Crocodile Shark - Biology and Ecology

Biology and Ecology

With a long body, small fins, and large liver rich in squalene and other low-density lipids, the crocodile shark is convergently similar to mesopelagic dogfish sharks such as the cookiecutter shark (Isistius brasiliensis). The liver may comprise a fifth of the shark's weight, and acts as an incompressible float that allows it to maintain neutral buoyancy in the water column with little effort. Like many other inhabitants of the mesopelagic zone, the crocodile shark apparently migrates closer to the surface at night to feed and descends into deeper water during the day, being rarely found above a depth of 200 m (660 ft) during daytime.

The large eyes of the crocodile shark, equipped with a reflective green or yellow retina and lacking an expanded iris, suggest that it is a nocturnal hunter that relies on sight to pick out the silhouettes or bioluminescence of its prey. Little is known of the crocodile shark's feeding habits; it is thought to be an active, fast-swimming predator based on its strong musculature, large tail, and behavior when captured. On one occasion, a crocodile shark off Cape Point, South Africa, jumped out of the water in pursuit of bait. Its diet consists of small to medium-sized bony fishes (including bristlemouths and lanternfishes), squid (including onychoteuthids, mastigoteuthids, pholidoteuthids, and cranchiids) and shrimp. Crocodile sharks are not known to be preyed upon by any other species.

The crocodile shark is aplacental viviparous and typically gives birth to litters of four, two pups to each uterus. The gestation period is unknown but believed to be long. The embryos have yolk sacs at 3–4 cm (1.2–1.6 in) long; once the yolk sac is fully absorbed they become oophagous: the mother produces large numbers of thin-walled egg capsules that contain 2–9 eggs each, which are then consumed by the unborn embryos. The abdomens of the embryos become characteristically distended with ingested yolk material, which can make up a quarter of the embryo's total weight. It is unclear how two crocodile shark fetuses manage to share a single uterus, when in some other oophagous mackerel sharks such as the sand tiger shark (Carcharias taurus), only one fetus survives in each uterus. The pups are born at approximately 40 cm (16 in) long; males attain maturity at 74–110 cm (29–43 in) and females at 89–102 cm (35–40 in). There is no defined reproductive season.