Cécile Berthe

Consistency in the supply of larval fishes among coral reefs in French Polynesia

For marine fishes with a bipartite life cycle, pelagic larval dispersal can shape the distribution, connectivity, composition and resilience of adult populations. Numerous studies of larval dispersal, and associated settlement and recruitment processes, have examined the relationship between population connectivity and oceanographic features. However, relatively little is known about spatial and temporal variation in the abundance of larvae settling among different reefs and the extent to which the species assemblage of larvae settling at one location is reflective of the assemblage in neighbouring areas. Here, using crest nets, which provide a non-selective measure of the total abundance and assemblage of larvae settling to a reef (i.e. larval supply), we collected larval coral reef fishes at five locations surrounding two spatially disparate French Polynesian islands: Moorea and Nengo-Nengo. Overall, larval settlement patterns were correlated with the lunar cycle, with larval abundance peaking during the new moon. Although there were some spatial differences in larval supply among the five monitored sites, settlement patterns were largely consistent, even at the species level, irrespective of factors such as coastline orientation or distance between sites. This study provides further insights into the mechanisms driving patterns of dispersal and settlement of larval fishes over large spatial scales.

Exposure to agricultural pesticide impairs visual lateralization in a larval coral reef fish

Lateralization, i.e. the preferential use of one side of the body, may convey fitness benefits for organisms within rapidly-changing environments, by optimizing separate and parallel processing of different information between the two brain hemispheres. In coral reef-fishes, the movement of larvae from planktonic to reef environments (recruitment) represents a major life-history transition. This transition requires larvae to rapidly identify and respond to sensory cues to select a suitable habitat that facilitates survival and growth. This ‘recruitment’ is critical for population persistence and resilience. In aquarium experiments, larval Acanthurus triostegus preferentially used their right-eye to investigate a variety of visual stimuli. Despite this, when held in in situ cages with predators, those larvae that previously favored their left-eye exhibited higher survival. These results support the “brain’s right-hemisphere” theory, which predicts that the right-eye (i.e. left-hemisphere) is used to categorize stimuli while the left-eye (i.e. right-hemisphere) is used to inspect novel items and initiate rapid behavioral-responses. While these experiments confirm that being highly lateralized is ecologically advantageous, exposure to chlorpyrifos, a pesticide often inadvertently added to coral-reef waters, impaired visual-lateralization. This suggests that chemical pollutants could impair the brain function of larval fishes during a critical life-history transition, potentially impacting recruitment success.

Snapshot recordings provide a first description of the acoustic signatures of deeper habitats adjacent to coral reefs of Moorea

Acoustic recording has been recognized as a valuable tool for non-intrusive monitoring of the marine environment, complementing traditional visual surveys. Acoustic surveys conducted on coral ecosystems have so far been restricted to barrier reefs and to shallow depths (10–30 m). Since they may provide refuge for coral reef organisms, the monitoring of outer reef slopes and describing of the soundscapes of deeper environment could provide insights into the characteristics of different biotopes of coral ecosystems. In this study, the acoustic features of four different habitats, with different topographies and substrates, located at different depths from 10 to 100 m, were recorded during day-time on the outer reef slope of the north Coast of Moorea Island (French Polynesia). Barrier reefs appeared to be the noisiest habitats whereas the average sound levels at other habitats decreased with their distance from the reef and with increasing depth. However, sound levels were higher than expected by propagation models, supporting that these habitats possess their own sound sources. While reef sounds are known to attract marine larvae, sounds from deeper habitats may then also have a non-negligible attractive potential, coming into play before the reef itself.

Influence of boat noises on escape behaviour of white-spotted eagle ray Aetobatus ocellatus at Moorea Island (French Polynesia)

The present study tested different sounds that could disturb eagle rays (Aetobatus ocellatus) during their foraging activities at Moorea, French Polynesia. Results showed that artificial white sound and single-frequency tones (40 Hz, 600 Hz or 1 kHz) did not have an effect on rays (at least 90% of rays continued to forage over sand), while playbacks of boat motor sound significantly disturbed rays during foraging activity (60% exhibited an escape behaviour). Overall, our study highlighted the negative effect of boat noises on the foraging activity of eagle rays. These noises produced by boat traffic could, however, have some positive effects for marine aquaculture if they could be used as a deterrent to repel the eagle rays, main predators of the pearl oysters.

Brain lateralization involved in visual recognition of conspecifics in coral reef fish at recruitment

In vertebrates, brain functional asymmetries are widespread and increase brain performance. Some species of fishes are known to have brain asymmetries; however, little information is available on brain lateralization in coral reef fishes and the impact this could have during the recruitment phase. In this study, soldierfish, Myripristis pralinia, at the larval and juvenile stage recognized conspecifics through visual cues. Larvae with the ablation of either the right or left telencephalic hemisphere lost the attraction towards conspecific cues. In contrast, juveniles with the ablation of the right (but not left) telencephalic hemisphere still displayed a preference towards conspecific visual cues. These results suggest the left telencephalic hemisphere is responsible for the lateralization process used in the visual recognition of coral reef fish juveniles. The determinism of lateralized perception of conspecifics during fish ontogeny may be a consequence of genetic factors, linked with the metamorphosis processes and/or environmental factors such as predation at recruitment.

