to determine their approximate ages. The oldest of the animals sampled, which was also the largest, had lived for 392 ± 120 years, and was consequently born between 1504 and 1744. The authors further concluded that the species reaches sexual maturity at about 150 years of age. Efforts to conserve Greenland sharks are particularly important due to their extreme longevity, long maturation periods, and the heightened sensitivity of large shark populations. Reproduction As recently as 1957, females were found not to deposit eggs in the bottom mud, but retain the developing embryos within their bodies so they are born alive (a process known as ovoviviparity) after an estimated gestation period of 8–18 years. About ten pups per litter is normal, each initially measuring some in length. Within a Greenland shark's uterus, villi serve a key function in supplying oxygen to embryos. It is speculated that due to embryonic metabolism dealing with reproduction, this only allows for a limited litter size of around 10 pups. It has been estimated that due to their extreme longevity, Greenland sharks can have 200 to 700 pups during their lifetime. Physiological adaptations Like other elasmobranchii, Greenland sharks have high concentrations of the two nitrogenous compounds urea and trimethylamine N-oxide (TMAO) in their tissues, which increase their buoyancy and function as osmoprotectants. TMAO also counteracts the protein-destabilizing tendencies of urea and of deep-water pressure. Its presence in the tissues of both elasmobranch and teleost fish has been found to increase with depth. The blood of Greenland sharks contains three major types of hemoglobin, made up of two copies of globin combined with two copies of three very similar subunits. These three types show very similar oxygenation and carbonylation properties, which are unaffected by urea, an important compound in marine elasmobranch physiology. They display identical electronic absorption and resonance Raman spectra, indicating that their heme-pocket structures are identical or highly similar. The hemoglobins also have a lower affinity for O compared to temperate sharks. These characteristics are interpreted as adaptations to living at great water depths. As food The flesh of the Greenland shark is toxic because of the presence of high concentrations of trimethylamine oxide (TMAO). If the meat is eaten without pretreatment, the ingested TMAO is metabolized into trimethylamine, which may be a uremic toxin. Occasionally, sled dogs that eat the flesh are unable to stand up because of this effect. Similar toxic effects occur