Genetic population structure of deep-water prawns Haliporoides triarthrus and langoustines Metanephrops mozambicus in the South West Indian Ocean : use of mitochondrial DNA to investigate metapopulation structure.

Abstract

Deep-water prawns Haliporoides triarthrus and langoustines Metanephrops mozambicus are endemic to the South West Indian Ocean (SWIO) region and make up the largest proportion of deep-water crustacean trawl catches in Mozambique and South Africa. Despite their economic importance to these fisheries, little is known about their distribution, biology and genetic population structure. The metapopulation genetic variation of H. triarthrus and M. mozambicus was assessed from 220 specimens per species collected from three sites in Mozambique (Bazaruto A, Boa Paz and Inhaca), two sites in western Madagascar (Morombe and Tulear) and one site in eastern South Africa (Durban). Two fragments of the mitochondrial region were amplified using universal primers ribosomal 16S subunit (16S) and mitochondrial cytochrome oxidase subunit I (COI). From H. triarthrus, fragments of 569 base pair (bp) (16S) and 1300 bp (COI) were amplified. A total of 207 sequences (16S) and 151 sequences (COI) were recovered, and 69 and 78 haplotypes identified, respectively. Metanephrops mozambicus 16S and COI genes produced similar fragment lengths, and 112 (16S) and 127 haplotypes (COI) were recovered. Both species demonstrated high genetic diversity and significant population differentiation in the SWIO region. Two sister-species (or subspecies) of H. triarthrus were identified, one occurring along the African continental shelf and the other off western Madagascar. Furthermore, individual populations making up each lineage were genetically structured, as indicated by the absence of shared haplotypes, and should be recognized as demographically distinct subspecies. Both species have undergone recent population expansions, likely since the late Pleistocene. The large anti-cyclonic and cyclonic eddies prevalent in the Mozambique Channel, and the boundary area between these eddies and upper Agulhas Current are likely factors driving larval retention or return process, thus giving rise to the observed genetically structured populations. The findings from this study are unique for the SWIO region, and may lead to a paradigm shift in the way that deep-water crustacean stocks are perceived by fisheries managers – instead of single shared stocks, they comprise of many isolated ones, in spite of the dispersal potential of larvae in strong ocean current regimes. Thus stocks should be managed as small independent units.