Greenland shark

The Greenland shark, known scientifically as *Somniosus microcephalus*, is a massive species of shark. It belongs to the Somniosidae family, which includes other "sleeper sharks." These sharks inhabit the cold waters of the North Atlantic and Arctic Oceans. They are famous for their incredible longevity. In fact, they have the longest lifespan of any known vertebrate. Scientists estimate they can live between 250 and 500 years.

To survive in freezing, deep environments, these sharks have unique biological adaptations. They are ectotherms, meaning their body temperature follows the water around them. Because they live in water near freezing, they are very sluggish. They have the lowest swim speed and tail-beat frequency for their size among all fish. Their average speed is only 1.2 kilometers per hour. This slow movement is likely linked to a very slow metabolism. This metabolic pace helps them achieve their extreme lifespans.

Feeding is a specialized process for these apex predators. They are generalist feeders, meaning they eat many different things. They consume fish like cod, halibut, and salmon, as well as seals and even minke whales. They also act as scavengers, eating carrion or rotting meat. To eat large carcasses, they use a rolling jaw motion. The 48 to 52 teeth in the upper jaw are thin and pointed. These act as an anchor for the prey. Then, the broad, square teeth in the lower jaw interlock to cut large chunks.

Their internal chemistry also helps them handle the deep ocean. Greenland sharks have high concentrations of urea and trimethylamine N-oxide, or TMAO, in their tissues. These nitrogenous compounds increase buoyancy. TMAO also acts as an osmoprotectant. It helps protect their proteins from being destabilized by urea and high deep-water pressure. Their blood contains three major types of hemoglobin. These proteins help carry oxygen. These specific hemoglobins have a lower affinity for oxygen than temperate sharks. This is another adaptation for life at great depths.

Researchers have learned much about these sharks through history and genetics. In 1936, a shark was tagged near Greenland and recaptured in 1952. This showed they grow at a rate of about 1 centimeter per year. Scientists estimate their age by using radiocarbon dating on crystals in their eye lenses. In 2024, the shark's genome was published. It is 6.45 billion base pairs long. This data helps scientists study how they live so long. They are looking at how certain genetic elements might play a role in their aging.

While they are huge, they are not a common threat to humans. They live in deep, remote areas. One possible attack on British fishermen occurred in August 1936, but it was never identified. However, their meat is quite special. It contains high levels of TMAO, which makes it toxic to mammals. In Iceland, people eat a treated version called kæstur hákarl. This is a pungent delicacy.

Greenland sharks also have interesting relationships with other creatures. They are often infested by a crustacean called *Ommatokoita elongata*. This parasite attaches itself to the shark's eyes. This can cause damage like ulceration or even blindness. Interestingly, this does not seem to reduce their lifespan. The sharks rely heavily on smell and hearing to hunt. Some scientists think the parasite might even help the shark by using bioluminescence to attract prey. This is a theory called a mutualistic relationship, though it is not yet verified.

Finally, these sharks have a much wider range than once thought. They typically prefer water near 0 degrees Celsius at depths of 200 to 700 meters. However, they have been found in the Gulf of Mexico at 25 degrees Celsius. They have even been spotted near coral reefs in Belize. They migrate annually based on temperature and depth. During winter, they move to shallower waters for warmth. In summer, they migrate to deeper areas. This movement helps them navigate the changing Arctic and Atlantic ecosystems.