The hunt is on- Adaptations of the Weddell seal
The Weddell Seal (Leptonychotes weddellii) is an abundant, non threatened species of seal and is one of the six species of seal that live in Antarctica, and amongst the four which are are specialists in ice-habitats. This seal has captured the imagination of many people with its large eyes and its chubby ‘bean-bag’ like body, but under the water they are one of the most efficient predators within the extreme environment of Antarctica.
It is the most southern dwelling mammal and inhabits coastal circumpolar fast ice, which exposes it to air temperatures of -10 to -60°C, and sea water temperatures around -1.8°C. Due to feeding beneath the ice, its diet comprises mostly of fish, prawns and octopus. To feed in the water under the ice, this Seal has developed specific adaptations which allow it to access, see and capture its prey efficiently in order to survive.
Diving and Gnawing…far from boring
With a streamlined body, the Seal is able to manoeuvre through the water and its thermally efficient skin and blubber layer make it a highly effective diving predator. Submersion can often extend for up to 80 minutes on a single breath, allowing hunt distances as far as 5km from a ‘breathing hole’. Even with dive times less than 25 minutes, the Seal is capable of diving between 100 and 350m with record depths around 700m. A ‘breathing hole’ is a crack in the ice which enables the Seal to haul out on the ice whilst having access the water below to hunt. These cracks are essential to these Seals as they facilitate their survival on the ice year round. During the warmer months, these cracks are more readily available, however, when the temperatures drop in the winter time, the holes freeze over. The Weddell seal then maintains the hole using its teeth. To do this, the seal opens its mouth and moves its head from side to side relatively vigorously. This method has a negative effect on the teeth and life of the seal and studies suggest that the condition of the teeth correlated to the early mortality of the adult Weddell seal.
A video of the ‘sawing action’ using teeth to maintain breathing holes Sourced from: jessiefudge13
Vision is a primary method of navigating and orientating the Weddell sea above and below ice. In order to hunt, in low light conditions with an exposure to snow, wind and cold water, the Weddell seal has specially adapted eyes. To protect the eyes against the ‘elements’ and salt in the water, it has a nictitating membrane. This membrane is similar to that of our domestic mammals but is more fundamental for the survival of this seal due to its reliance on its vision for the detection and intake of food.
At the back of the seals eye, behind the retina , it has a tapetum layer which is identifiable as the glow you may see in a cats eyes during dim lighting. This layer increases the light reflected onto the retina which aids the vision of the seal in dim lighting and with deep dives under the ice, which is most prevalent during the austral winter.
As a point of reference, studies have shown that the eyes of pinnipeds, the sub-order in which the weddell seal belongs to, are similar to the eyes of nocturnal mammals in that they have higher light sensitivity and can adapt to darkness fast.
Capturing its prey
Once below the ice, the challenges of low lighting are prevalent with regards to locating and perusing prey. Although the eyes of the Weddell seal are adapted to these low light conditions, light is still necessary to locate prey. The image above gives an idea of the light at the surface of the water however it is noticeable that, under the ice, viability is reduced. One method that the Weddell seal has adapted is the ability to position itself in the water to use the light through the ice as a form of ‘back lighting’ The seal also swims under the prey which creates a silhouette of the prey.
Once the seal has located its prey, it will then peruse, however, the fish are able to hide in cracks and crevices in the ice. To overcome this, the seal has been observed blowing bubbles out of the its nostrils into crack in the ice in order to essentially ‘flush out’ the prey items to then catch. If the seal is unable to flush out the prey and it is hidden in loosely compacted platelet ice, the seal will pursue the fish into the ice numerous times until successful. A further method of prey capturing and consumption is the action of sucking in prey which are up to 21cm in length. Once the seal has caught its prey, depending on its size, the seal may consume it wholly, partially or will ‘stash’ the catch at a breathing hole. A reason for conserving the catch is linked to the size of the prey, for example a toothfish, seen in the image above, can weigh up to 100 kg. Due to the difficulty in finding and successfully catching prey, the behaviour of preserving or defending prey aggressively indicates survival instincts.
To conclude, the Weddell seal has developed specific and effective adaptations to living in this extreme environment in order that it might survive- some of which can induce early mortality. These adaptations make it one of the successful predators able to inhabit the harsh environment of Antarctic.