Chairman of the 10th IPFC : Prof. David LECCHINI

Research thematic : FROM THE OCEAN TO A REEF HABITAT: HOW DO THE LARVAE OF CORAL REEF FISHES FIND THEIR WAY HOME?

Animals constantly make decisions: they choose mates, select a place to live or forage, and they decide whether to engage in activities which expose them to risk of predation. These decisions necessitate the perception of some environmental cues in order to optimize the social interactions. Thus, results of simple decisions based on the perception of some cues may influence an individual’s life span, a species’ population biology, and community structure. While there is a lot of information about decision-making in breeding, foraging and habitat selection, little is known on habitat selection for fish species at settlement, especially in coral reefs.

The life cycle of most coral reef fish species includes a planktonic larval phase which usually lasts from three to six weeks, followed by a sedentary reef phase for the juveniles and adults.During the pelagic phase, the larvae may move far from their native reef due to currents and/or their swimming abilities. Then larvae return to the reef (natal or not) to continue their development into juveniles, then to adults. At the end of its pelagic phase, a species-specific change in morphology and physiology, called metamorphosis, occurs during which fish lose many of the characteristics that enhance survival in the pelagic environment and develop other features suited to their new reef environment (transformation from larval to juvenile stage). Moreover, a fish larva must choose  a suitable reef habitat to settle , habitat that will facilitate post-settlement survival and growth. As fish larvae are subjected to strong selective pressures to choose the suitable reef habitat, many fish species show specific preference of habitat at settlement based on the presence/absence of specific substrates, conspecifics, predators and competitors for food and space. After several weeks or months in the settlement habitat, juveniles move into the adult population and become sexually mature.

As it is unlikely that successful settlement is solely a matter of chance (i.e. to find a suitable habitat), one of the greatest challenges facing the fish larvae is how to locate the relatively rare patches of coral reef habitat on which they settle and ultimately reside as adults. The answer must lie partly in the sensory modalities of fishes at settlement. Habitat selection is only possible if fish larvae could detect some environmental cues to select a suitable reef habitat at settlement. David Lecchini works on the importance of information perception in coral reef fish larvae at settlement since 2000 in the Pacific (French Polynesia, New Caledonia, Fiji, Samoa, Salomon and Ryukyus islands).

Until two decades ago, it was generally assumed that larval behaviors and sensory abilities at settlement were considered too feeble to significantly affect dispersal outcomes. Since then,David Lecchini (but also other researchers as D. Dixson, E. parmentier, J. Leis) showed that recognition of suitable reef habitats by fish larvae at settlement is based on a combination of sensory cues: visual, chemical and acoustic cues used by fish larvae to detect an island, a reef, a micro-habitat, a conspecific or some predators. These different cues are used at the different scales. David lecchini showed some contradictory results about the distance of transmission and of detection of chemical and acoustic cues in coral reefs. Lastly, as global and regional environmental changes have stressed coral reefs to such an extent that they are either destroyed or in decline, David Lecchini studied the effects of both anthropogenic and environmental stressors on information perception and response capacities in coral reef fish larvae. If polluted seawater disrupts the larval abilities to find the suitable reef habitat, fish larvae may spend more time in the planktonic environment, resulting in increased energetic costs and predation risk, and consequently a lower larval settlement.  David Lecchini put thus the hypothesis that as the stability of fish communities is dependent, in part, on the stability of social interactions, the disruption of “larvae-habitat relationships” can have major consequences for larval settlement into adult population with further repercussions for the ecosystem as a whole. Overall, larval settlement of coral reef fish is  an excellent example of the complexity of interactions between an organism and its environment as without perceiving environmental cues, fish larva would have very little chance of selecting a suitable reef habitat. Moreover, understanding the relationship between reef state and settlement potential will allow management planning for the maintenance of coral reefs that are increasingly degraded.

