Uncertainty in empirical estimates of marine larval connectivity

International audienc

Giant clams of New Caledonia : new species and spatial scales of connectivity among Tridacna maxima and Hippopus hippopus

Malgré leur rôle fonctionnel au sein des écosystèmes coralliens, les populations de bénitiers (Tridacninae) sont en déclin à travers le monde en raison de leur surexploitation. En Nouvelle-Calédonie, 1 à 9 tonnes sont prélevées par an. Dans ce contexte, cette thèse a pour principaux objectifs de fournir des indications quant aux connectivités démographiques et évolutives des populations de deux espèces de bénitiers en Nouvelle-Calédonie, Tridacna maxima et Hippopus hippopus afin d'estimer les échelles spatiales de dispersion larvaire, comprendre leur dynamique et proposer des mesures de conservation adaptées. La connectivité des populations a été estimée via l'analyse de marqueurs génétiques microsatellites spécifiquement développés pour ces deux espèces. L'échantillonnage a permis l'identification de deux nouvelles espèces en Nouvelle-Calédonie. A l'échelle d'un récif et de quelques kilomètres, les tests de parenté montrent des taux d'auto-recrutement variables, jusqu'à 8% pour T. maxima dans le lagon sud-ouest et 29% pour H. hippopus dans le lagon nord-est. Les échelles spatiales de dispersion larvaire varient selon les espèces, jusqu'à au moins 35km pour H. hippopus. A l'échelle du territoire, les populations de bénitiers sont peu structurées, indiquant une connectivité évolutive certaine. Néanmoins, des différences significatives ont été observées entre la Grande Terre et Chesterfield, les Iles Loyautés et les atolls d'Entrecasteaux pour H. hippopus et entre la côte ouest et les Îles Loyautés pour T. maxima. Ces résultats fournissent d'importants éléments de réponse relatifs aux échelles spatiales de dispersion des bénitiers en Nouvelle-Calédonie.Despite their functional role in coral reef ecosystems, giant clams (Tridacninae) are in decline worldwide due to their overexploitation. In New Caledonia, 1 to 9 tons are harvested per year. In this context, the main objectives of this thesis are to provide information about the demographic and evolutionary population’s connectivity for two species of giant clams in New Caledonia, Tridacna maxima and Hippopus hippopus, in order to estimate the spatial scales of larval dispersal, understand their population dynamics and propose appropriate conservation measures. The connectivity of populations was estimated by analyzing microsatellite genetic markers specifically developed for these two species. Sampling allowed the identification of two new species in New Caledonia. At reef scale and up to a few kilometers, parentage analysis showed that giant clam populations have varying self-recruitment rates, up to 8% for T. maxima in southwest lagoon and 29% for H. hippopus in northeastern lagoon. The spatial scales of larval dispersal varied for the two species, up to at least 35km for H. hippopus. At the scale of the territory, giant clam populations showed a weak genetic structure, highlighting evolutionary connectivity among sampled sites. However, significant differences were observed between the Mainland and Chesterfield, Loyalty Islands and Entrecasteaux for H. hippopus and between the west coast and the Loyalty Islands for T. maxima. These results provide important answers related to the spatial scales of larval dispersal in giant clams in New Caledonia

First record of the Devil Clam, Tridacna mbalavuana Ladd 1934, in New Caledonia

International audienc

Isolation and characterization of fifteen microsatellite loci for the giant clam Tridacna maxima

Fifteen polymorphic microsatellite markers were developed for Tridacna maxima in order to assess self-recruitment and larval dispersal within and among MPAs in New Caledonia and provide baseline data for conservation management of this species. Number of alleles varied from 5 to 25 per locus, observed and expected heterozygosities ranged from 0.171 to 0.860 and 0.591 to 0.935 respectively. Significant deviations from HWE were detected in eight loci. Cross-amplifications were tested in five other species of Tridacnidae (T. crocea, T. squamosa, T. derasa, T. noae and Hippopus hippopus)

Isolation and characterization of fifteen microsatellite loci for the giant clam Hippopus hippopus (family Tridacnidae)

Fifteen polymorphic microsatellite markers were developed for Hippopus hippopus in order to assess the effectiveness of population replenishment within marine protected areas in New Caledonia. Number of alleles varied from 2 to 11 per locus, observed and expected heterozygosities ranged from 0.300 to 0.866 and 0.495 to 0.858 respectively. Significant deviations from HWE were detected in two loci. Cross-amplifications were tested in four other species of Tridacnidae

Chapitre 12. Les bénitiers, joyaux des récifs néo-calédoniens

Le manteau des bénitiers héberge des microalgues qui leur donnent ces couleurs chatoyantes. Tridacna maxima © IRD/S. Andréfouët Des « coquillages » imposants mais encore mal connus Les bénitiers sont des mollusques marins de la classe des bivalves – tout comme les huîtres, moules, palourdes etc. – habitant les eaux côtières chaudes de la zone Indo-Pacifique. On les trouve préférentiellement dans les faibles profondeurs des lagons et sur les pentes externes des récifs-barrières – jusqu’à 30 m ..

