When a half-dozen researchers recently ventured into the Gulf of Mexico to investigate the effects of the Deepwater Horizon oil spill on marine life, they certainly weren’t expecting to find a polar shark. And yet, there it was, among the 44 species characterized during the 1,750-mile journey of the RV Apalachee: a 12-foot-long Greenland Shark.

When the Apalachee set sail on its surveying journey in late July, the plan was to catch as many fish and shark species as possible from the Gulf’s depths, taking tissue samples in order to determine how the infamous BP oil spill has affected large organisms over the last three years. At the other end of the biological spectrum, microbes seem to have consumed products of the oil spill much faster and more completely then many doomsayers had predicted, but what about the macrofauna? As predators, were they concentrating toxins found in their food sources?

Polycycling aromatic hydrocarbons, or PAHs, are the most dangerous constituents of oil, according to Jim Gelsleichter, the University of North Florida Professor handling much of the toxicology analysis for the study. PAHs are hydrophobic molecules, making them difficult to process in the cell’s decidedly aqueous metabolic environment. Adding lots of electrical charges to the molecules makes them easier to work with, but brings the added danger of causing PAHs to complex with DNA, another charged biomolecule. This interference can alter gene expression and cell cycle dynamics, often leading to runaway cell division; several PAHs are classified as carcinogens.

Chip Cotton, a Florida State University researcher involved with the expedition, explains that “the levels of exposure to PAHs in the Gulf are about equivalent to what you’d expect in an urban harbor, a Baltimore or a Miami.” Cotton believes this is both good news – “it wasn’t catastrophic” – and bad news – “we typically like to think of the deep sea as being pristine.”

When investigating the effects of these elevated concentrations, Gelsleichter looks for biological signals of PAH metabolism rather than the molecules themselves, as they are quickly processed by the fish of interest. He describes two phases of metabolic processing: one that oxidizes the molecules, and a second that attaches water-soluble molecules to the oxidation product. “In an area that is oil-exposed,” he says, “you should see higher levels of the genes for the enzymes involved in these two phases, and higher levels of the proteins themselves.”

Ever since the spill, that’s exactly what he’s seen, but the most recent set of samples are telling a new story: “there’s been a decline in enzyme levels,” notes Gelsleichter, “and a decline in associated metabolite levels.” This is not to say that the danger has passed: “it will probably be a decade or longer to see real long-term effects,” a scientific reality that ensures years of Gulf of Mexico deep-sea fishing expeditions.

And while the Apalachee’s goal was to examine the health of the entire deep-sea macrofaunal community, their Greenland Shark catch was particularly unexpected. The bizarre find seems to run counter to the more common pole-ward biome shifts that have been accompanying global warming: as the planet heats up, species must move away from the equator to experience the temperatures they’re used to. But it’s unlikely anything allegorical is going on, since the deep-water conditions at which the shark was caught are pretty similar around the world (~4 °C). In Greenland, however, cold temperatures persist closer to the surface, allowing the sharks to feast on surface-based organisms and making them more visible to ships, fishermen, and taxonomists. “It’s a very common phenomenon among deep sea fishes,” explains Cotton, “that as you approach the tropics, their distribution becomes deeper and deeper. No way this was the only Greenland shark in the Gulf.”