The Weird Lifestyle of the Longest Living Vertebrates on Earth
Living in the cold deep waters of the Atlantic and Arctic oceans can be a difficult endeavor; there is little food, the water is colder and more saline than the surface water and mates are hard to come by. To cope with these conditions organisms evolve to age more slowly to maximize the time taken to grow, the true king of this longevity is without a doubt the Greenland Shark.Reaching sexual maturity at an estimated 150 years, and estimated to live up to 514 years the Greenland Shark is often claimed to be the longest living vertebrate.
The Greenland Shark is a strange looking animal named Somniosus microcephalus due to it’s small head, as micro means small and cephalus refers to the head or brain (Image Credit; wikicommons)
What is a Greenland Shark?
Greenland sharks are members of the family Somniosidae, commonly known as the sleeper sharks with their closest relatives being the Southern Sleeper Shark. Greenland sharks like other sleeper sharks live in deeper waters below 200 meters from the surface.
Greenland sharks are generally safe from hunting by humans since their flesh has been historically known to be toxic by the Inuit people, possibly due to high levels of trimethylamine which is dangerous to most animals. The toxicity of the flesh however has recently been questioned with research finding that in small doses it is fine to eat and the Swedish people after processing the meat consume it in a dish known as Kæstur hákarl.
Kæstur hákarl is prepared by soaking the meat in salt water then aging it in special barns, without this treatment the meat could be dangerous to eat (Image Credit; WikiCommons)
Reproduction and Survival
As the Greenland Shark takes 150 years to reach sexual maturity then it must be incredibly vulnerable to exploitation since population recovery will take a long period of time. The Greenland Shark to an extent does not need any adaptations to avoid predators since it has no known predators due to it’s large size, however it is commercially exploited by many national fisheries. When young are observed both oviparity and vivparity (egg laying and live births) have been reported, these findings are the result of stomach analysis of very young individuals however due to a lack of easy to find populations little is actually known about the reproduction or early life of the Greenland Shark. Later analysis of adult females found that they were capable of giving birth to yolk dependent young suggesting that they give birth to lecithotrophic offspring (young which rely on a yolk-sac in order to feed until they reach such a size that they can feed themselves). However due to a deficit in research the top number of pups per litter is unknown, however a litter of 10 has been observed.
Unique Parasites and Their Effects
The deep seas are often so barren that parasites can rarely find a host so they are often very rare, this unfortunately for the Greenland Shark is not the case. A copepod crustacean (Ommatokoita elongata). has evolved specifically to inhabit the the eye of the Greenland shark. This relationship was initially thought to be mutualistic benefiting both the copepod and it’s host however, as observed now it’s attachment site is opaque; this renders the eyes far less sensitive with large blind spots present, so the Greenland Shark has permanently reduced vision.
An eye with associated parasite as you can see scar tissue builds up around the entry point causing further issues with vision (Image Credit)
Whilst the reduced vision may initially be thought to damage the overall fitness of the animal, however research suggests that the Greenland Shark actually does not use it eyesight in hunting and as it has no known predators the lack of eyesight does not effect the animal. The effects or lack thereof caused by Ommatokoita elogata are due to the Greenland Shark’s reliance on olfactory senses; such as chemical cues, and electromagnetic radiation when foraging and hunting the reliance on these senses both makes the Greenland Shark effective at hunting in the dark and allows it to hunt without eyesight and since it has no known predators, predator evasion is not needed.
What does a Greenland Shark Eat?
A Greenland Shark’s diet is mainly comprised of fish that it encounters in the water column or of animals it finds on the seafloor, and this was historically thought to be the only extent of the species’ diet. More recent studies have found a range of food sources including mammals, jellyfish, other sharks and many other groups; this supports the idea that Greenland Sharks are opportunistic feeders eating whatever they can find rather than a specialised predator targeting a specific species or group. This opportunistic lifestyle is best suited to those who live in the deep sea, as often animals can go months without finding any food so to eat anything means that an organism is likely to feed as often as it finds food
Greenland Sharks are one of the few species in our oceans that we have not exploited on a large scale, whilst this means the organism itself is not directly killed by our practices. Overfishing of important prey species and ocean acidification are still a threat to all life in the sea, however at least for now within the Arctic, the Greenland Shark rules the deep.