Boat noise prevents soundscape-based habitat selection by coral planulae

Understanding the relationship between coral reef condition and recruitment potential is vital for the development of effective management strategies that maintain coral cover and biodiversity. Coral larvae (planulae) have been shown to use certain sensory cues to orient towards settlement habitats (e.g. the odour of live crustose coralline algae - CCA). However, the influence of auditory cues on coral recruitment, and any effect of anthropogenic noise on this process, remain largely unknown. Here, we determined the effect of protected reef (MPA), exploited reef (non-MPA) soundscapes, and a source of anthropogenic noise (boat) on the habitat preference for live CCA over dead CCA in the planula of two common Indo-Pacific coral species (Pocillopora damicornis and Acropora cytherea). Soundscapes from protected reefs significantly increased the phonotaxis of planulae of both species towards live CCA, especially when compared to boat noise. Boat noise playback prevented this preferential selection of live CCA as a settlement substrate. These results suggest that sources of anthropogenic noise such as motor boat can disrupt the settlement behaviours of coral planulae. Acoustic cues should be accounted for when developing management strategies aimed at maximizing larval recruitment to coral reefs.

Taxonomic validation of Encheliophis chardewalli with description of calling abilities

Encheliophis chardewalli was described from a single cleared and stained specimen. Twelve years later, additional specimens were found in the lagoon of Moorea (French Polynesia) in association with their host, the sea cucumber Actinopyga mauritiana. These fish were used to consolidate the species diagnosis, to validate species status and to record sound production. This species is remarkable because of its ability to penetrate inside the cloaca of sea cucumbers having anal teeth and the fact this species is largely unknown despite it lives in lagoons in 1m depth. Encheliophis chardewalli produced three sound types: long regular calls made of trains of numerous pulses, short irregular calls characterized by a constant lowering of its pulse period and short regular call (or knock) made of 3 to 6 pulses. Comparison with other sympatric Carapini supports a large and distinct repertoire. Morphological characteristics could be the result of reduced body size allowing to penetrate inside a new host, thus avoiding competition and conflict with other larger sympatric Carapini species.

Unprecedented massive reproduction aggregation of Gymnodoris ceylonica

Opisthobranchs are hermaphrodite benthic animals that occur in seas worldwide. Mostly seen alone or in small groups, mass aggregations (from 30 up to hundred individuals) of nudibranchs have been reported for feeding or reproduction purposes (Claverie and Kamenos 2008). Gymnodoris ceylonica occurs in the Indo-Pacific area and is known to migrate by dozens to reproduce (Huang 2010). Here we report an exceptional massive aggregation of G. ceylonica occurring in the lagoon of Temae, Moorea Island, French Polynesia (Fig. 1). More than a thousand individuals were documented to reproduce from March 2 to 4, 2018, starting one day after the full moon, over a sand area of 500 m (east point: 17 29¢53.04†S 149 45¢23.88†O/west point: 17 29¢54.25†S 149 45¢31.31†O) and at 2 m depth. Individuals were gathering on or under rocks associated with massive egg string deposition that were mostly loosely made (Fig. 1). G. ceylonica also aggregated on a large surface ‘‘aggregation field’’. Coordinated movements of nudibranchs were reported over the sand area with dozens of individuals migrating in the same direction (Fig. 2). This direction changed over day time: G. ceylonica were migrating to the barrier reef in the morning and to the shoreline in the afternoon. Migrating individuals were either full or empty of eggs irrespective of the direction (Fig. 2). With their nocturnal and cryptic activity, nudibranch biology still remains unclear and such events may help to better understand the biology of these species.

Chafing behavior on a patch of sandy bottom by ocellated eagle ray (Aetobatus ocellatus)

Hosting parasites is costly, and many coral reef fish species are known to clean themselves through chafing behaviors and/or using cleaning stations (e.g., Losey et al. 1999; O’Shea et al. 2010). In elasmobranchs, Papastamatiou et al. (2007) showed that sharks (Carcharhinus amblyrhynchos) use cleaning stations and are used as cleaners by smaller reef fish (rainbow runner—Elagatis bipinnulata). O’Shea et al. (2010) showed that manta rays use the cleaning stations in the Great Barrier Reef and in the Coral Sea. Although eagle rays are known to host ectoparasites (Marie and Justine 2006), how they remove them is not known as they have not been reported as regular users of cleaning stations. Here, we report two observations of ocellated eagle rays (Aetobatus ocellatus) (Berthe et al. 2016) chafing on patches of sandy bottom in French Polynesia. The first observation was made on Moorea Island (17°29′ 28.39′′S 149°54′41.70′′W) in May 2011 (Suppl data 1). A female was swimming in a channel, and then started to rub its body against the sand, chafing each side (presumably fins and/or gills), one after the other, over the course of about 30 seconds. The second observation was made on Fakarava Atoll (16°31′8.27′′S 145°27′ 37.13′′W) in June 2014 (Suppl data 2). Two female eagle rays were swimming together over a patch of sandy bottom at 15–20 m depth. One of them started chafing on the sand, first the left side of its body (presumably fins and/or gills—Fig. 1a), then the right (Fig. 1b), and left again (Fig. 1c) over the course of about 20 seconds. Overall, we propose that chafing behaviors in eagle rays may function to fulfil their maintenance requirements either as a primary strategy or in addition to using cleaning stations. Ritter showed that Caribbean reef sharks (Carcharhinus perezi) and blacktip sharks (Carcharhinus limbatus) chafed against sand ripples with either a parallel or perpendicular swim direction, rather than a transverse swim direction. Communicated by R. Serrao Santos