Shark Spotlight: The Carolina Hammerhead

Written by Michelle Scott The Carolina Hammerhead

Taxonomy

The Order Carcharhiniformes includes 295 species from 51 genera and nine families: Carcharhinidae (requiem sharks), Hemigaleidae (weasel sharks), Leptochariidae (barbeled houndshark), Proscylliidae (finback catsharks), Pseudotriakidae (false catsharks), Pentanchidae and Scyliorhinidae (catsharks), Sphyrnidae (hammerhead sharks) and Triakidae (houndsharks). This order contains some of the most well-recognized shark species, including the Tiger Shark (Galeocerdo cuvier) and the Bull Shark (Carcharhinus leucas), as well as the Hammerhead Sharks (family Sphyrnidae). The Carolina Hammerhead (Sphyrna gilberti), a sister species of the Scalloped Hammerhead (S. lewini), is named after the former curator of the Florida Museum of Natural History, Dr Carter Gilbert. In 1967, Dr Gilbert caught what he believed was an anomalous Scalloped Hammerhead with fewer vertebrae than previously recorded. But it wasn’t until 2013 that it was confirmed to be a different species through genetic analysis and morphological data.

Morphology

The Carolina Hammerhead is a cryptic species, morphologically similar to the Scalloped Hammerhead, but genetically distinct. Sphyrna gilberti can be differentiated from other species within the taxonomic group by having a cephalofoil with median indentation, the presence of an inner narial groove, straight rear margins on the pelvic fins, and 91 or fewer precaudal vertebrae. The maximum size of this species is unknown, but it is recorded to a total length (TL) of at least 69 cm (TL). Neonates have also been sampled, and the size at birth is reported at around 39 cm (TL).

Distribution and habitat

The Carolina Hammerhead is presumably a rare species and is thought to have a more restricted distribution than its congener, the Scalloped Hammerhead. However, its full distribution and depth range are unknown since S. gilberti is co-distributed with its sister species S. lewini compounding the difficulties in establishing distribution ranges. This is also likely due to the misidentification of S. gilberti as S. lewini in fisheries monitoring programs. With the exception of three individuals captured off southern Brazil, Carolina Hammerheads are only known from specimens collected off the east coast of the USA from North Carolina to Florida in the western Atlantic. This species may be migratory, similar to other members of the genus Sphyrna and is thought to be a coastal neritic and possibly semi-oceanic pelagic shark.

Conservation measures and IUCN Red List status

The Carolina Hammerhead has been assessed as Data Deficient on the IUCN Red List of Threatened Species. Considering they occur in similar habitats, Carolina Hammerheads may be susceptible to similar fishing pressures as the Scalloped Hammerhead. This includes capture in longlines, gillnets, trammel nets, and trawls. Hammerhead sharks of all species are utilized for their fins, liver oil, skin, cartilage and jaws. The similarity between the Carolina and Scalloped Hammerheads causes difficulty when establishing management actions for both species. Not only due to the distributional overlap but also because there is evidence of hybridization. The effects of hybridization provide a challenge in establishing conservation measures when a species is threatened because of the complexities in confirming causes of declines (whether they are natural or anthropogenic). If hybridization reduces fitness, then this could threaten the Carolina Hammerhead population through wasted reproductive efforts. Carolina Hammerheads are not currently considered in management plans, and more data are needed regarding habitat use, distribution, and relative abundance of the species. The similarity between the S. gilberti and S. lewini means that the Carolina Hammerheads have been included in previous assessments of Scalloped Hammerhead populations. Future decisions regarding the conservation status of the Carolina Hammerhead will not only have to consider the presence and status of Scalloped Hammerheads but should also consider the potential consequences of continued hybridization between these species.

References Barker, A. (2020). Molecular ecology and conservation genomics of Scalloped (Sphyrna lewini) and Carolina Hammerheads (S. gilberti). hdl.handle.net/1969.6/89425 Barker, A.M. , Adams, D.H. , Driggers W.B., Frazier, B.S., & Portnoy, D.S. (2019). Hybridization between sympatric hammerhead sharks in the western North Atlantic Ocean. Biology Letters, 15(4), 20190004. Quattro, J. M., DRIGGERS, W. B. I., Grady, J. M., Ulrich, G. F., & Roberts, M. A. (2013). Sphyrna gilberti sp. nov., a new hammerhead shark (Carcharhiniformes, Sphyrnidae) from the western Atlantic Ocean. Zootaxa, 3702(2), 159-178. VanderWright, W.J., Carlson, J., Pollom, R. & Dulvy, N.K. 2020. Sphyrna gilberti. The IUCN Red List of Threatened Species 2020: e.T152783714A172115852. DOI:10.2305/IUCN.UK.2020-3.RLTS. T152783714A172115852.en. Downloaded on 17 November 2021.