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| | Adaptations of Seals to the Marine Environment
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| Seals need to be streamlined in order to move through the water with the minimum amount of resistance or "drag". External structures are absent e.g. ear flaps are internal, limbs are reduced and the male genitalia and female mammary glands are withdrawn into the body.
All seals are covered in a layer of blubber, a thick layer of fat found between the muscle and the skin. Its thickness varies with age, body size and energy balance. It is built up when food is abundant and used as an energy source when required, e.g. during lactation and migration. It
may be up to 6 cm thick over the chest of adult grey seals.
| Blubber gives the body of seals a smooth outline, and thus also helps to reduce drag. As an insulating layer, it is superior to hair in the water, as it does not readily collapse under pressure. It thus retains its insulating properties at depth, allowing seals to live in waters only slightly above freezing. In young pups, hair is still the primary insulator until a layer of blubber is produced.
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Seals´ eyes are large and adapted to life in the water, with a surface (cornea) that is specially strengthened.The shape and composition of the cornea, coupled with a nearly spherical
lens, allows accurate focusing underwater. The light receptive lining of the eye, the retina,
is specially adapted to function in the dim light found in the depths of the ocean. On land, seals´ sight is not so efficient. Vision in bright light with the pupil constricted is good, but in dim light with the pupil dilated, images are blurred.
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Seals have to come ashore to give birth, suckle their young and to moult (the annual replacement of the coat). They will also haul our between feeding bouts at any time of the year. On land, seals are clumsy movers. Keeping their hind flippers clear of the ground, the animals "hump" forwards on their chest and hindquarters, often aided by their fore flippers.
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Usually grey seals dive for no longer than 10 minutes. Recovery time from a dive of this length is quick and the seal can soon dive again. If a seal dives for longer (up to half and hour), more time has to be spent recovering on the surface before diving again. Oxygen needed to complete these dives is not stored as free air in the lungs. If it was, seals would be too buoyant to dive efficiently and would also be prone to the "bends".
| The bends is an extremely painful and potentially fatal condition which occurs in divers when nitrogen, from inhaled air,dissolved under pressure in the blood, comes out of solution too
rapidly as the pressure is released on surfacing. Instead, oxygen is efficiently stored attached to pigments in the muscle (myoglobin) and blood (haemoglobin). A large blood volume also
increases the amount of oxygen that can be stored prior to diving, and the heart rate slows
by a third, preserving oxygen during dives.
The seal propels itself through the water by a fish-like lashing of the two hind flippers, with the toes spread in a web. The front flippers help achieve a sudden spurt of acceleration, if required. Both sets of flippers are used for steering and the front flippers are also used for stabilising the animal when it is stationary.
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Grey seals will eat crabs and squid, but they mainly feed on fish, which they will dive as
deep as the sea bed to recover. Prey is located by sight, sound and possibly vibrations which are picked up seals, sensitive whiskers, and then captured by short bursts of speed, extension of the neck and a snapping of the jaws. Small fish are swallowed whole, but large fish are held in
the fore flippers and ripped apart.
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Killer whales occasionally prey on grey seals in British waters. Man, however, is the greys
seal´s major predator. In Britain, hunting for seals for food has been carried out for
centuries. Commercial hunting for skins began in the eighteenth century and by 1914
only 500 grey seals were thought to be left. Laws passed in 1914 and 1970 limited hunting
to outside the breeding season, allowing grey seal numbers to recover. This has brought
them into direct conflict with fishermen. Seal culls were carried out in the 1960´s and
1970´s, but have since then ceased due to lack of proof that seal numbers affect catches.
Seals are still shot legally all year round if caught near fishing nets.
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