Multiple paternity in two populations of finetooth sharks (Carcharhinus isodon) with varying reproductive periodicity.

The mechanisms underlying polyandry and female mate choice in certain taxonomic groups remain widely debated. In elasmobranchs, several species have shown varying rates of polyandry based on genetic studies of multiple paternity (MP). We investigated MP in the finetooth shark, Carcharhinus isodon, in order to directly test the encounter rate hypothesis (ERH), which predicts that MP is a result of the frequency of encounters between mature conspecifics during the breeding season, and should therefore increase when more time is available for copulation and sperm storage. Female finetooth sharks in the northern Gulf of Mexico (GoM) have been found to reproduce with both annual periodicity and biennial periodicity, while finetooth sharks from the northwestern Atlantic Ocean have only been found to reproduce biennially, allowing us to compare mating opportunity to frequency of MP. Our results show high rates of MP with no significant difference in frequency between females in the GoM (83.0%) and Atlantic (88.2%, p = .8718) and varying but nonsignificant rates of MP between females in the GoM reproducing annually (93.0%) and biennially (76.6%, p = .2760). While the ERH is not supported by this study, it remains possible that reproductive periodicity and other physiological factors play a role in determining rates of MP in elasmobranchs, with potential benefits to individuals and populations.