Defense Date

8-9-2021

Document Type

Thesis

Degree Type

Master of Science

Degree Name

Biological Sciences

First Advisor

Mahmood Shivji, Ph.D.

Second Advisor

Andrea Bernard, Ph.D.

Third Advisor

Kevin Feldheim, Ph.D.

Keywords

elasmobranch, shark, philopatry, microsatellites, mitochondrial DNA, conservation, Eastern Pacific, Galápagos

Abstract

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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