Common skate

Common skate
Scientific classification
Kingdom: Animalia
Class: Chondrichthyes
Subclass: Elasmobranchii
Superorder: Batoidea
Order: Rajiformes
Family: Rajidae
Genus: Dipturus
Species: D. batis
Binomial name
Dipturus batis
(Linnaeus, 1758)

The common skate or blue skate (Dipturus batis[1]) is the largest skate in the world attaining a length of more than 2.5 m (8.2 ft).[2] Historically, it was one of the most abundant skates in the northeast Atlantic Ocean and the Mediterranean Sea. Despite its name, today it appears to be absent from much of this range.[3] Where previously abundant, fisheries directly targeted this skate, elsewhere it is caught incidentally as by-catch. Previously assessed as Endangered globally and Critically Endangered in shelf and enclosed seas in the 2000 IUCN Red List, it has been uplisted to Critically Endangered globally in 2006.[4] In 2009, research showed what was formerly listed as a single species, D. batis, should be instead classified as two separate species, D. flossada, and the flapper skate, D. intermedia.[5][6]

Description

The common skate can grow to 2.85 m (9.4 ft)[7] long and weigh 100 kg (220 lbs),[8] making it the largest skate in the world. Overall shape features a pointed snout and rhombic shape, with a row of spines or thorns along the tail.[9] The top surface is generally colored olive-grey to brown, often with a pattern of spots, and the underside is lighter blue-grey.[7]

Growth and Reproduction

Males usually grow up to a length of 125 cm. Female have yet to be accurately measured for size, it is reported to be approximately 150 cm. For every male Common Skate there is approximately one female Common Skate, so the sex ratio is 1:1, however this changes depending on the season and geography.When juveniles are born, they are measured at 21–22 cm long.[10] It takes a female Common Skate around 11 years to reach sexual maturity. Once it has reached sexual maturity, they will only reproduce every other year. They mate in the spring and during the summer they lay approximately 40 eggcases which they put in sandy/muddy flats. The embryos then develop for 2–5 months before leaving their eggcase.[11]

Eggcase

Eggcases are measured up to 25 cm long (excluding the horns) and 15 cm wide. They are covered in close-felted fibers and often washed up on the shore. They have distinct lateral keels and deep anterior fields.[11] Eggcase hunts have been done to see the general distribution of the Common Skate. Eggcases were only found in northern Scotland and the north of Ireland. In the 19th and 20th centuries eggcases were usually seen along the entire British coastline in high numbers, but now they are only found in a few selective areas.[12]

Range/Habitat and Ecology

The Common Skate is a bottom dwelling species. People will most likely find them in waters with a depth of approximately 200m. But, common skates can be found anywhere up to 600m. They are now found in those depths at the waters off northwestern Scotland and in Celtic Sea, and along the edge of the continental shelf. The historic range of D. batis covered much of the continental shelf of the North-east Atlantic. It went from Madeira and some of northern Morocco in the south, all the way to Iceland and northern Norway in the north. This range also included the Mediterranean Sea. When the twentieth century started common skates were considered to have a wide range over the shallower waters of the continental shelf surrounding the British Isles. Now their population and range is severely depleted and fragmented, with disappearances being reported on several.[10][13]

Status as an Endangered Species

Currently, the Common Skate is listed as a Critically Endangered species. It is threatened both in the Atlantic Ocean and Mediterranean Sea. In both cases, it seems likely due to fishing trawls that are after things like hake or anglerfish, and who simply catch the Common Skate by accident. Due to the popularity and profitability of trawling fishing, "it is unlikely that fishing effort will decrease".[14] As of 2014 it is extinct in the Baltic Sea.[15]

Because the Common Skate is a long-lived and slow to mature creature, it is difficult for it to repopulate quickly.[16] That, coupled with danger from careless fishing trawls, means that not only is the Skate population drastically decreasing, but that it is likely the Common Skate will soon disappear entirely unless more is done to preserve it.[13]

