The scientists looking at the strangely colored animal.

Provided by Andrej A. GAJIĆ

The deep sea is often imagined as a place where color does not matter. After all, sunlight fades quickly with as you plunge down into the depths, leaving much of the ocean in dim blues and eventually darkness. But for the animals living there, color can still mean the difference between staying hidden and being seen. Recently, researchers examining a rarely encountered deep-sea predator discovered something unexpected: a sevengill shark whose unusual coloration may have come with a serious energetic cost. That single individual is now helping scientists rethink what pigmentation really means in the lives of sharks.

“When Glorimar Franqui-Rivera, Ph.D. of the University of Puerto Rico first sent me the images from her systematic deep-sea surveys in Puerto Rico, I immediately realized we were looking at something extraordinary. The pigmentation pattern did not fit any of the previously described anomalies in elasmobranchs,” said General director of the Sharklab ADRIA Research Center, Andrej A. Gajić, Ph.D. res. vet. med. lead author of the new study. “At that point, it became clear that this was not just a visual irregularity but potentially something biologically meaningful. We initiated the process to secure the specimen for detailed analysis, and with the support of the Ministry of Environment in Albania, we were able to transfer it to our center and conduct extensive examinations.”

The shark in question was a sharpnose sevengill shark, Heptranchias perlo, a species most people have never heard of. Although wide ranging across tropical and temperate regions of the Atlantic, Indian and Pacific Oceans, they are a relatively uncommon species. They have an elongate, slender body with seven gill slits (hence the common name), a single dorsal fin positioned far back near the tail and distinctive fang-like upper teeth paired with comb-like lower teeth, and can reach about 4.6 feet (1.40 meters) in total length. Adults are typically brownish-grey above and paler below with white fin margins, while juveniles show darker fin tips and blotches along the lower sides, and freshly caught individuals may appear fluorescent green. A carnivorous predator (they feed mainly on squid, bony fishes and crustaceans), although they are not directly targeted by fisheries, they are regularly taken as bycatch in deepwater operations across its range. This one was retrieved as incidental bycatch from nearly 400 meters deep off Puerto Rico. Yet instead hauling out an individual with their typical brownish-grey back and pale underside, this one was… different. It was a mosaic of dark blotches, pale regions and completely pigment-free white patches scattered across its body.

It was not albino.

It was not leucistic.

It was something else entirely.

Pigmentation in sharks is controlled largely by melanophores, specialized cells that produce dark pigments. These cells help create countershading, one of the most important camouflage strategies in the ocean. Seen from above, a darker back blends with deeper water. Seen from below, a lighter belly blends with the brighter surface. This simple gradient reduces visibility from multiple angles and helps sharks both avoid predators and approach prey undetected. If camouflage fails, animals may need to spend more energy escaping predators or searching longer for food. That makes coloration more than just a cosmetic quirk but part of an animal’s energetic strategy. “Most reports treated abnormal pigmentation primarily as a descriptive phenomenon, often without clear biological implications. Some studies suggested potential ecological costs, but others, including our own work on deep-sea species such as the angular rough shark, showed that conditions like albinism or leucism do not necessarily compromise overall health. As a result, the functional significance of pigmentation anomalies remained largely unresolved,” explained Gajić.

Thus, the team delved deeper into the mysteries of this creature, going past the unusual appearance to seeing what exactly was inside its body. Compared with similar-sized males, this individual weighed far less than expected and its liver was smaller than normal and contained dramatically reduced lipid reserves. “In sharks, the liver is central to energy storage, metabolism, and buoyancy regulation, particularly in deep-sea species where lipid reserves are critical for survival,” Gajić pointed out. So, losing those reserves is a serious signal that something is wrong. Its stomach was also empty. Together, these clues suggest the shark had been living in a prolonged state of energetic depletion. But the question is why. And was this shaped by an unknown environmental or developmental factor earlier in life? Currently, we do not know.

Unlike uniform color changes such as albinism, this shark showed a patchwork pattern combining darker-than-normal regions with partially pigmented zones and areas lacking pigment entirely. These transitions were gradual rather than sharply defined, creating a mottled appearance never before documented in this species or any deep-sea shark like it. That kind of disruption might be interfering with its camouflage more strongly than previously recognized pigmentation disorders. “Even at depth, light gradients and visual detection remain ecologically relevant. Many sharks rely on countershading to remain undetected by both prey and predators. Disrupting this balance can reduce foraging efficiency and increase exposure risk, particularly during vertical movements or when interacting with visually adapted or bioluminescent prey,” said Gajić, who believes that as environmental pressures intensify, we may begin to observe more deviations from typical biological patterns (however, detecting meaningful trends will require long-term monitoring, detailed clinical assessments, and consistent documentation).

Gajić says that if pigmentation reflects underlying physiological stress, then the liver provides one of the most direct indicators of energetic status. But since full histological analysis was not feasible in this case, they applied a complementary approach by examining liver lipid composition and comparing it with conspecific individuals from both Puerto Rico and the Adriatic, allowing the team to directly test whether the anomaly was associated with measurable physiological consequences. “The individual was visibly emaciated and in compromised condition. However, the extent of depletion we observed in both condition indices and lipid signatures was still striking and confirmed that the anomaly was linked to a severe chronical disruption of energy balance.” Compared with similarly sized males, its condition factor was about 37% lower than expected, while the hepatosomatic index (a key indicator of liver-based energy reserves in deep-sea sharks) was reduced by more than 60%, pointing to markedly diminished internal energy storage. This pattern was also seen via biochemical evidence with total hepatic lipid content being only 16% compared with 56% in a phenotypically normal conspecific, representing roughly a 70% proportional reduction. Spectroscopic analyses further supported their hunch, showing weakened triglyceride signals and features consistent with lipid degradation. Together, these independent metrics indicate pronouced chronic energetic stress, suggesting the individual had experienced prolonged depletion of energy reserves.

The deep sea still holds more unknowns than answers. And this is especially true for deep-sea species like the sharpnose sevengill shark, where scientists rarely encounter individuals at all. Sometimes a single specimen is the only window into an entire biological question. And that one shark can change how researchers frame future investigations.

The article can be read in full here. The research team is deeply grateful to the The Explorers Club and Save Our Seas Foundation for their generous support in funding this work.