Résumé
Designing appropriate management plans requires knowledge of both the
dispersal ability and what has shaped the current distribution of the
species under consideration. Here we investigated the evolutionary history
of the endangered gray reef shark (Carcharhinus amblyrhynchos) across its
range by sequencing thousands of RAD-seq loci in 173 individuals in the
Indo-Pacific (IP). We first bring evidence of the occurrence of a range
expansion (RE) originating close to the Indo-Australian Archipelago (IAA)
where two stepping-stone waves (east and westward) colonized almost the
entire IP. Coalescent modeling additionally highlighted a
homogenous connectivity (Nm~10 per generation) throughout the
range, and an isolation-by-distance model suggested the absence
of barriers to dispersal despite the affinity of C.
amblyrhynchos to coral reefs. This coincides with
long-distance swims previously recorded, suggesting that the strong
genetic structure at the IP scale (FST ~ 0.56 between its ends)
is the consequence of its broad current distribution and organization in a
large number of demes. Our results strongly suggest that management plans
for the gray reef shark should be designed on a range-wide rather than a
local scale due to its continuous genetic structure. We
further contrasted these results with those obtained previously
for the sympatric but strictly lagoon-associated Carcharhinus
melanopterus, known for its restricted dispersal ability. C.
melanopterus exhibits a similar RE dynamic but is characterized by
stronger genetic structure and a non-homogeneous connectivity largely
dependent on local coral reefs availability. This sheds new light on shark
evolution, emphasizing the roles of IAA as a source of biodiversity and of
life history traits in shaping the extent of genetic structure and
diversity.