High clonal propagation and low population connectivity in the holothurian Stichopus chloronotus from the Indo-Pacific
Sea cucumber populations are declining worldwide through overfishing. This is also the case for the greenfish Stichopus chloronotus in spite of its moderate commercial value. We studied asexual propagation of this fissiparous species at different geographic scales as well as the genetic population structure within and among two marine provinces: the Western Indian Ocean and the Tropical Southwestern Pacific. A total of 18 populations (n = 809 individuals) were sampled in both provinces encompassing three ecoregions (New Caledonia, western and northern Madagascar, Mascarene Islands) and genotyped using nine microsatellite loci, allowing the identification of repeated multi-locus genotypes. Populations presented low clonal richness with few clones and a high number of individuals per clone. No clones were shared among different islands. Within islands, members of the same clone could be found 3–15 km apart, indicating that asexual reproduction allows population maintenance at reef scales. Assignment tests and individual-based networks identified two isolated New Caledonian populations, while a third population was less differentiated to those from the Western Indian Ocean than to the two former. Regional ocean currents could explain these patterns. Within the Western Indian Ocean, differentiation indices were always high, indicating restricted gene flow among populations. Possible overfishing of S. chloronotus stock is evidenced by very low sample sizes (and in some cases, the absence) in spite of high sampling efforts [all around New Caledonia, Chesterfield/Bampton/Bellona Plateau (Coral Sea), north-east of Madagascar, Scattered Islands (Mozambique Channel)].
Sampling in New Caledonia (HM) was carried out during COBELO (http://dx.doi.org/10.17600/14003700), BIBELOT (http://dx.doi.org/10.17600/13100100), and CHEST (http://dx.doi.org/10.17600/15004500) and POST-BLANCO oceanographic campaigns on board of RV Alis (IRD), and in the north-east and north-west of Madagascar during MAD (http://dx.doi.org/10.17600/16004700) oceanographic campaign on board of RV Antea (IRD). We gratefully thank Marion Verhiest and Grégory Lasne for sampling in Noumea (NCA2) and Claude Payri for sampling in Surprise Island (NCA3). Sampling was carried out in the south of Madagascar (HM) in collaboration with the Institut Halieutique des Sciences Marines (IHSM; Tulear), in the north of Madagascar (HM) in collaboration with the Centre National de la Recherche Océanographique (CNRO; Nosy Be) and in Rodrigues Island (HM) with the collaboration of the Rodrigues Regional Assembly and the South-East Marine Protected Area (SEMPA) supported by project Biodiversity (POCT FEDER fund); in Scattered Island (HM) by program BIORECIE (financial supports from INEE, INSU, IRD, AAMP, FRB, TAAF, and the foundation Veolia Environnement); in Mayotte (HM) by program SIREME (FED). We acknowledge the Plateforme Gentyane of the Institut National de la Recherche Agronomique (INRA, Clermont-Ferrand, France) for guidance and technical support. Agathe Pirog was supported by a doctoral fellowship from the region Reunion. Anne Latreille was supported by a French ATER position from Reunion Island University. We also thank two anonymous reviewers for their constructive comments.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
We declare that all applicable international, national and/or institutional guidelines for sampling, care and experimental use of organisms for the study have been followed and all necessary approvals have been obtained: New Caledonia: authorization n° 2432-2012/ARR/DENV, n° 2660-2013/ARR/DENV, n° 60912-25-28-2012/JJC, n° 60455-15-25/JJC, n° 6161-37/PR. Reunion Island: authorization n° 20-2015/DEAL/SEB/UBIO. Madagascar: authorization n° 16/1040-AE/SG/DAJC/SAG/NAV/FRANCE. Rodrigues Island: authorization n° MU 140897/Regional Assembly. Mayotte: authorization n° 2016-31/DMSOI. Juan de Nova: authorization n° 2013-66/TAAF.
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