First record of Piscicapillaria bursata (Nematoda: Capillariidae), a parasite of hammerhead sharks Sphyrna spp., in the western Atlantic Ocean.

Examination of 32 spiral valves from neonate specimens of hammerhead shark Sphyrna spp. (Carcharhiniformes: Sphyrnidae) captured between June and August 2018 off the Atlantic coast of South Carolina, USA, revealed the presence of the capillariid nematode Piscicapillaria bursata (Capillariidae) in the Carolina hammerhead S. gilberti, the scalloped hammerhead S. lewini, and their hybrids. This is the second find of this parasite originally described from hammerhead sharks off Australia, its first record from the western Atlantic Ocean, and its first record in a new host species and in hybrids.

Hybridization between sympatric hammerhead sharks in the western North Atlantic Ocean.

Hybridization between closely related species has been documented across a wide range of taxa but has not been well studied in elasmobranchs. Hammerhead sharks have drawn global conservation concern because they experience some of the highest mortality rates among sharks when interacting with fisheries. Here we report on the detection of hybrids between the globally distributed scalloped hammerhead ( Sphyrna lewini) and recently described Carolina hammerhead ( S. gilberti) which are only known from the western Atlantic Ocean. Using a genomics approach, 10 first-generation hybrids and 15-17 backcrosses were detected from 554 individuals. The identification of backcrosses demonstrates hybrids are viable, and all backcrosses but one involved a scalloped hammerhead. All hybrids but one possessed Carolina hammerhead mtDNA, indicating sex-biased gene flow between species. Repeated hybridization and backcrossing with scalloped hammerheads could lead to the loss of endemic Carolina hammerheads.