The hooded seal or bladdernose seal (Cystophora cristata) is a large phocid found only in the central and western North Atlantic, ranging from Svalbard in the east to the Gulf of St. Lawrence in the west. The seals are typically silver-grey or white in color, with black spots that vary in size covering most of the body. Hooded seal pups are known as "blue-backs" because their coats are blue-grey on the back with whitish bellies. This coat is shed after 14 months of age when the pups molt. It is the only species in the genus Cystophora.

Naming

The generic name Cystophora means "bladder-bearer" in Greek, from the species' unusual sexual ornament – a peculiar inflatable bladder septum on the head of the adult male. This bladder hangs between the eyes and down over the upper lip in the deflated state. In addition, the hooded seal can inflate a large balloon-like sac from one of its nostrils. This is done by shutting one nostril valve and inflating a membrane, which then protrudes from the other nostril.

Size

thumb|Skull of a hooded seal

Adult males are long on average, can grow to 3.5 m, and weigh . Sexual dimorphism is obvious from birth and females are much smaller: long and weighing . The color is silvery; the body is scattered with dark, irregular marks. The head is darker than the rest of the body, and without marks.

Distribution and habitat

Hooded seals live primarily on drifting pack ice and in deep water in the Arctic Ocean and North Atlantic. Although some drift away to warmer regions during the year, their best survival rate is in colder climates. They can be found on four distinct areas with pack ice: near Jan Mayen Island (northeast of Iceland); off Labrador and northeastern Newfoundland; the Gulf of St. Lawrence; and the Davis Strait (off midwestern Greenland). Males appear to be localized around areas of complex seabed, such as Baffin Bay, Davis Strait, and the Flemish cap, while females concentrate their habitat efforts primarily on shelf areas, such as the Labrador Shelf. Hooded seals are known to be a highly migratory species that often wander long distances, as far west as Alaska and as far south as the Canary Islands and Guadeloupe.

Diet

The diet of the hooded seal is composed primarily of various amphipods (crustaceans), euphausiids (krill), and fish, including Atlantic argentine, capelin, Greenland halibut, flounder, polar cod, herring, and redfish. They also are known to eat octopus, squid, sea stars, shrimp, other crustaceans, and mussels. Relative to the other species, hooded seals consume 3 times the proportion of redfish; percentages of capelin were similar in relation to closely related species.

Behavior

Hooded seals tend to feed in relatively deep waters ranging from , and dive from 5 to 25 minute durations. However, some dives can go deeper than and as long, or longer, than 52 minutes. Diving is rather continuous, with approximately 90% of their time spent submerged during the day and night, although dives during the day are generally deeper and longer. Dives during the winter are also deeper and longer than those in the summer. It is known that the hooded seal is generally a solitary species, except during breeding and molting seasons. During these two periods, they tend to fast as well. The seals mass annually near the Denmark strait around July, at the time of their molting periods, to mate. Hooded seals are a relatively unsocial species compared to other seals, and they are typically more aggressive and territorial. They demonstrate aggression by inflating the "hood" (which is explained in the "Nasal Cavity" section below). They frequently migrate and remain alone for most of the year, except during mating season. The hood begins to inflate as the seal makes its initial breath prior to going underwater. It then begins to repetitively deflate and inflate as the seal is swimming. The purpose of this happening is for acoustic signaling, meaning that it occurs when the seal feels threatened and attempts to ward off hostile species when competing for resources such as food and shelter. It also serves to communicate their health and superior status to both other males and females they are attempting to attract.

Breeding and life cycle

There are four major breeding areas for the hooded seal: the Gulf of St. Lawrence; the "Front" east of Newfoundland; Davis Strait (between Greenland and northern Canada); and the West Ice near Jan Mayen. Male hooded seals are known to have several mates in a single mating season, following the hypothesis that they are polygynous. While some males will defend and mate with just one female for long periods of time, others will be more mobile and tend to mate with multiple females for shorter periods of time, generating maximum offspring within the population. Most males reach sexual maturity by 5 years of age.

Throughout all areas, the hooded seals whelp in late March and early April and molt from June to August.

Offspring

Hooded seal pup (next to researcher) on ice in the Gulf of St. Lawrence|thumb

Pups are about long at birth and weigh about . They are born on the ice from mid-March to early April with a well-developed blubber layer and having shed their pre-natal coat. They are born with a slate blue-grey coat (giving them the name "blueback"), with a pale cream color on the belly, which they will molt after about 14 months. Nursing of the pup lasts for an average of only 4 days, the shortest lactation period of any mammal, during which the pup doubles in size, gaining around /day. This is possible because the milk that they drink has a fat content of 60%. The female pup will mature between ages 3 and 6, whereas the male pup will mature between ages 5 and 7.

Early development

Researchers find that due to a pup's differing needs in regards to sustaining work and foraging while under water compared to adults, the skeletal and cardiac muscles develop differently. Studies show that cardiac blood flow provides sufficient oxygen to sustain lipolytic pathways during dives, remedying their hypoxic challenge. Cardiac tissue is more developed than skeletal muscles at birth and during the weaning period, although neither tissue is fully developed by the end of the weaning period. Pups are born with fully developed hemoglobin stores (found in blood), but their myoglobin levels (found in skeletal tissue) are only 25–30% of adult levels. These observations demonstrate that pup muscles are less able to sustain either aerobic or anaerobic ATP production during dives than adults. This is due to the large stores of oxygen, either bound to hemoglobin or myoglobin, which the seals rely on to dive for extended periods of time. This could be a potential explanation for pups' short weaning period as diving is essential to their living and survival. This process of fur removal in the womb and alternatively using blubber may be advantageous because water cannot penetrate blubber like it does with fur, therefore blubber can insulate the seals in both water and air fairly uniformly.

Conservation practices, brought about by international cooperation and the formation of the Northwest Atlantic Fisheries Organization (NAFO) led to hooded seal population increases. It is now required to hold a license to hunt hooded seals in international waters and each license is set a quota. Total allowable catch of hooded seals are set at 10,000 annually.<gallery>

File:FMIB 34394 Hood Seals.jpeg|Mother with pup.

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Predators of hooded seals include killer whales, polar bears and Greenland sharks.

References

  • Animal Diversity Web: Hooded Seal
  • SCS: Hooded Seal
  • CRESLI hooded seal
  • Smithsonian Institution – North American Mammals: Cystophora cristata