Hunting in the the twilight zone: are Greenland sharks capable of active predation?
The Greenland shark (Somniosus microcephalus) is the only native species of arctic shark. The Greenland shark grows slowly, and reaches an average total length of >500cm; suggestive of a life span much greater than that of any known living vertebrate. An article in Science recently showed that through radiocarbon dating of eye lens nuclei, Greenland sharks can attain a life span averaging at least 272 years; with sexual maturity arising at around 150 years of age. Are slow growth rates demonstrated by this species an adaptation to living in a cold environment?
Species distribution and habitat
Greenland sharks typically inhabit arctic and subarctic waters with temperatures varying between -2 and 7 °C. Greenland sharks are widely distributed in Arctic waters and are frequently caught as by-catch in some fisheries. Despite this, they have rarely been observed due to their preferred habitat in the bathybenthic environment, i.e. above the sea floor, typically frequenting depths with potential pressures of >4000psi.
The species does however occasionally make vertical excursions into the water column (Figure 1). Sharks have been known to leave the sea bottom and “stalk” divers returning to the surface; indicative of the shark’s potential hunting ability to ambush prey in conditions of low visibility. Considered to be a resident species, a recent study involving pop-up archival transmitting tags (PATs) in the Canadian Arctic and the north-western Atlantic, discovered that Greenland sharks are a wider ranging species than what was previously suspected. Tagging results indicated that sharks travelled between 315 and 1615km at an average depth of 1816m; with mature individuals moving into deeper waters away from the continental shelf. Such movement and diving patterns would be consistent with the theory that mature individuals migrate offshore to mate and/or to give birth. Alternatively, individuals may be following a “scent trail”, either to scavenge or to seek live prey. This would be consistent with the observations of short term vertical excursions made by sharks.
Contrary to their docile appearance, scientists and local Inuit have found the remains of seals within the stomachs of Greenland sharks – promoting the idea that Greenland sharks can somehow hunt seals. Here, arctic underwater cinematographer Adam Ravetch discusses the Greenland shark, presented by One World One Ocean.
Arctic underwater cinematographer Adam Ravetch discusses the deep-dwelling and seldom seen Greenland shark, providing comments on the species’ habitat, association with an ocular parasite, dentition, and potential hunting capabilities. Video credit: One World One Ocean/ YouTube (channel)
Combating the cold: consequences to feeding
Despite their large size, Greenland sharks ironically have small fins and lack an anal fin, ultimately reducing maneuverability. Thought to weigh around 900 kilos, the Greenland shark is lethargic, somewhat sluggish is appearance. But due to an asymmetrical caudal fin, the Greenland shark is capable of short bursts of speed. In cold environments, locomotory muscular function is generally depressed in ectothermic fishes, i.e. “cold-blooded” organisms whose regulation of body temperature is dependent upon the external environment, such as water temperature. Making species typically seen as prey items vulnerable to predation from avian or mammalian organisms due to their constantly elevated body temperature.
Paradoxically, the reversal of the usual predator-prey thermal pattern has been observed by analysing the swim speed and tail-beat frequency of Greenland sharks. Amongst all known fishes studied to date, Greenland sharks have the slowest swimming speed (0.34 ms-1) for their size; a direct result of a low tail-beat frequency. The low tail-beat frequency may be attributed to low environmental temperatures experienced by the sharks. Low temperatures may have a depressing effect on muscle function, to which the species has apparently not compensated for. However water viscosity must also be taken into account, i.e. the density of water increases linearly with depth and declining temperature. The external medium is therefore harder to move through, consequently having an effect upon swimming speed. How then, can Greenland sharks predate live seals?
It can be hypothesised that they may be targeting sleeping seals resting at breathing holes within the sea ice. Greenland sharks may be exploiting the seal’s behaviour, enabling them to exert little energy and to stage an ambush, though such behaviour has yet to be observed. The “corkscrew controversy” suggested that the appearance of mutilated seals with wounds resembling corkscrew-like twists may potentially be the result of Greenland sharks actively predating upon seals (Figure 2). However the corkscrew wound has not definitively been associated with Greenland sharks; such injuries could actually be the result of accidental collision with vessels associated with offshore construction or drilling activities. Further research would be required to determine whether Greenland sharks are behind the occurrence of such injuries.
The Greenland shark’s dentition is comparable to that of the Cookiecutter shark, Isistius brasiliensis. The upper jaw is comprised of slender, pointy teeth lacking serrations, whereas teeth on the lower jaw are substantially larger, broader, interlocking and cusped (Figure 3). Whilst feeding, the narrow upper teeth act as anchors, whereas the lower teeth slice into the prey. The shark cuts out a round “plug” of flesh via the shark swinging its head in a circular motion. The Greenland shark, therefore, would appear to be more than capable of taking a bite out of a mammalian prey item.
Hunting in the twilight
What makes the species even more incredible? It’s thought that over 90% of Greenland sharks are completely blind. Less than 10% of the Arctic population of Greenland sharks do not play host to a copepod parasite, Ommatokoita elongata (Figure 4). The female of this species attaches itself permanently to the cornea of the eye, creating ocular lesions. Either one or both eyes can be encompassed by the parasite – over time impairing the shark’s eyesight or rendering it completely sightless. However, parasitic infections do not significantly affect the shark. Acute vision is generally impaired at depth where the habitat is devoid of light; the shark will therefore rely upon other senses, i.e. smell, in order to conduct searches for food in the darkness.
So can this blind shark, living in almost complete darkness, actually hunt living prey? Previous studies dictate that the Greenland shark possesses the slowest swimming speeds of any studied fish for their size, suggesting that they aren’t capable of actively capturing a swimming seal. Alternatively, Greenland sharks may be an ambush predator, slowly stalking sleeping seals resting at breathing holes. So if they are in fact hunting sleeping seals, maybe vision isn’t all that important. That being said, the behaviours of Greenland sharks are a still a mystery to science. Do they hunt? It appears so. Do they scavenge? Yes. Time and further studies may reveal the truth behind the Greenland shark’s hunting capabilities.