Smooth hammerhead

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Smooth hammerhead
CITES Appendix II (CITES)[2]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Carcharhiniformes
Family: Sphyrnidae
Genus: Sphyrna
Species:
S. zygaena
Binomial name
Sphyrna zygaena
Range of the smooth hammerhead
Synonyms
  • Squalis pictus* Blainville, 1816
  • Squalus carolinensis* Blainville, 1816
  • Squalus zygaena Linnaeus, 1758
  • Zygaena malleus Valenciennes, 1822
  • Zygaena subarcuata Storer, 1848
  • Zygaena vulgaris Cloquet, 1830

* ambiguous synonym

The smooth hammerhead (Sphyrna zygaena) is a

temperate waters and occurs worldwide at medium latitudes. In the summer, these sharks migrate towards the poles following cool water masses, sometimes forming schools
numbering in the hundreds to thousands.

The second-largest hammerhead shark after the

viviparous and gives birth to litters of 20–40 pups. A relatively common shark, it is captured, intentionally or otherwise, by many commercial fisheries throughout its range; its fins are extremely valuable for use in shark fin soup. This shark is potentially dangerous and has likely been responsible for a few attacks on humans, though it is less likely to encounter swimmers than other large hammerhead species due to its temperate habitat
.

Taxonomy and phylogeny

The Swedish natural historian

type specimen. The name was later changed to Sphyrna zygaena.[3] The specific epithet zygaena originates from the Greek word zygòn, meaning "yoke", referring to the shape of its head.[4] The Greek name zýgaina had already been used for the hammerhead shark by Aristotle in the second book of his History of Animals.[5] Other common names for this species include common hammerhead, common smooth hammerhead, round-headed hammerhead, or simply hammerhead.[6]

Eusphyra blochii

Sphyrna mokarran

Sphyrna zygaena

Sphyrna lewini

Sphyrna tudes

Sphyrna media

Sphyrna tiburo

Sphyrna corona

Phylogenetic tree of hammerhead sharks.[7]

Studies based on

Phylogenetic analyses based on nuclear and mitochondrial DNA have concluded differently: while the smooth and great hammerheads are closely related, they are not as closely related to the scalloped hammerhead as the other Sphyrna species. Furthermore, the smooth hammerhead is among the more basal hammerhead species, indicating that the first hammerheads to evolve had large cephalofoils.[7][8]

Description

The scalloped hammerhead (left) and the smooth hammerhead (right) differ in cephalofoil shape.
Upper teeth
Lower teeth

The second-largest hammerhead next to the great hammerhead, the smooth hammerhead typically measures 2.5–3.5 m (8.2–11.5 ft) long, with a maximum recorded length and weight of 5 m (16 ft) and 400 kg (880 lb) respectively.[9] The smooth hammerhead differs from other large hammerheads in the shape of its cephalofoil, which has a curved front margin without an indentation in the center. The cephalofoil is wide but short, measuring 26–29% of the body length across. The nostrils are located near the ends of the cephalofoil, with long grooves running towards the center. There are 26–32 tooth rows in the upper jaw and 25–30 tooth rows in the lower jaw. Each tooth is triangular in shape, with smooth to weakly serrated edges.[9]

The body is streamlined, without a dorsal ridge between the two

dermal denticles are densely packed, each with 5–7 horizontal ridges (3 in juveniles) leading to a W-shaped rear margin. The back is dark brownish gray to olive in color, in contrast to the simple brown of most other hammerheads, becoming lighter on the flanks. The belly is white, and sometimes the pectoral fins have dark edges underneath.[9]

Distribution and habitat

Of the hammerhead sharks, the smooth hammerhead is the species most tolerant of temperate water, and occurs worldwide to higher

Galapagos Islands, Ecuador, and Chile, with a report in 1954 of two unidentified hammerheads off British Columbia which probably were smooth hammerheads. This species is usually considered to be amphitemperate (absent from the tropics) in distribution, though there are rare reports from tropical waters such as in the Gulf of Mannar off India, and off southern Mozambique. Its presence in the tropics is difficult to determine due to confusion with other hammerhead species.[3] Although generally preferring subtropical and warm temperate regions, a study of captures in the West Atlantic and Gulf of Mexico off the United States recorded smooth hammerheads in water temperatures ranging from 7.5 to 27.5 °C (45.5–81.5 °F).[14]

Compared to the scalloped and great hammerheads, the smooth hammerhead stays closer to the surface, in water less than 20 m (66 ft) deep. However, it has been recorded diving to a depth of 200 m (660 ft). It prefers inshore waters such as

freshwater habitats, such as the Indian River in Florida. In the summer, smooth hammerheads migrate poleward to stay in cooler water, heading back towards the equator in winter.[15]

Biology and ecology

A migrating smooth hammerhead swimming with its dorsal fin exposed

Adult smooth hammerheads are either solitary or form small groups. They may come together in great numbers during their annual migrations; schools of over a hundred juveniles under 1.5 m (4.9 ft) long have been observed off the eastern Cape of

parasites of the smooth hammerhead include the nematodes Parascarophis sphyrnae and Contracaecum spp.[9]