Key words: perception of information; coral reef fish; larval settlement; habitat selection; coral reef degradation

Contact

Centre de Recherches Insulaires et Observatoire de l’Environnement (CRIOBE – www.criobe.pf)

Laboratoire d’Excellence CORAIL, USR 3278 CNRS – EPHE – UPVD

Associate Director of the Institute for Pacific Coral Reefs (IRCP – www.ircp.pf)

E-mail : david.lecchini@ephe.sorbonne.fr

Replenishing Tetiaroa’s lagoon through capture, culture and restocking of fish and crustacean post-larvae

The project aims to replenish the Tetiaroa lagoon by rearing and releasing fish and crustaceans caught at post-larval stage. The marine post-larvae will be caught using nets set up on the reef crest of Tetiaroa. The post-larvae will be kept in aquarium at Tetiaroa research center in cages or in the lagoon between 1 to 3 months according to species, and then released in the lagoon of Tetiaroa. Released fish and crustacean will be tagged by external tags or implantations of magnetic bars in the flesh of fish and crustaceans. This tagging will allow to estimate, several months after being released, the proportion of marine post-larvae raised involved the adult stock of fish and crustaceans at Tetiaroa. The replenishment of fish and crustacean will be conducted in the different parts of the Marine Protected Area at Tetiaroa. Overall, the implementation of this project is part of a responsible approach to management of the resource in the context of sustainable development on Tetiaroa and is part of The Tetiaroa Sustainable and Conservation Plan

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Ecological importance of brain lateralization in coral reef fish facing local stressors

Behavioral lateralization, which reflects brain asymmetry and specialization, is ecologically advantageous in a rapidly changing environment. The coral reef fish recruitment process involves an environmental transition, from the ocean to the reef, and is critical for population replenishment and sustainability. Here, Acanthurus triostegus larvae held in aquaria preferentially used the right eye to investigate a positive (conspecific) or negative (predator) visual stimulus. However, when held in in situ cages with predators, the larvae that previously preferred their left eye experienced the highest survival rates. This supports the “brain’s right hemisphere” theory: the right eye is used to categorize stimuli while the left eye is used to inspect novel items and execute rapid responses. Lastly, when exposed to a pesticide (chlorpyrifos) visual lateralization was lost. This study highlights how a reef pollutant could impact sustainability by altering the brain functionality of fish populations during a critical life-history transition, bringing a new understanding of the relationship between reef health and recruitment potential.

Acoustic indices provide information on the status of coral reefs: an example from Moorea Island in the South Pacific

Different marine habitats are characterised by different soundscapes. How or which differences may be representative of the habitat characteristics and/or community structure remains however to be explored. A growing project in passive acoustics is to find a way to use soundscapes to have information on the habitat and on its changes. In this study we have successfully tested the potential of two acoustic indices, i.e. the average sound pressure level and the acoustic complexity index based on the frequency spectrum. Inside and outside marine protected areas of Moorea Island (French Polynesia), sound pressure level was positively correlated with the characteristics of the substratum and acoustic complexity was positively correlated with fish diversity. It clearly shows soundscape can be used to evaluate the acoustic features of marine protected areas, which presented a significantly higher ambient sound pressure level and were more acoustically complex than non-protected areas. This study further emphasizes the importance of acoustics as a tool in the monitoring of marine environments and in the elaboration and management of future conservation plans.

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A first acoustical examination of the deeper environments adjacent to coral reefs

1. Coral reefs are severely threatened and assessment of their biodiversity appears to be one of the most needed yet difficult challenges encountered by ecologists and conservation biologists. Marine habitats are particularly difficult to access and visual surveys may target a limited number of species, and are limited in space and time. Acoustic monitoring has now been recognized as a valuable instrument for remote and non-intrusive monitoring complementing traditional visual census methods. However, acoustic surveys conducted on coral reefs have so far been restricted to relatively shallow depths (10-30m).
2. Monitoring deeper habitats in the vicinity of coral reefs is crucial for current conservation efforts. The description of their soundscapes may contribute to our understanding of the states of coral reef ecosystems. By recording and characterizing the acoustic features of four habitats, located at different depths (from 10 to 100m), this study aims to provide insight into variations in spectral characteristics and sound levels with depth, and to evaluate the vocal activity of these under explored habitats. The studies were carried out along three transect, from the barrier reef to the reef drop-off, on the north coast of Moorea Island, French Polynesia.
3. The analysis of the spectra showed differences between transects and habitats. Barrier reefs proved to be the noisiest habitat. Sound levels within other habitats decreased with their distance from the barrier reef and with increasing depth. However, when comparing similar habitats, their different spectral characteristics might not result only from the decrease with distance of sound from the barrier reef. The sandy plains along two of the transects displayed higher sound levels than similar habitats along the third transect, despite being further away from the barrier reef.
4. The present study is a first attempt to appraise the acoustic features of deeper habitats and gain information on their status. The results highlight the need for further studies to provide a more detailed acoustic description of the ecology of coral reef neighbouring environments and to follow their evolution in the face of global warming