Smalleye Hammerhead - Taxonomy and Phylogeny

Taxonomy and Phylogeny

Despite being one of the most easily recognizable sharks, the smalleye hammerhead has had a long history of taxonomic confusion that still remains to be fully resolved. Its scientific name originated in 1822, with French zoologist Achille Valenciennes' description of Zygaena tudes in the scientific journal Memoires du Museum National d'Histoire Naturelle; the specific epithet tudes is Latin for "hammer". Valenciennes made reference to three specimens: one from Nice in France, one from Cayenne in French Guyana, and one from the Coromandel Coast of India. However, for over two centuries taxonomists believed Valenciennes' account matched the great hammerhead, which thus became known as Zygaena (later Sphyrna) tudes. The smalleye hammerhead was known by a different name, Sphyrna bigelowi, coined by Stewart Springer in a 1944 issue of Journal of the Washington Academy of Sciences.

In 1950, Enrico Tortonese examined the Nice and Cayenne specimens of S. tudes (the Coromandel specimen having been lost in the interim) and concluded that they were not great hammerheads but rather the same species as S. bigelowi. Carter Gilbert concurred in his 1967 revision of the hammerhead sharks, noting that while the lost Coromandel specimen was probably a great hammerhead, none of the existing material belonged to that species. Thus, Sphyrna tudes became the accepted name for the smalleye hammerhead, taking precedence over S. bigelowi because it was published earlier, and the great hammerhead received the next available name Sphyrna mokarran. Gilbert designated the Nice specimen as the lectotype that would define S. tudes, having priority over the Cayenne specimen (the paralectotype). This was meant to stabilize the name but had the opposite effect.

In 1981, Jean Cadenat and Jacques Blache revisited the type specimens of S. tudes and found that the lectotype from Nice is likely not a smalleye hammerhead but rather a fetal whitefin hammerhead (S. couardi, likely a synonym of the scalloped hammerhead, S. lewini). This would also explain the anomalous locality of the Nice specimen, as the smalleye hammerhead is not otherwise known outside of the Americas. By the rules of binomial nomenclature, Sphyra tudes should then become the valid name for the whitefin hammerhead, taking precedence over S. couardi, and the smalleye hammerhead would revert to being Sphyrna bigelowi. Taxonomists though have been reluctant to change the names again, preferring to keep the smalleye hammerhead as S. tudes. For this solution to have official status would require a decision by the International Commission on Zoological Nomenclature (ICZN), to reject the Nice specimen as the lectotype and designate the Cayenne specimen in its place. The relevant petition to the ICZN has not yet been put forth.

Until the first detailed study of the smalleye hammerhead was carried out in 1985–86 by José Castro of Clemson University for the Food and Agriculture Organization of the United Nations (FAO), its distinctive golden coloration was unknown to science. The color fades after death and the pigments leech into the preservative, resulting in the "yellowish cast" of museum specimens being regarded as an artifact of preservation. The names "yellow hammerhead" or "golden hammerhead" are used by fishermen in Trinidad for this shark, and the latter was promoted for wider usage by Castro. Another common name for this species is the curry shark. Phylogenetic analyses based on nuclear and mitochondrial DNA have found that the hammerheads with the smallest cephalofoils are the most derived members of their lineage. The closest relative of the smalleye hammerhead appears to be the scoophead (S. media), and the two of them in turn form a clade with the sister species pair of the scalloped bonnethead (S. corona) and the bonnethead (S. tiburo), and that the three form a clade with the bonnethead (S. tiburo).