Whatever it was, the creature was feisty. The fishermen had spent the last half-hour trying to pull it toward the surface. Hooked under the water, it could be a red snapper or an amberjack, they thought — powerful, large game fish known for their strong fighting ability.
What emerged was something entirely different.
“We saw an orange glow below the water and I said, ‘My God, what is this?’” said Garvin Watson, 43, a sport fishing guide and hotel owner in Barra del Parismina, a small town located on the Caribbean coast of Costa Rica. “We were all screaming like crazy.”
A bright, tangerine-colored nurse shark wriggled alongside the boat. It was unlike any other shark that Mr. Watson, the descendant of a long line of fishers, had ever encountered.
“I still get goose bumps simply remembering what happened,” he said.
Mr. Watson caught the six-foot nurse shark — mostly docile animals whose snouts have a hint of catfish to them — in August last year, and it garnered some attention after he posted photos of the animal on social media. Researchers quickly noticed, too, realizing this could be the first recorded case of an orange shark.
“I saw the photos and I was impressed,” said Daniel Arauz Naranjo, a marine biologist and executive director of Costa Rica’s Rescue Center for Endangered Marine Species. “I was like, ‘Wow, this is something strange.’”
This month, Mr. Arauz Naranjo and his colleagues published a study on the shark in the peer-reviewed journal Marine Biodiversity. They concluded that the nurse shark’s odd pigmentation — the intense yellow-orange hue on its body and ghostly white eyes with no visible iris — might be the result of two rare genetic conditions taking place at the same time: albinism and xanthism.
The first condition strips any pigment from the body, including the eyes. The second causes an excess of yellow pigments.
Seeing both conditions at once can be extremely rare, mostly because animals with them might become easy targets for predators.
“It’s like carrying a warning sign with you all the time,” said Arturo Angulo, an ichthyologist at the University of Costa Rica who was not involved in the study.
Nurse sharks, Mr. Angulo added, typically rely on their brownish color to camouflage on the sea floor, which is useful for hiding from threats, but also for surprising prey they vacuum with their large throat.
But he has documented the same color anomaly in some freshwater fish called cichlids. Gene mutations are the most plausible explanation, but researchers think other factors, such as stress, diet or pollutants in the water might also play a role.
Clearly, though, the shark discovered last year had no trouble surviving.
Mr. Arauz Naranjo speculated that the murky waters in the area might have helped it when it was smaller, hiding its flashy colors from hungry predators. “This guy was lucky to have lived in this environment and to not have run into any other larger sharks,” he said.
The discovery has raised questions about whether this is nothing but an isolated case or whether it might signal an emerging trend in the local population of nurse sharks.
“I find it amazing that at this point in time somebody can still go out to sea, near the coast, and discover something that hasn’t been recorded for science yet,” Mr. Arauz Naranjo said. “The sea still has surprises for us.”
Where the shark is now, only it — and any other sea life surprised to see an orange predator swimming around — would know.
Mr. Watson said a group of biologists had contacted him, aiming to dive at the same site where he spotted the animal next year — hoping to find it again, or at least study whether something in the environment could be related to its peculiar color.
But the thought of keeping the nurse shark never crossed Mr. Watson’s or his crew’s minds, he said.
Before letting it go, though, he approached the creature and kissed its sandpaper-like back, he said.
“Thank you for making this trip such a happy experience. Thank you for being here at the exact right moment, by my boat. Thank you for taking my bait,” Mr. Watson remembers telling the animal. “Thank you. We love you.”
Emiliano Rodríguez Mega is a reporter and researcher for The Times based in Mexico City, covering Mexico, Central America and the Caribbean.
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