As far as ocean predators go, Atlantic nurse sharks are pretty nondescript. Brownish-yellow skin, characteristic round heads and a pair of catfish-like barbels to help detect prey make the species easy to identify. But there's at least one nurse shark out there breaking the mould. Recreational divers exploring waters off the coast of Honduras earlier this year came across a particularly unusual-looking shark. Instead of the uniform brown typical of the species, this individual looked more like a speckled egg. It was later determined that the spotty character was exhibiting signs of piebaldism – a genetic condition that results in a partial lack of body pigmentation.
Skin pigmentation deficiencies are particularly rare in marine animals. According to researchers from Beneath The Waves (BTW) who – together with members of the Caribbean Shark Coalition – wrote a research paper on the oddly coloured nurse shark, this is the first time piebaldism has been documented in the species (it should be noted, however, that a similar-looking nurse shark was spotted in the Caribbean in 2016).
Although rare, oddly coloured marine creatures do turn up from time to time. A spotty requiem shark was caught by an angler in Texas back in 2012 and an all-white nurse shark believed to be leucistic was photographed in Western Australia last year. Australia is also home to a famous albino humpback whale affectionately known as 'Migaloo'.
Leucistic, piebald or albino?
Leucism, piebaldism and albinism fall under a suite of skin pigmentation disorders called hypomelanosis. Although animals afflicted with these deficiencies share similarities, the conditions can be differentiated.
True albino animals show a total loss of pigment as a result of an absence or complete dysfunction of the protein pigment melanin. This extends to the irises so albino animals will have red eyes. The condition has been seen in white sharks before, as well as in dogfish, and a number of cetaceans like pilot whales, Pacific white-sided and Risso's dolphins, and humpbacks (and a selection of non-marine species as well).
Leucistic animals have a "genetic disorder caused by expression of recessive alleles that affect melanin metabolism prior to birth," the authors explain in a paper describing the recently discovered piebald nurse shark. This results in either a total or partial loss of pigment, but will not produce the spooky red eyes seen in albino animals.
Finally, piebald animals show a partial loss of body pigment which usually results in a spotty or patchy appearance. The eyes are not affected.
Can animals with skin pigment deficiencies survive in the wild?
Animals with albinism, leucism or piebaldism can have a tougher time surviving the challenges of life in the wild. They may be ultra sensitive to light, could have health problems associated with genetic mutations and can have trouble blending into their natural habitats. Given the rarity of skin pigment conditions in wild animals, it's unclear exactly how much of an impact they have on survival.
The Honduras nurse shark was estimated to be around six feet in length, which is average for the species according to the study authors, leading them to theorise that the mottled shark may be faring well. "We can assume that it has been able to hunt and avoid predators successfully to reach maturity," they outlined in a press release. Nurse sharks are unfussy eaters and can survive in a number of different conditions so the speckled shark's resilience and adaptability were somewhat expected. Other species afflicted with pigment deficiencies may not have coped as well.
More research is required to gain a better understanding of how animals are affected by skin colouration, the authors suggest. And citizen science may have a role to play. The latest research paper was only possible thanks to a collaboration between recreational divers and marine biologists – something that BTW hopes can be fostered in the future.
Header image: NOAA Photo Library
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