The smooth hammerhead is an active-swimming predator that feeds on bony fishes, rays, sharks (including of its own species), cephalopods, and to a lesser extent

dolphins.[20]

Like other hammerheads, the smooth hammerhead is viviparous: once the young exhaust their supply of

gestation period of 10–11 months.[15] Birthing occurs in shallow coastal nurseries, such as Bulls Bay in North Carolina.[21] The pups measure 50–61 cm (20–24 in) long at birth; females reach maturity at 2.7 m (8.9 ft) long and males at 2.1–2.5 m (6.9–8.2 ft) long, depending upon locality.[9] Off South Africa, newly mated females have been caught in February and females with full-term embryos in November; off the east coast of Australia, birthing takes place between January and March, with ovulation taking place around the same time.[18] This shark is thought to live for 20 years or more.[9]

Human interactions

The smooth hammerhead is potentially dangerous to humans. As of 2008, the International Shark Attack File lists 34 attacks attributable to large hammerhead sharks, 17 of them unprovoked (1 fatal).[22] However, due to the smooth hammerhead's occurrence in temperate regions where people are less likely to enter the water, it was likely responsible for a minority of these attacks.[9] Off southern California, this species has been reported to steal catches from sport fishers and divers.[3]

Smooth hammerheads are caught by commercial fisheries throughout the world, including those off the

Chinese medicine.[6]

Many other fisheries from every ocean also take smooth hammerheads as bycatch, and they are caught in some numbers by recreational anglers. Smooth hammerheads are also killed by entanglement in shark nets used to protect beaches. Fewer than 10 smooth hammerheads were caught annually in the nets off KwaZulu-Natal, South Africa, from 1978–1990. In contrast, in the nets off New South Wales, Australia, smooth hammerheads comprised 50% of the 4,715 sharks captured from 1972–1990.[18] At present, this species remains relatively common and has been assessed as "Vulnerable (VU)" by the World Conservation Union.[1] Off New Zealand, it is a prohibited target species and is the most abundant shark along the northwest coast. In June 2018 the New Zealand Department of Conservation classified the smooth hammerhead as "Not Threatened" with the qualifier "Secure Overseas" under the New Zealand Threat Classification System.[23]

It also does not appear to have been negatively impacted by fishing off southern Australia.

elasmobranchs were added to Appendix 2 of CITES, meaning international trade (including in parts and derivatives) is regulated by the CITES permitting system.[24]

References

  1. ^
    doi:10.2305/IUCN.UK.2019-3.RLTS.T39388A2921825.en
    . Retrieved 19 November 2021.
  2. ^ "Appendices | CITES". cites.org. Retrieved 2022-01-14.
  3. ^
    ISBN 92-5-101384-5
    .
  4. ISBN 0-679-72210-6
    .
  5. ^ Aristotle (350 BCE). "Ch. XI.11". History of Animals (Book II)
  6. ^ a b Froese, Rainer; Pauly, Daniel (eds.) (2008). "Sphyrna zygaena" in FishBase. January 2008 version.
  7. ^
    PMID 20138218
    .
  8. ^ Cavalcanti, M.J. (2007). "A Phylogenetic Supertree of the Hammerhead Sharks (Carcharhiniformes: Sphyrnidae)". Zoological Studies. 46 (1): 6–11.
  9. ^ a b c d e f g h i j k l Bester, C. Biological Profiles: Smooth Hammerhead. Florida Museum of Natural History Ichthyology Department. Retrieved on October 19, 2008.
  10. doi:10.1017/S1755267206000984
    .
  11. ^ Kelly, B (21 July 2019). "Hammerhead shark spotted in Irish waters for the first time ever". Galway Daily. Retrieved 10 December 2019.
  12. Illustreret Videnskab
    . 9 February 2015. Retrieved 4 November 2017.
  13. ^ Carl, H.; J.G. Nielsen; P.R. Møller (2005). "En kommenteret og revideret oversigt over danske fisk". Flora og Fauna. 110 (2): 29–39.
  14. ^ Grace, M.; T. Henwood (1997). "Assessment of the Distribution and Abundance of Coastal Sharks in the U.S. Gulf of Mexico and Eastern Seaboard, 1995 and 1996". Marine Fisheries Review. 59 (4): 23–32.
  15. ^
    ISBN 0-520-23484-7
    .
  16. doi:10.1578/AM.31.1.2005.83
    .
  17. JSTOR 1446449
    .
  18. ^
    ISBN 2-8317-0700-5
    .
  19. doi:10.2989/025776191784287808
    .
  20. doi:10.1111/mms.12169
    .
  21. ISBN 0-7637-3313-X
    .
  22. ^ ISAF Statistics on Attacking Species of Shark. International Shark Attack File, Florida Museum of Natural History, University of Florida. Retrieved on May 18, 2009.
  23. OCLC 1042901090
    .
  24. ^ "CITES conference takes decisive action to halt decline of tropical timber, sharks, manta rays and a wide range of other plants and animals". Cites. 14 March 2013.

External links

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