The egg layers, like some of the dogfishes, encapsulate the embryo attached to its nutritive yolk sac in a horny casing to protect it. When this food is exhausted, the young shark hatches and swims free to fend for itself in a more dangerous world. The embryos are not connected to the womb. In placental viviparous sharks, the embryos develop in the uterus and in time become attached to the uterine wall, an arrangement analogous to the placenta in mammals. Gestation can take a year or longer and a litter usually has from four to ten pups depending on the species.
All sharks go to great lengths to protect their developing young before birth.
Such strategies are necessary because sharks are slow to reach sexual maturity, many large pelagic sharks taking 12 to 20 years. It is easy to see why overfishing can devastate shark populations. The deep sea holds many species in surprising forms and about which relatively little is known. The eyes in these deep water sharks are often emerald green and adapted to perceive the luminescence of other animals that also dwell where sunlight never reaches. All sharks can be considered flesh eaters although a few feed like baleen whales, filtering the water for plankton and small fish.