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Journalism of Courage
A rare, vividly orange nurse shark has been discovered off the coast of Costa Rica, baffling scientists and prompting renewed interest in marine pigmentation anomalies.
The six-foot-long shark was caught during a sport fishing trip near Tortuguero National Park, at a depth of 37 metres. Unlike the typical brown colouring that nurse sharks use for camouflage along the ocean floor, this shark’s vibrant orange hue made it stand out starkly against its surroundings.
According to researchers from the Federal University of Rio Grande, this peculiar colouration is due to xanthism—also known as xanthochroism—a rare condition of pigmentation. “Further research is needed to explore potential genetic or environmental factors influencing this rare pigmentation anomaly in sharks,” the researchers noted, according to The Independent.
Xanthism causes excessive yellow or golden hues due to a lack of red pigmentation. It has never been documented before in cartilaginous fish, the group of species that includes sharks, rays, and skates, in the Caribbean.
In addition to its golden-orange skin, the nurse shark also showed signs of albinism, particularly a pair of striking white eyes. These physical traits could make it more vulnerable in the wild. Normally, nurse sharks rely on their brown skin to blend into the seabed. Bright colouring and white eyes remove that natural defence.
However, the presence of both xanthism and albinism traits raises interesting biological questions. While some scientists believe that animals with xanthism are at a survival disadvantage due to their visibility to predators, this discovery challenges that theory.
The origin of the pigmentation is still uncertain. While xanthism is often the result of a genetic mutation, environmental factors such as diet could also play a role. Researchers believe this finding could lead to broader questions about the genetic diversity of local nurse shark populations. The full study was published in the journal Marine Biodiversity.
Xanthism is considered extremely rare across the animal kingdom. It’s previously been observed in a small number of fish, reptiles, and birds. For instance, vivid yellow morphs have appeared in freshwater fish, while parrots and canaries have been seen with golden feathers. Occasional reports have also emerged of yellow snakes and lizards exhibiting the condition.
On the opposite end of the pigmentation spectrum is axanthism, a condition marked by a lack of yellow pigment.
