For the past two decades, pelagic thresher sharks (Alopias pelagicus) have been known to visit a shallow coastal seamount in the Philippines where they interact with cleaner wrasse Labroides dimidiatus and Thalassoma lunare (Oliver et al. 2011). Known for possessing an elongate caudal fin that evolved for prey capture (Oliver et al. 2013), pelagic thresher sharks mature late, have low fecundity, and are classified as vulnerable to overexploitation by the International Union for Conservation of Nature. Parturient sharks have rarely been observed in the wild, and little is known of the reproductive strategies of most oceanic species. Here, we present photographic evidence that we captured on April 4, 2013, as the first record of a thresher shark giving birth.
Pelagic thresher sharks have been described as having a cosmopolitan distribution with metapopulations occupying appropriate habitats across the globe (Tsai et al. 2010). Current evidence suggests that the gender and maturity dynamics of thresher sharks that visit Monad Shoal collectively comprise a single unit stock, which incorporates male, female, juvenile, transitional, and mature sharks, and includes gravid females (current data). While interactions among thresher sharks and cleaner fish are well documented at this site (Oliver et al. 2011), this is the first time cleaners have been seen interacting with a marine organism while it gives birth.
We are grateful to Matthew Spencer, Howard Nelson, and Alison Beckett for providing helpful feedback.