Evidence of early chemotaxis contributing to active habitat selection by the sessile giant clam Tridacna maxima

Finding a suitable habitat that will maximize the likelihood of settlement is one of the greatest challenges faced by coral reef species. This is the case for giant clams, which exhibit high sensory abilities coupled with a fast decline in locomotion after settlement that may curtail subsequent searches for an optimal environment. The goal of this study was to evaluate the ability of early giant clams to use and discriminate contrasted reef signals. We investigated the chemosensory ability and subsequent locomotion of the widely-distributed giant clam Tridacna maxima, ranging from five-day-presettlement larvae to six-month-old juveniles. Choice experiments were conducted to investigate the response of larvae and juveniles to chemical cues that were potentially associated with conspecifics, and with "favorable" and "unfavorable" reef habitats. Our results suggested that T. maxima may rely on both conspecific and habitat cues to detect suitable reef areas. Both larvae and juveniles showed a strong preference for and actively moved towards conspecific effluents. Two- to six-month-old juveniles were capable of efficiently discriminating effluents from "favorable" vs. "unfavorable" reefs, even without the presence of conspecifics, offering the first direct support for active habitat selection based on chemotaxis in giant clams. Our results expand the range of giant clam species known to exhibit post-settlement locomotion and demonstrate that juveniles (up to six months in the present study) have unexpectedly retained their locomotive ability. Despite a marked reduction in displacement occurring between four and six months, the ability of juveniles to move away from unsuitable areas using effective chemotaxis has positive implications for conservation-oriented initiatives, such as clam restoration projects

Phylogeography of Noah’s giant clam

International audienceNoah’s giant clam (Tridacna noae), recently resurrected from synonymy with T. maxima, occurs from Christmas Island to the Northern Line Islands and from the Ryukyu Islands to New Caledonia. We used mitochondrial and microsatellite markers to investigate the phylogeographic structure and demographic history of T. noae over most of its geographical range. Results from the two types of markers reveal a consistent population structure, partitioning T. noae into three distinct lineages: (1) eastern half of the Indo-Malay archipelago and Western Australia, (2) Melanesia and Micronesia, and (3) Central Polynesia. Demographic expansion initiated between 300,000 and 400,000 years ago, as was detected for each haplogroup. This pattern, which is congruent with other co-occurring Tridacna species, indicates a shared evolutionary history with expansion from past refuges following late-Pleistocene sea-level changes

Distribution of Noah's giant clam, Tridacna noae

Previously confused with the small giant clam Tridacna maxima, the recently resurrected Noah's giant clam, Tridacna noae, has been reported from the Taiwanese and the Ryukyu archipelagoes. Our recent underwater observations now extend its distribution to Dongsha (northern South China Sea), Bunaken (Sulawesi Sea), Madang and Kavieng (Bismarck Sea), the Alor archipelago (Sawu Sea), Kosrae (Caroline Islands), New Caledonia, the Loyalty Islands and Vanuatu (Coral Sea), Viti-Levu (Fiji), Wallis Island, and Kiritimati (Northern Line Islands). Published mitochondrial DNA sequences retrieved from open-access databases also indicate its presence in eastern Negros (Philippines), in the Molucca Sea, at Ningaloo Reef (Western Australia), and in the Solomon Islands. Noah's giant clam is thus a widely distributed Indo-West Pacific species. Wherever research has been done on small giant clams throughout T. noae's range, the inadvertent confusion of T. noae with T. maxima might have led to overestimating actual T. maxima densities and to errors in estimating demographic parameters

Considering reefscape configuration and composition in biophysical models advance seascape genetics.

Previous seascape genetics studies have emphasized the role of ocean currents and geographic distances to explain the genetic structure of marine species, but the role of benthic habitat has been more rarely considered. Here, we compared the population genetic structure observed in West Pacific giant clam populations against model simulations that accounted habitat composition and configuration, geographical distance, and oceanic currents. Dispersal determined by geographical distance provided a modelled genetic structure in better agreement with the observations than dispersal by oceanic currents, possibly due to insufficient spatial resolution of available oceanographic and coastal circulation models. Considering both habitat composition and configuration significantly improved the match between simulated and observed genetic structures. This study emphasizes the importance of a reefscape genetics approach to population ecology, evolution and conservation in the sea