We're going to take it slow this Wet Beast Wednesday and talk about Somniosus microcephalus, the Greenland shark. Its one of the sleeper sharks, a family named for their very slow movement and generally low-activity lifestyles. Greenland sharks follow the rule that slow and steady wins the race. With a top speed of 2.6 km/h (1.6 mph), the Greenland shark has the slowest speed of its size of all fish. They also have a slow metabolism which correlates with a very long lifespan. The lifespan of these sharks isn't known for sure, but given that they grow at a rate of about 1 cm per year and can reach around 6 meters in length, they can live a pretty long time. Normal methods of aging sharks don't work on the GS. Sharks are usually aged by counting growth rings on fin rays or vertebrae, but Greenland sharks don't have fin spines and their vertebrae are too sift to form growth rings. In 2016, a paper titled "Eye Lens Radiocarbon Reveals Centuries of Longevity in the Greenland Shark (Somniosus microcephalus)" by Nielsen et al. was published that described a new aging method in which crystals in the eye lens could be radiocarbon dated to provide and age estimate. The oldest shark they tested was aged at 392 ± 120 years old. If the lifespan falls on the upper end of that range, it could give the GS a maximum age of 500 to 600 years old. The same paper also estimated that they don't reach sexual maturity until around 150 years old. The GS is officially listed as the longest-lived of all vertebrates.
It looks like somebody tried to draw a shark from memory (image: a Greenland shark swimming under ice)
The extreme age and slow lifestyle of the sharks is linked to their habitat. Despite the name, they are not only found around Greenland. In fact, they live in very cold waters (0.6 - 12 degrees C or 31 - 54 degrees F) in the north Atlantic and Arctic Oceans. They are the only shark species to live in arctic waters all year round. It is likely that they have a much larger habitat range than we know as they spend most of their time in very deep water. One specimen was found as far south as the Caribbean. It's possible that the north Atlantic and Arctic Oceans are the only places in their range that are cold enough for them to tolerate surface conditions and in the rest of their range they stay in deep, cold water. Being an ectothermic (cold-blooded) animal living in very cold conditions requires a very slow metabolism and slow metabolisms are linked to longer lifespans. A slow metabolism also requires less energy, which is very advantageous for seep-sea animals, who may be forced to go for months or longer between meals. The size of the shark may also be linked to its habitat. At up to 7 meters (23 feet) long and 1,400 kg (3090 lbs), the GS is one of the largest fish in the world. This size could be an example of deep-sea gigantism, a phenomenon in which deep-sea animals grow much larger than their shallow-water relatives. There are many proposed explanations for deep sea gigantism, including cold temperatures inducing growth (Bergmann's rule), improved foraging ability, defense against predators, and greater dissolved oxygen availability in colder and deeper waters. Because of their abyssal habitats, Greenland sharks are rarely observed in the wild and we know little about their natural behavior.
(Image: a scuba diver swimming next to a Greenland shark)
What does a large carnivore living in food-scarce deep water eat? Just about everything. Greenland sharks are generalists who will eat almost any meat. They are both scavengers and active predators, with a diet consisting primarily of squid as juveniles and fish as adults. Most of what we know about their diet comes from the stomach contents of dead specimens as they have very rarely been observed feeding in the wild. Seals also make up a large portion of their diet. As it's very unlikely that such a slow shark could chase down and catch a seal for lunch, the shark likely targets sleeping seals. Greenland sharks are scavengers who likely get a lot of nutrition from carrion. They have very sensitive noses that they use to track down rotting meat. Stomach contents have revealed meat from various animals, including polar bears, moose, and reindeer. They are known to follow fishing boats to snatch up any scraps. Greenland sharks are apex predators who play an important role in the Arctic ecosystem.
(Image: a Greenland shark captured by a fishing boat)
As mentioned above, Greenland sharks are estimated to reach sexual maturity at around 150 years old. They are ovoviviparous, meaning that the eggs hatch while still in the mother, who then gives live birth. A typical litter has around 10 pups (likely the maximum given the metabolic requirements of reproduction) that are about 38-42 cm (15-16.5 in) long. Due to their lifespan, it is estimated that a single female could produce between 200 and 700 pups during her life. Unconfirmed reports have been made of them hybridizing with other sleeper sharks.
(Image: a 100-year old juvenile shark that washed up on a beach in England)
Greenland sharks are often found infested by the parasitic copepod Ommatokoita elongata. This parasite attaches itself to the cornea of the eye and infests both the Greenland shark and the Pacific sleeper shark and dangles out of the eye like a thread. The presence of the copepod damages the eye, leaving the shark almost if not entirely blind. This doesn't actually seem to hurt the shark that much as they rely mostly on smell and hearing. One hypothesis stated that the copepod may bioluminesce to attract prey to the shark, but this remains unconfirmed.
(Image: a close-up of a Greenland shark eye infested with a copepod)
Now to address the elephant seal in the room: Greenland sharks stink. All elasmobranchs have concentrations of the compounds urea and trimethylamine N-oxide (TMAO) in their bodies, but the GS has a higher concentration of these compounds than other species. The compounds are waste products retained by elasmobranchs to aid in buoyancy and the GS has more because they also help resist the pressure of the deep ocean. The concentration of urea is high enough that you can smell it if the shark is near you. Urea is a key component of urine. Greenland sharks smell like piss. The high levels of TMAO makes Greenland shark meat toxic to humans. Despite this, people still eat the meat, but it must be treated first. This can be done by boiling the meat multiple times or by fermenting it. A national dish of Iceland, kæstur hákarl, consists of Greenland shark meat that has been buried underground in gravelly sand and pressed with stones, then left to ferment for 6-12 weeks. It is then dug up and cut into strips, which are hung up and left to dry for several months. Apparently first time eaters are known to gag because of how much ammonia is in the meat. I don't think I'll try any.
(Image: hákarl hung up to dry)
Because of their deep-sea habitat, Greenland sharks have relatively few interactions with humans. In the past, They were hunted for their liver oil, but in modern days, there is no fishery for them. Greenland sharks are classified as vulnerable by the IUCN. Their primary threat is bycatch and they are likely being affected by ocean warming and loss of sea ice in the Arctic. Because of their slow maturity and low fecundity, Greenland sharks are particularly vulnerable to population loss. There are no recorded attacks on humans. The species features in the legends of the Inuit. The legendary first Greenland shark was named Skalugsuak and one story says it was born after a woman washed her hair with urine (a lice remedy) and dried it with a cloth. The cloth then blew into the ocean and turned into Skalugsuak, who still smells like urine. Another legend says that the shark lives in the urine pot of Sedna, the sea goddess, explaining its smell.
(image: scientists tagging and releasing a small Greenland shark)
