Molecular ecology and evolution of elasmobranch reproductive strategies




Swift, Dominic

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Elasmobranchs are a diverse group of cartilaginous fishes consisting of sharks and batoids that exhibit a variety of reproductive strategies. Elasmobranch reproductive biology has been studied in the wild for many decades but molecular techniques have been used more recently to broaden understanding. Though polyandry has been demonstrated to be widespread, the benefits to females are unclear. Similarly, multiple species have been shown to re-use nurseries – which may increase juvenile survival – yet the impacts of this behavior on population structure require further study. Molecular studies using high-throughput sequencing can help to address knowledge gaps; however, the application of these techniques to study elasmobranchs is limited. Therefore, this dissertation examined elasmobranch reproductive strategies using high throughput approaches. The first chapter reviewed research on elasmobranch reproductive strategies and outlined how high-throughput data can help to address knowledge gaps. For the three other chapters, the blacktip shark (Carcharhinus limbatus) was studied to advance understanding of mate choice and nursery use and inform management. Chapter two assessed for MHC-associated mate choice. Evidence of assortative choice for mhc1a was observed in four of six litters but further study is needed to validate this observation. Chapter three examined the influence of philopatry on the genetic population structure of blacktip sharks using young-of-the-year sampled in United States waters. Regional philopatry by males and females has contributed to the formation of three genetically distinct units that closely align with fishing stocks. Furthermore, philopatry by females to environmentally heterogenous estuaries where offspring are born appears to have resulted in fine-scale adaptive structure within management units. Chapter four assessed the genetic stock structure and movement of blacktip sharks sampled across the western North Atlantic Ocean to evaluate the potential for multinational fisheries management. The blacktip shark stock in the western Gulf of Mexico might straddle U.S. and Mexican waters, and stocks in Cuba and The Bahamas are much more genetically diverged compared with other stocks. Moreover, five blacktip sharks were determined to have moved across stock boundaries, but the majority of individuals were sampled in the region of their natal stock. The research provides novel insights into elasmobranch reproductive strategies and is a basis for additional studies of mate choice and nursery use. There is preliminary evidence that MHC is involved in mate choice by blacktip sharks; however, additional research is necessary to examine the benefits and mechanisms associated with MHC-mediated choice. Further, there is evidence that female natal philopatry facilitates local adaptation to nursery conditions, but more research is required to provide direct evidence of this behavior and determine the distribution of putatively adaptive loci across the genome. Future studies should examine how mating systems and patterns of habitat use can generate, maintain, and disperse adaptive variation because this is vital for resilience to environmental change. Understanding disparities in abundance and dispersal potential between continental and insular populations would be particularly informative for management.



Blacktip shark, fisheries, local adaptation, mate choice, MHC, philopatry



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