Genetic Population Dynamics of the Critically Endangered Scalloped Hammerhead Shark (Sphyrna lewini) in the Eastern Tropical Pacific Ocean

The scalloped hammerhead shark, Sphyrna lewini, is a Critically Endangered, migratory species known for its tendency to form large aggregations of mostly adult females, especially in the heavily-fished Eastern Tropical Pacific (ETP) Ocean. This species forms iconic, visually spectacular, seasonal aggregations in the northern Galápagos Islands, and it is hypothesized based on telemetry studies that pregnant females from these aggregations migrate to mainland coastal nurseries for parturition. This study investigated the population genetic dynamics of the scalloped hammerhead across most of its coastal and offshore distribution (Mexico to Ecuador) in the ETP, focusing on young-of-the-year animals sampled from nursery areas and adults from the Galápagos aggregations. Using nuclear microsatellites and mitochondrial control region sequences as markers, we found little evidence of population structure among 12 ETP locations, suggesting that females are not philopatric to specific nurseries in this region, and that scalloped hammerheads in the ETP comprise a single genetic stock. However, the ETP sharks showed strong matrilineal differentiation from scalloped hammerheads in the Central Pacific. Despite the high level of fisheries exploitation in the ETP, the scalloped hammerheads here still possess greater genetic diversity compared to conspecifics from other parts of their global distribution. The adults in the Galápagos aggregations showed negligible relatedness, suggesting that kinship does not play a role in the formation of the repeated, annual groupings at this remote offshore location. The data from this study increase the global mitochondrial sequence dataset available for the scalloped hammerhead by almost a third; a phylogeographic analysis based on this largest mitochondrial sequence dataset available revealed that scalloped hammerheads globally 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 architecture 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

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Genetic population dynamics of endangered scalloped hammerhead sharks in the Eastern Tropical Pacific

The globally distributed, coastal-pelagic scalloped hammerhead shark (Sphyrna lewini) is widely exploited for its fins and meat. Drastic population declines have resulted in its categorization as Endangered on the IUCN Red-List and inclusion under the US Endangered Species Act. This species is famously recognized for its tendency to form seasonal aggregations of mostly adult females around seamounts and offshore islands in the Eastern Tropical Pacific (ETP), including the northern Galápagos Islands. Female scalloped hammerheads in the ETP are also known to utilize shallow mangrove bays along the Central and South American mainland for parturition. My study objectives are to determine: 1) if parent-offspring relationships exist between the adult sharks in the Galápagos and young-of-year (YOY) sharks sampled in coastal nursery areas of Panama, Ecuador, Costa Rica, Colombia and the Galápagos Islands, 2) genetic relatedness of YOY animals sampled in individual coastal nursery areas, and 3) population structure of YOY sharks to test for hypothesized female philopatry to coastal parturition areas. Results to date using five biparentally inherited nuclear microsatellite and maternally-inherited mitochondrial DNA sequence markers show little genetic population structure among mainland sampled sites in the ETP; however, significant pairwise genetic differences were found in the mitochondrial DNA between the Galápagos YOY and all mainland YOY samples. No parent-offspring relationships were found between the adult Galápagos aggregation (n=130 females) and YOY individuals (n=320) in the ETP. Analysis of the five biparentally-inherited markers found 33 full-sibling pairs and 1208 half-sibling pairs among the 5 nurseries sampled. The high percentage of half-siblings suggests a polyandrous mating system in scalloped hammerheads in the ETP, consistent with similar findings for this species in the Indian Ocean. Analysis with additional microsatellite markers is ongoing to examine the comparative genetic diversity of the iconic Galápagos aggregation animals to scalloped hammerheads from other regions

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-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\u27 global distribution. A phylogeographic analysis of available, globally sourced, mitochondrial control region sequence data (n = 1818 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