Cow Sharks

Little is known about the mysterious cow shark as it spends most of its life in secret living in the deepest, coldest, darkest parts of the ocean well below the depth most divers reach. Some species, however, do come up to shallow water to give birth and to feed at certain times of the year.

They are known scientifically as hexanchiform sharks as they have six or seven pairs of gills. The name means six arches, even though some have seven pairs. Most sharks only have five pairs. This is the characteristic that links them to Triassic period sharks making them the most primitive species of shark.

Appearance

The cow shark is one of the biggest sharks in the ocean reaching up to 18 feet in length. They have a stout build with underslung jaws with thorn like teeth in the upper jaw and unique teeth shaped like cockscombs in the lower jaw. Cow sharks are brownish in colour that is sometimes so dark it seems black.

These sharks have an anal fin and a single dorsal fin which is far back on the body. Their eyes are small and bluish green. The females are usually bigger than the males but both grow very big.

The largest hexanchoid is the Bluntnose that grows to more than fifteen-and-a-half feet in length. The smallest is the Sharpnose Sevengill which only grows to four-and-a-half feet in length.

These sharks often appear slow and cumbersome but are capable of an amazing burst of speed when attacking their prey.

There are four species of cow shark, the Bluntnose Sixgill, the Big-eyed Sixgill, the Broadnose Sevengill and the Sharpnose Sevengill.

Bigeyed Sixgills (Hexanchus nakamurai)

These cow sharks can reach up to 1.8 metres in length and usually weigh about 20 kg. They are small and slim in appearance with narrow heads, blunt, pointed snouts and fluorescent green eyes. Their bodies are spindle-shaped (fusiform) with a small dorsal fin set back right over their pelvic fins. They are light brown-grey in colour on top and lighter underneath, their fins have white trailing margins. The young have black tipped upper tail fin (caudal fin).

Bluntnose Sixgill (Hexanchus griseus)

This is the biggest of the hexanchoids with a massive body and a long, powerful tail. The single dorsal fin is at the rear of its body, just in front of its anal fin. Its eyes are small and set to either side of its wide, short-snouted head.

Broadnose Sevengill (Notorynchus cepedianus)

This shark has seven gill slits, with the posterior one being very short. It has a bluntly rounded snout, a single dorsal fin that is set far back on its body. It has a large body and the upper part is sprinkled with black and, occasionally, white spots.

Sharpnose Sevengill (Heptranchias perlo)

The other species of cow shark with seven gill slits is the Sharpnose Sevengill. As its name suggests, it has a sharp snout. When young, these sharks have black markings on the tip of their dorsal fins and part of their tail. These markings fade with age. They are brownish grey to olive in colour on the top of their bodies which fades to a lighter colour on their ventral or stomach surface. The tip of their dorsal fin and caudal lobes has a black blotch when they are young but fades at they get older. Their eyes are large and fluorescent green.

Reproduction

This is a species of shark that is ovoviviparous, meaning that it does not lay eggs but the female retains the eggs in her body until they hatch and are born alive. The gestation period is about 12 months and the pups are about 18–21 inches in length at birth. Males reach maturity at about four to five years old and the females at 11. The largest hexanchoid litter is produced by the Bluntnose Sixgill, which gives birth to between 22 and 108 pups at a time, each about 28 inches in length.

Habitat

The cow shark lives only in marine environments, mainly in cold waters from boreal through to temperate and tropical zones of the Indian, Pacific and Atlantic Oceans. They usually inhabit the deep outer continental shelves and slope waters but can be found in intertidal estuaries, rocky reefs, sandy plains and kelp forests. They live in depth of up to 150 feet.

The Bluntnose Sixgill is known to inhabit depths of more than 300 feet; in fact, it is known to go as deep as 6,150 feet. Every night this species of cow shark, like many deep-sea inhabitants, migrates vertically to the surface to feed and give birth then returns to the depth before dawn. They can only feed at the surface at night because their eyes are adapted to the dark and so cannot bear even a moderate amount of light.

The Broadnose Sevengill Shark is unlike other cow sharks in that it lives in shallow coastal waters. These sharks can adapt easily to living in public aquariums, so more is known about this species behaviour than other hexanchoids. This species is found in temperate seas worldwide, where it can coordinate its movements to the tide cycle, moving in and out of shallow bays with the rising and falling of the tide which is important for its feeding and breeding behaviour.

Diet

Juvenile cow sharks eat small sharks, teleost fish, such as salmon, anchovies and sturgeon, and bat rays whilst adults prey on sharks, rays, sea lions, dolphins, seals, teleost fishes, carrion, octopuses, skates, lampreys, octopuses, mollusks, and crustaceans.

The Broadnose Sevengill, with its distinctive peppering of dark spots, is an effective predator and scavenger. It hunts in coastal bays and anywhere to a depth of 165 feet where it preys on everything it can find, including gastropods, rays, crabs, sharks, teleost fish, seals, octopuses and lampreys.

This species has also been known to attack humans. It has bitten divers both in captivity and in the wild; it has behaved aggressively towards spear fishermen and has attacked swimmers in shallow waters. One specimen had human remains in its stomach but whether it preyed on this person or scavenged the body is not known. This is the only species of cow shark that is thought to pose a danger to humans as the other species are rarely encountered because they prefer to live at great depths and very rarely visit coastal waters.

Adult Broadnose Sevengills only feed sporadically, once every 5 to seven days. They eat about 0.2% of their body weight each day on average, whilst the newborn pups eat about ten times what the adults eat and feed far more often.

References

http://www.elasmo-research.org/education/ecology/sandy-broadnose_7gill.htm

http://en.wikipedia.org/wiki/Cow_shark