity for and swimming response to DMS would allow a fish larva to locate its source and enhance its ability to find settlement habitat. Moreover, it may help them locate prey accumulating in fronts, eddies, and thin layers, where DMS is also produced.

Pre-settlement coral-reef fish larvae respond to magnetic field changes during the day.

Observations of coral-reef fish larvae have revealed remarkably consistent orientation behaviour while swimming offshore, requiring large-scale orientation cues. However, the mechanisms underlying this behaviour are still being investigated. One potential large-scale cue for orientation is the Earth's geomagnetic field. Here, we examined the effect of magnetic field manipulations on the orientation behaviour of coral-reef fish during the pelagic larval phase. In the absence of visual cues, individual larvae responded to a 90deg shift of the horizontal component of the magnetic field within a Helmholtz coil with a comparable shift in orientation, demonstrating that they use a magnetic compass for orientation. Our findings suggest that geomagnetic field information guides swimming behaviour of larval fish in the pre-settlement phase. The ability to use large-scale sensory cues allows location-independent orientation of swimming, a behaviour that influences dispersal and connectivity of fish populations, which has important ecological implications for anthropogenic development of marine areas.

Behavioural lateralization and shoaling cohesion of fish larvae altered under ocean acidification

Recent studies have shown that the behaviour and development of coral reef fish larvae is hampered by projected future CO2 levels. However, it is uncertain to what extent this effect also occurs in temperate species. The effects that elevated pCO2 (~2000 µatm) levels, which are expected to occur in coastal upwelling regions in the future, have on shoaling behaviour and lateralization (turning preference) of fish, were tested in temperate sand smelt Atherina presbyter larvae. The hypothesis that behavioural changes are caused by interference of high CO2 with GABA-A receptor function was tested by treating larvae with a receptor antagonist (gabazine). Routine swimming speed did not differ between control and high pCO2, but exposure to high pCO2 for 7 days affected group cohesion, which presented a more random distribution when compared to control fish. However, this random distribution was reversed after 21 days of exposure to high CO2 conditions. Lateralization at the individual level decreased in fish exposed to high pCO2 for 7 and 21 days, but gabazine reversed this decline. This adds to the growing body of evidence that the effects of a more acidified environment on fish larvae behaviour are likely due to altered function of GABA-A receptors. Overall, our results suggest that future pCO2 levels likely to occur in temperate coastal ecosystems could have an adverse effect on temperate larval fish behaviour.

Limited Capacity for Faster Digestion in Larval Coral Reef Fish at an Elevated Temperature.

The prevalence of extreme, short-term temperature spikes in coastal regions during summer months is predicted to increase with ongoing climate change. In tropical systems, these changes are predicted to increase the metabolic demand of coral reef fish larvae while also altering the plankton communities upon which the larvae feed during their pelagic phase. The consequences of these predictions remain speculative in the absence of empirical data on the interactive effects of warm temperatures on the metabolism, postprandial processes and growth responses of coral reef fish larvae. Here, we tested the effect of increased temperature on the metabolism, postprandial performance and fine-scale growth patterns of a coral reef fish (Amphiprion percula) in the latter half of its ~11-d larval phase. First, we measured the length and weight of fed versus fasted larvae (N = 340; mean body mass 4.1±0.05 mg) across fine temporal scales at a typical current summer temperature (28.5°C) and a temperature that is likely be encountered during warm summer periods later this century (31.5°C). Second, we measured routine metabolic rate (Mo2 routine) and the energetics of the postprandial processes (i.e., digestion, absorption and assimilation of a meal; termed specific dynamic action (SDA)) at both temperatures. Larvae fed voraciously when provided with food for a 12-hour period and displayed a temperature-independent increase in mass of 40.1% (28.5°C) and 42.6% (31.5°C), which was largely associated with the mass of prey in the gut. A subsequent 12-h fasting period revealed that the larvae had grown 21.2±4.8% (28.5°C) and 22.8±8.8% (31.5°C) in mass and 10.3±2.0% (28.5°C) and 7.8±2.6% (31.5°C) in length compared with pre-feeding values (no significant temperature effect). Mo2 routine was 55±16% higher at 31.5°C and peak Mo2 during the postprandial period was 28±11% higher at 31.5°C, yet elevated temperature had no significant effect on SDA (0.51±0.06 J at 28.5°C vs. 0.53±0.07 J at 31.5°C), SDA duration (6.0±0.6 h vs. 6.5±0.5 h), or the percent of total meal energy used for SDA (SDA coefficient: 10.1±1.3% vs. 13.0±1.7%). Our findings of higher Mo2 routine but similar SDA coefficient at high temperature provide the first empirical evidence that coral reef fish larvae may have to secure more food to attain similar growth rates during warm summer periods, and perhaps with chronically warmer conditions associated with climate change.

