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Abstract
Due to continued overexploitation and anthropogenic change, hammerhead sharks (Carcharhiniformes: Sphyrnidae) have experienced drastic declines over most of their geographic range. To date, there are ten described species of hammerhead sharks distributed in two genera (Sphyrna and Eusphyra). It has been assumed that the hammerheads represent a monophyletic group and that the head shape evolved gradually, starting from a carcharhinid ancestor. Interestingly there are four species of small hammerhead sharks (< 150cm >TL) that are only distributed in the American continent and have been poorly studied and assessed in fisheries (the smalleye hammerhead S. tudes, the scoophead shark S. media, and the scalloped bonnethead shark - S. corona, and the bonnethead shark - S. tiburo). These rare small sharks face significant threats from gillnetting, overfishing, and misidentification in fisheries.
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The bonnethead shark, Sphyrna tiburo, is a small coastal shark distributed in the Western Atlantic from North Carolina (U.S) to southern Brazil, and in the eastern Pacific Ocean from California (U.S) to southern Brazil. Previous genetic studies based on mitochondrial markers revealed that bonnethead sharks comprise a species complex with at least two lineages in the Northwestern Atlantic and the Caribbean (S. tiburo and S. aff. tiburo, respectively), and one in the eastern Pacific (S. tiburo vespertina). Despite several efforts to provide a taxonomical placement their phylogeny and key aspects of their biology remain unresolved. Bonnethead sharks are assessed as Endangered by the International Union for Conservation of Nature (IUCN) and the family Sphyrnidae was listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) to regulate their international trade as there is constant misidentification of the species in fisheries as they “look-alike”.
Chapter II focuses on resolving the phylogeography and population genetics of the two lineages identified in the Atlantic (S. tiburo from the Western Atlantic, and S. aff. tiburo from the Caribbean and the Southwestern Atlantic). Based on two mitochondrial markers [control region (mtCR) and cytochrome oxidase I (COI)] and landscape genetics, the findings confirmed that S. aff. tiburo and S. tiburo have been on independent evolutionary paths, we detected significant population differentiation within and between the two lineages, identified isolation by distance (IBD) pattern, and updated the distribution range for each lineage.
Following these results, in Chapter III, I analyzed 67 morphological traits, two meristic characters (prevertebral counts and teeth counts), two mitochondrial markers (COI and mtCR), and 12 microsatellite loci (nuclear markers) to describe S. aff. tiburo as a new species (aka Sphyrna alleni sp. nov., Shovelbill shark; in press). Results indicate that prevertebral counts, teeth distribution, and head shape are the morphological characters to distinguish between S. alleni sp. nov., and S. tiburo. Moreover, both nuclear and mitochondrial markers evidenced that the two species have been on independent evolutionary trajectories for at least 3.5 Mya.
In Chapter IV, I use the same methods as Chapter III to investigate if the Pacific bonnethead shark (S. tiburo vespertina) from the Eastern Pacific constitutes a third cryptic species for the bonnethead complex. I also discussed the origin and drivers of speciation analyzing the role of the closure of the Isthmus of Panama in separating these lineages. The findings indicate that prevertebral counts, head shape, and both nuclear and mitochondrial markers clearly evidence that the Pacific bonnethead shark is a species and should be resurrected as S. vespertina (Springer, 1940).
Finally, this dissertation is a baseline for fisheries management for small hammerhead sharks since they are not protected or managed for sustainability in most of their distribution ranges. We also make a call for reassessing the conservation status by the UICN and to promote other studies focused on resolving the phylogeny and origins of these underrepresented sharks.