Diet

Like the rest of the skates, the common skate is a bottom feeder. Their diet typically consists of crustaceans and clams, oysters, snails, and small fish.[17] The size of the individual can affect its diet. Larger ones eat larger things like fish.[18] The bigger the skate is the more food that will be needed to sustain its large body size. For common skates their activity level determines how much it eats, the more active it is the more it eats.[19] In addition to crustaceans and clams, skates can also eat bristle worms, sand eels, crabs, and other species of flatfish.[20] The common skate does not only feed on creatures at the bottom of the ocean, as some do ascend to higher depths to feed on mackerel, herring, and other species of fish.[21] Once a common skate has caught a prey it envelopes it before consuming it. The common skate captures its prey by going rapidly upward and gripping the fish before returning to the seabed to consume it [11]

Taxonomy

Recent discoveries have shown that the Common Skate is actually made of two more specific species. They are either the Blue Skate (Dipturus flossada) or the Flapper Skate (Dipturus intermedia). The D. intermedia takes a while longer to grow in to its full length, but reaches a larger size. The size at 50% maturity has been reported to be 197.5 cm for females and 185.5 cm for males. The D. flossada is recorded to be at 122.9 cm for females and 115 cm for males. Due to the slight difference in size, they are easily confused for one another. The best way to identify the species is through their eye color. The D. intermedia has dark green/olive eyes while the D. flossada has a pale yellow color.[11][22]

Notes

  1. ↑ Sometimes listed as Raja batis which was the name given by Linnaeus, however this name is no longer considered valid.
  2. ↑ Florida Museum of Natural History. "Ray and Skate: Basic Questions". Retrieved 2007-10-29.
  3. ↑ Brander, K. (1981). "Disappearance of common skate Raja batis from Irish Sea". Nature. 290 (5801): 48–49. doi:10.1038/290048a0.
  4. ↑ "Marine Species." ICUN Global Marine Programme. 2003. ICUN. 26 November 2006 <http://www.iucn.org/themes/marine/marine_species.htm>.
  5. ↑ Is 80-Year-Old Mistake Leading to First Species to Be Fished to Extinction?, ScienceDaily 17 Nov 2009
  6. ↑ "BBC News - Skate may be fished to extinction". bbc.co.uk.
  7. 1 2 Froese, Rainer and Pauly, Daniel, eds. (2007). "Dipturus batis" in FishBase. October 2007 version.
  8. ↑ Kindersley, Dorling (2001,2005). Animal. New York City: DK Publishing. ISBN 0-7894-7764-5. Check date values in: |date= (help)
  9. ↑ ARKive. "Common skate - Dipturus batis". Retrieved 2007-10-29.
  10. 1 2 "Dipturus batis". Archived from the original on 27 June 2014. Retrieved 20 October 2013.
  11. 1 2 3 4 http://www.sharktrust.org/en/factsheets/59/common-skate.html
  12. ↑ "The Shark Trust - Common/Flapper Skate". sharktrust.org.
  13. 1 2 http://qsr2010.ospar.org/media/assessments/Species/P00477_common_skate.pdf
  14. ↑ "Dipturus batis". iucnredlist.org.
  15. ↑ HELCOM (2013). "HELCOM Red List of Baltic Sea species in danger of becoming extinct" (PDF). Baltic Sea Environmental Proceedings (140): 72.
  16. ↑ "Descriptions and articles about the Blue Skate (Dipturus batis) - Encyclopedia of Life". Encyclopedia of Life.
  17. ↑ "The Benthic Zone: The Sea Floor". Retrieved 23 October 2013.
  18. ↑ "Dipturus batis". Retrieved 23 October 2013.
  19. ↑ "Movement and behaviour patterns of the critically endangered common skate Dipturus batis revealed by electronic tagging". Journal of Experimental Marine Biology and Ecology. 380: 77–87. doi:10.1016/j.jembe.2009.07.035.
  20. ↑ "BIOTIC Species Information for Dipturus batis". Retrieved 23 October 2013.
  21. ↑ "Common Skate". Retrieved 23 October 2013.
  22. ↑ http://www.sfpa.ie/Portals/0/Corporate%20Affairs/Information%20Leaflets//Industry%20Advice%20Common%20Skate.pdf
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