Small-Boat Noise Impacts Natural Settlement Behavior of Coral Reef Fish Larvae.

After a pelagic larval phase, settlement-stage coral reef fish must locate a suitable reef habitat for juvenile life. Reef noise, produced by resident fish and invertebrates, provides an important cue for orientation and habitat selection during this process, which must often occur in environments impacted by anthropogenic noise. We adapted an established field-based protocol to test whether recorded boat noise influenced the settlement behavior of reef fish. Fewer fish settled to patch reefs broadcasting boat + reef noise compared with reef noise alone. This study suggests that boat noise, now a common feature of many reefs, can compromise critical settlement behavior of reef fishes.

Chemical spying in coral reef fish larvae at recruitment

When fish larvae recruit back to a reef, chemical cues are often used to find suitable habitat or to find juvenile or adult conspecifics. We tested if the chemical information used by larvae was intentionally produced by juvenile and adult conspecifics already on the reef (communication process) or whether the cues used result from normal biochemical processes with no active involvement by conspecifics (“spying” behavior by larvae). Conspecific chemical cues attracted the majority of larvae (four out of the seven species tested); although while some species were equally attracted to cues from adults and juveniles (Chromis viridis, Apogon novemfasciatus), two exhibited greater sensitivity to adult cues (Pomacentrus pavo, Dascyllus aruanus). Our results indicate also that spying cues are those most commonly used by settling fishes (C. viridis, P. pavo, A. novemfasciatus). Only one species (D. aruanus) preferred the odour of conspecifics that had had visual contact with larvae (communication).

The influence of various reef sounds on coral-fish larvae behaviour.

The swimming behaviour of coral-reef fish larvae from 20 species of 10 different families was tested under natural and artificial sound conditions. Underwater sounds from reef habitats (barrier reef, fringing reef and mangrove) as well as a white noise were broadcasted in a choice chamber experiment. Sixteen of the 20 species tested significantly reacted to at least one of the habitat playback conditions, and a range of responses was observed: fishes were (1) attracted by a single sound but repelled by none (e.g. white-banded triggerfish Rhinecanthus aculeatus was attracted by the barrier-reef sound), (2) repelled by one or more sounds but attracted by none (e.g. bridled cardinalfish Pristiapogon fraenatus was repelled by the mangrove and the bay sounds), (3) attracted by all sounds (e.g. striated surgeonfish Ctenochaetus striatus), (4) attracted and repelled by several sounds (e.g. whitetail dascyllus Dascyllus aruanus was attracted by the barrier-reef sound and repelled by the mangrove sound) and (5) not influenced by any sound (e.g. convict surgeonfish Acanthurus triostegus). Overall, these results highlight two settlement strategies: a direct selection of habitats using sound (45% of the species), or a by-default selection by avoidance of certain sound habitats (35%). These results also clearly demonstrated the need to analyse the influence of sounds at the species-specific level since congeneric and confamilial species can express different behaviours when exposed to the same sounds.

Towards sustainable exhibits - application of an inorganic fertilization method in coral reef fish larviculture in an aquarium

Coral reef fish are collected from the wild and exhibited in aquaria worldwide. Some of the fish spawn in captivity; however, the eggs are usually neglected. In this study, we collected the eggs spawned naturally in the exhibit tanks, hatched and cultured them indoor in 2000-L fibreglass tanks (initial density = 18 000 egg tank−1). We applied an inorganic fertilization method commonly used in freshwater fish culture in raising these coral reef fish larvae. We maintained inorganic phosphorus concentration at 100 μg P L−1 and inorganic nitrogen at 700 μg N L−1 daily in the fertilized group (n = 4), while the control tanks (n = 4) were fed with rotifers (10 ind mL−1). Chlorophyll a at particle sizes of both 0.45–20 μm and >20 μm, as well as NH3-N, NO3-N, and PO4-P concentrations were significantly higher in the fertilized group than the control. Zooplankton in the size groups of 10–50 μm (mainly flagellates) and 50–100 μm (mainly ciliates) were abundant (about 10~60 ind mL−1) during 3–7 days in fertilized tanks. The average larval fish survival rate at 21 day after hatch in fertilized group was consistently higher than the control in two trials. The experiments demonstrated that the inorganic fertilization approach can be successfully adapted for coral reef fish culture in an aquarium to achieve sustainable exhibits.