Bigeye thresher

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Bigeye thresher
Temporal range: 16–0 
Ma[1]
 Middle Miocene to Present
CITES Appendix II (CITES)[3]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Lamniformes
Family: Alopiidae
Genus: Alopias
Species:
A. superciliosus
Binomial name
Alopias superciliosus
(R. T. Lowe, 1840)
Range of the bigeye thresher
Synonyms

Alopecias superciliosus R. T. Lowe, 1840
Alopias profundus Nakamura, 1935

The bigeye thresher (Alopias superciliosus) is a species of

scientific name
is derived.

The large eyes of the bigeye thresher are

range; the meat is not highly regarded but the skin, fins, and liver oil are valued. It has been assessed as Vulnerable by the International Union for Conservation of Nature
(IUCN).

Taxonomy and phylogeny

British biologist

scientific name to be resurrected. The specific epithet superciliosus is from the Latin super meaning "above", and ciliosus meaning "eyebrow", referring to the distinct lateral grooves above the eyes.[6][7]

An

Ma) have been found in the Hokuriku region of Japan.[1]

Distribution and habitat

The bigeye thresher has a virtually circumtropical distribution. In the western

Galapagos Islands.[5] Analysis of mitochondrial DNA has shown that Atlantic and Indo-Pacific populations are somewhat genetically divergent from each other.[9]

Bigeye threshers are usually found over the continental shelf and in the open sea, though they are occasionally encountered in shallow coastal waters. They occur in surface temperatures of 16–25 °C (61–77 °F), but have been tracked as far down as 723 m (2,372 ft), where the temperature is only 5 °C (41 °F).[6][10] Little is known of their geographical movements; one individual has been documented moving from New York to the Gulf of Mexico, a straight-line distance of 2,767 km (1,719 mi).[11]

Description

The eyes of the bigeye thresher can measure up to 10 cm (3.9 in) across in adults. Each eye is taller than wide, with a bulbous upper portion. The orbits extend onto the

dermal denticles, with the smaller ones more numerous and interspersed amongst the larger ones. The smaller denticles taper to a point.[7]

Up to half the body length is taken up by the long upper lobe of the caudal fin, which is broader than in other threshers. The large

pectoral fins have a curved anterior margin and broad tips. The first dorsal fin is placed further back than in the other thresher sharks, with the free rear tip located above or just before the pelvic fins. Its coloration is a deep, metallic violet to purplish brown above and creamy white below.[6][7] This color rapidly fades to a dull gray after death.[12] Most bigeye threshers are 3.3–4.0 m (10.8–13.1 ft) long and weigh 160 kg (350 lb).[7] The largest known bigeye thresher measured 4.9 m (16 ft) long and weighed 364 kg (802 lb), and was caught near Tutukaka, New Zealand, in February 1981.[12]

Biology and ecology

The size and upward orientation of the bigeye thresher's eyes are adapted to search for the silhouettes of prey in dim light.[6] This species is one of a handful of shark species that conducts a diel vertical migration, spending daytime in deeper water between 300–500 m (980–1,640 ft), beneath the thermocline where the temperature ranges from 6 to 12 °C (43 to 54 °F), and ascending above it to water less than 100 m (330 ft) deep during nighttime.[11] This migration likely relates to finding prey at night and avoiding predators during the day. The sharks' daytime swimming patterns are usually steady, while at night they have a pattern of slow ascents and rapid descents.[10]

Bigeye threshers are likely preyed upon by larger sharks and

tapeworm Litobothrium janovyi.[14][15] Sea lampreys (Petromyzon marinus) have been seen attached near the cloaca of this shark. The bigeye thresher appears to be an ecological competitor of the blue shark (Prionace glauca), and the numbers of the two species are negatively correlated such as that only one of two occurs in any given location.[12]

  • Dorsal view of a bigeye thresher, showing upward-facing eyes and prominent lateral grooves.
    Dorsal view of a bigeye thresher, showing upward-facing eyes and prominent lateral grooves.
  • Teeth, male
    Teeth, male
  • Teeth, male
    Teeth, male
  • Teeth, female
    Teeth, female
  • Teeth, female
    Teeth, female

Feeding

The enormous eyes of the bigeye thresher allow them to hunt in dim light.

