Giant Clam connectivity in New-Caledonia
All underwater pictures © S. Andréfouët
In New Caledonia, the current state of the giant clam resource seems to indicate a decrease in the densities of giant clams due to increases in fishing pressure linked to the demographic development of the territory. Without sustainable management of this resource, this observation is likely to worsen.
With a final goal of providing scientific data to improve conservation actions and management strategies for the giant clams, the BeN-Co project aimed to
- obtain genetic data on the spatial scale of New Caledonia for two species of giant clams, Tridacna maxima and Hippopus hippopus, in order to investigate the genetic structure of these populations and possibly identify genetically differentiated giant clam stocks,
- assess the role of habitat fragmentation in the connectivity of T. maxima populations, used as a model, in the New Caledonia & Vanuatu area,
- from the results obtained for T. maxima, deduce innovative conservation approaches for giant clam populations, and
- molecularly determine the species of giant clams found on market stalls in order to identify the main species of giant clams exploited in New Caledonia.
The BeN-Co project was partially funded by the ZONECO program, New Caledonia, the LabEx CORAIL through the related project TriMax, the WWF through the related project Hippopus, and the ANR through the related project COMPO.
The sampling (underwater biopsies) was carried out as follows:
– Mission on the Golden Shadow of the Living Ocean Foundation from October 27 to November 26, 2013 (sampling: Ile des Pins, Prony, Récif Cook and Récifs d’Entrecasteaux);
– « COBELO » campaign on the R/V ALIS (IRD), from November 08 to 17 of 2013;
– « BIBELOT » campaign on the R/V ALIS (IRD), from February 14 to 28 of 2014;
– “North” mission with IRD’s DIODON boat and local fishermen’s boats, from 03 to 06 March 2014;
– Daily fieldtrips in the South Lagoon of Nouméa and in Yaté, depends in part on the “COMPO” project.
Additional samples were collected in Chesterfield by Jonh Butscher (IRD, LOCEAN) and Efate (Vanuatu) by Pascal Dumas (IRD, ENTROPIE).
Genetic structure and population connectivity of Giant clams in New Caledonia
Our results show that the populations of giant clams T. maxima and H. hippopus are not genetically homogeneous throughout New Caledonia, although the levels of genetic differentiation are generally low. The greatest differences were observed for H. hippopus between Surprise atoll (Entrecasteau) and the rest of the populations sampled in New Caledonia, and to a lesser extent, between the Loyalty Islands and Grande-Terre.
For T. maxima, our markers have demonstrated their power by revealing a strong genetic difference between Vanuatu (Efate) and New Caledonia. For this species, although the territory-wide population cannot be considered homogeneous, slight genetic differentiations have been observed, with a more pronounced difference between Chesterfield and the rest of the territory.
For the species T. maxima, our results show that this observed genetic structure is better explained by a model of isolation by distance when all of the habitat plots and the abundance of giant clams within the plots are integrated in the model (van Wynsberge et al. 2017). These results suggest that, when combined with habitat fragmentation and the distribution of the resource, a simple dispersion model based on isolation by geographical distance can constitute a first proxy for the dispersion nucleus in order to identify the major trends in the structuring of populations.
Exploited giant clam species in New Caledonia
The molecular identification of the giant clam species found at the Nouméa city market stalls confirmed that the species most threatened by fishing in New Caledonia are H. hippopus (84% of the individuals molecularly identified) and to a lesser extent T. derasa (8.6%). In a smaller proportion, we also found T. squamosa (4.3%), and more surprisingly T. maxima (2.8%), yet more rarely fished because of their shell embedded in the reef.
Finally, during sampling for the project, we were able to observe for the first time two species of giant clams which were not referenced in New Caledonia, T. mbalavuana (Tiavouane and Fauvelot 2017) and T. noae (Borsa et al. 2015, Fauvelot et al. 2019), which raises the number of giant clam species present in New Caledonia from 5 to 7 species.
Related references
Fauvelot C, Andréfouët S, Grulois D, Tiavouane J, Wabnitz CCC, Magalon H, Borsa P (2019) Phylogeography of Noah’s giant clam. Marine Biodiversity 49:521–526 https://link.springer.com/article/10.1007/s12526-017-0794-0
Fauvelot C, Borsa P, Andréfouët S, Tiavouane J, Van Wynsberge S, Dumas P (2018) Les bénitiers, joyaux des récifs néo-calédoniens. In : Payri Claude (ed.), Moatti Jean-Paul (pref.). Nouvelle-Calédonie : archipel de corail.Marseille (FRA) ; Nouméa : IRD ; Solaris, 2018, p. 95-100. ISBN 978-2-7099-2632-4
Fauvelot C, Dumas P, Tiavouane J (2018) Les bénitiers, une ressource à préserver. In : Payri Claude (ed.), Moatti Jean-Paul (pref.). Nouvelle-Calédonie : archipel de corail.
Marseille (FRA) ; Nouméa : IRD ; Solaris, 2018, p. 221-222. ISBN 978-2-7099-2632-4
Van Wynsberge S, Andréfouët S, Gaertner-Mazouni N, Tiavouane J, Grulois D, Lefèvre J, Pinski ML, Fauvelot C (2017) Considering reefscape configuration and composition in biophysical models advance seascape genetics PLOS ONE 12:e0178239 https://doi.org/10.1371/journal.pone.0178239
Neo ML, CCC Wabnitz, RD Braley, GA Heslinga, Fauvelot C, S Van Wynsberge, S Andréfouët, C Waters, AS-H Tan, ED Gomez, MJ Costello & PA Todd (2017) Giant clams (Bivalvia: Cardiidae: Tridacninae): A comprehensive update of species and their distribution, current threats and conservation status. Oceanography and Marine Biology: An Annual Review, Volume 55, 87-388. https://www.taylorfrancis.com/books/e/9781315272276/chapters/10.1201/b21944-5
Tiavouane J, Fauvelot C (2017) First record of the Devil Clam, Tridacna mbalavuana Ladd 1934, in New Caledonia. Marine Biodiversity 47:781–782 https://doi.org/10.1007/s12526-016-0506-1
Borsa P, Fauvelot C, Andrefouët S, Chai T.T., Kubo H., Liu L.L (2015) On the validity of Noah’s giant clam Tridacna noae (Röding, 1798) and its synonymy with Ningaloo giant clam Tridacna ningaloo Penny & Willan, 2014.Raffles Bulletin of Zoology 63 :484-489. https://archive.org/details/raffles-bulletin-zoology-63-484-489
Borsa P, Fauvelot C, Tiavouane J, Grulois D, Wabnitz C, Abdon Naguit M.R, Andrefouet S (2015) Distribution of Noah’s giant clam, Tridacna noae. Marine Biodiversity 45: 339-344 http://doi-org-443.webvpn.fjmu.edu.cn/10.1007/s12526-014-0265-9
Grulois D, Tiavouane J, Dumas P, Fauvelot C (2015) Isolation and characterization of fifteen microsatellite loci for the giant clam Tridacna maxima. Conservation Genetics Resources 7: 73-75 https://doi.org/10.1007/s12686-014-0290-9
Tiavouane J, Jacob T, Dumas P, Fauvelot C (2014) Isolation and characterization of fifteen microsatellite loci for the giant clam Hippopus hippopus (family Tridacnidae). Conservation Genetics Resources 6(3): 735-737 https://doi.org/10.1007/s12686-014-0203-y
Wabnitz CCC, Fauvelot C (2014) Tridacna noae is back. SPC Fisheries Newsletter #145 – September–December 2014 link
Cécile FAUVELOT 