Tuesday, 4 December 2018

Carcharhinus melanopterus: Leucism in Blacktip Reef Sharks.

Leucism is a pigment deficiency in which a prenatal enzyme deficiency results in low levels of the pigment melanin, although it is not completely absent as in albinism. Typically this results in a partial or total loss of pigmentation across all or part of the body, but normal pigmentation of the eyes. Leucism is known in a variety of vertebrate groups, including Mammals, Amphibians, Crocodilians, Turtles, Squamates, Teleost Fish and Sharks.

In a paper published in the journal Coral Reefs on 12 February 2018, Andrew Bruckner and Georgia Coward of Coral Reef CPR, describe an instance of leucism in Blacktip Reef Sharks, Carcharhinus melanopterus, from Baa Atoll in the Maldives.

Blacktip Reef Sharks are usually yellow-brown in colour, with a white belly and black on the tips of their dorsal fin, the edges of their pectoral fins, and the upper lobe of their tails. An unusual colour variation was observed in eight young Sharks in a nearshore nursery used by Sharks under a year old, alongside about 25 Sharks with normal colouration. These Sharks had mottled colouration, though they still had the black tips and edges to their fins, and normal eye colouration, strongly suggesting that the condition was a form of leucism. The Sharks were observed for six months and their pigmentation did not alter.

Abnormal pigmentation in four Carcharhinus melanopterus individuals. (a) Partial body leucism and normal pigmentation. (b) Normal pigmentation and extreme case of abnormal pigmentation. (c) Two individuals with abnormal pigmentation. (d) Abnormal pigmentation between the head and trunk and on the fins. Bruckner & Coward (2018). 

Leucism is potentially harmful to small Sharks such as Blacktips, as it removes their natural camouflage, making them more vulnerable to predators. The condition can be caused by a variety of factors, including inbreeding, environmental stress or exposure to high temperatures, all of which are potential factors in the Maldives. The islands suffered from heavy overfishing, which reduced population sizes for many Fish. There is also a current construction boom in the Maldives, with land reclamation projects and building on reefs, largely associated with tourism developments. The area also suffered from elevated seawater temperatures in 2015-16 due to an El Niño event.

The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.
 

Movements of air masses and changes in precipitation in an El Niño weather system. Fiona Martin/NOAA.

See also...

https://sciencythoughts.blogspot.com/2018/08/tourist-killed-by-shark-on-red-sea.htmlhttps://sciencythoughts.blogspot.com/2018/05/teenager-bitten-by-shark-off-florida.html
https://sciencythoughts.blogspot.com/2018/04/massive-ghost-net-seen-with-thousands.htmlhttps://sciencythoughts.blogspot.com/2018/04/edinburgh-schoolboy-attacked-by-shark.html
https://sciencythoughts.blogspot.com/2018/04/edinburgh-schoolboy-attacked-by-shark.htmlhttps://sciencythoughts.blogspot.com/2018/02/british-tourist-attacked-by-shark-while.html
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