The bigeye thresher has larger teeth than other threshers and feeds on a wider variety of prey. Known food items include

frigate mackerel (Auxis rochei), suggesting that these sharks follow concentrations of their prey from place to place. It is said that the bigeye thresher uses its long tail to smack down birds.[12]

Life history

Like other

maximum lifespan of this species is believed to be 19 years for a male and 20 years for a female.[7] A female bigeye thresher is estimated to produce only 20 young over her entire life.[16]

Thermoregulation

There is conflicting evidence on whether the bigeye thresher is

aerobic red muscles responsible for generating heat in the common thresher, these muscles are arranged in two strips along the flanks just beneath the skin, as opposed to near the core of the body. There is also no blood vessel countercurrent exchange system (the rete mirabile) in the trunk to limit the loss of metabolic heat to the water. Based on these differences, the authors questioned earlier measurements and concluded it was unlikely that the bigeye thresher maintains an elevated body temperature.[19] The bigeye thresher does possess a highly developed rete system around its brain and eyes. This is thought to function in buffering those sensitive organs against temperature changes during the shark's daily migrations up and down the water column, which can be as much as 15–16 °C (27–29 °F).[11]

Human interactions

Bigeye threshers are often caught on longlines.

The bigeye thresher shark is rarely encountered by divers underwater and poses no danger. This species is or was taken by longline

smoked, or dried and salted, though it is not highly regarded due to its mushy texture. The skin is used to make leather products, the liver oil for vitamins, and the fins for shark fin soup.[5]

In the waters of the

over-exploitation due to its low lifetime fecundity.[6] All three thresher shark species were assessed as Vulnerable by the International Union for Conservation of Nature (IUCN) in 2007.[2] In New Zealand, the Department of Conservation has classified the bigeye thresher shark as "Not Threatened" under the New Zealand Threat Classification System.[20]

References

  1. ^
    doi:10.1111/j.1440-1738.1994.tb00115.x
    .
  2. ^
    doi:10.2305/IUCN.UK.2019-3.RLTS.T161696A894216.en
    . Retrieved 19 November 2021.
  3. ^ "Appendices". Convention on International Trade in Endangered Species of Wild Fauna and Flora. Retrieved 2022-01-14.
  4. ^ Lowe, R. T. (1839). "Description of some new species of Madeiran fishes, with additional information relating to those already described". Proceedings of the Zoological Society of London. 8: 36–39 – via Biodiversity Heritage Library.
  5. ^
    ISBN 92-5-104543-7
    .
  6. ^
    ISBN 0-520-23484-7
    .
  7. ^ a b c d e f g Jensen, C. Bigeye Thresher. Florida Museum of Natural History. Retrieved December 21, 2008.
  8. JSTOR 1446753
    .
  9. ^ Trejo, T. (2005). "Global phylogeography of thresher sharks (Alopias spp.) inferred from mitochondrial DNA control region sequences". MSc thesis. Moss Landing Marine Laboratories, California State University.
  10. ^
    doi:10.3354/meps265255
    .
  11. ^ a b c Weng, K.C. & Block, B.A. (2004). "Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia". Fishery Bulletin – National Oceanic and Atmospheric Administration. 102 (1): 221–229.
  12. ^ a b c d e f Martin, R.A. Biology of the Bigeye Thresher (Alopias superciliosus). ReefQuest Centre for Shark Research. Retrieved on December 21, 2008.
  13. OCLC 38468784
    . Retrieved 13 July 2021.
  14. ^ Cressey, R. (1964). "A new genus of copepods (Caligoida, Pandaridae) from a thresher shark in Madagascar". Cahiers O.R.S.T.O.M. Océanographie. 2 (6): 285–297.
  15. S2CID 4828139
    .
  16. ^
    S2CID 13641670
    .
  17. JSTOR 1444380
    .
  18. doi:10.1093/icb/11.1.137
    .
  19. PMID 16272248
    .
  20. OCLC 1042901090
    .

External links

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