Genetic population dynamics of the critically endangered scalloped hammerhead shark (Sphyrna lewini) in the Eastern Tropical Pacific

The scalloped hammerhead shark, Sphyrna lewini, is a Critically
Endangered, migratory species known for its tendency to form iconic and
visually spectacular large aggregations. Herein, we investigated the
population genetic dynamics of the scalloped hammerhead across much of its
distribution in the Eastern Tropical Pacific (ETP), ranging from Costa
Rica to Ecuador, focusing on young‐of‐the‐year animals from putative
coastal nursery areas and adult females from seasonal aggregations that
form in the northern Galápagos Islands. Nuclear microsatellites and
partial mitochondrial control region sequences showed little evidence of
population structure, suggesting that scalloped hammerheads in this ETP
region comprise a single genetic stock. Galápagos aggregations of adults
were not comprised of related individuals, suggesting that kinship does
not play a role in the formation of the repeated, annual gatherings at
these remote offshore locations. Despite high levels of fisheries
exploitation of this species in the ETP, the adult scalloped hammerheads
here showed greater genetic diversity compared with adult conspecifics
from other parts of the species' global distribution. A
phylogeographic analysis of available, globally sourced, mitochondrial
control region sequence data (n = 1,818 sequences) revealed that scalloped
hammerheads comprise three distinct matrilines corresponding to the three
major world ocean basins, highlighting the need for conservation of these
evolutionarily unique lineages. This study provides the first view of the
genetic properties of a scalloped hammerhead aggregation, and the largest
sample size‐based investigation of population structure and phylogeography
of this species in the ETP to date.