Off the coast of Tortuguero National Park in Costa Rica, fishermen accidentally caught a two-meter-long nurse shark (Ginglymostoma cirratum) with a unique, intense yellow-orange coloration, even though sharks of this species are typically grayish-brown.

Biologists later studied the shark and concluded that the rare hue was caused by xanthism—a pigmentation abnormality that favors yellow or orange. However, xanthism has never before been observed in nurse sharks.

Questions for Researchers This discovery has raised questions about the causes of such anomalies. Could the pigmentation be the result of a spontaneous genetic mutation affecting pigment-producing cells, or is it an inherited recessive trait that only appears very rarely?

Could a diet rich in specific pigments, like what happens with some birds, play a role? Or do environmental stressors, pollutants, and changes in water chemistry affect pigment expression in sharks, as they sometimes do in other marine species (e.g., corals)?

Researchers have yet to find the answers to all these questions.