Lamna nasus

The porbeagle (Lamna nasus) is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent is the closely related salmon shark (L. ditropis). The porbeagle typically reaches 2.5 m (8.2 ft) in length and a weight of 135 kg (298 lb); North Atlantic sharks grow larger than Southern Hemisphere sharks and differ in coloration and aspects of life history. Gray above and white below, the porbeagle has a very stout midsection that tapers towards the long, pointed snout and the narrow base of the tail. It has large pectoral and first dorsal fins, tiny pelvic, second dorsal, and anal fins, and a crescent-shaped caudal fin. The most distinctive features of this species are its three-cusped teeth, the white blotch at the aft base of its first dorsal fin, and the two pairs of lateral keels on its tail. The porbeagle is an opportunistic hunter that preys mainly on bony fishes and cephalopods throughout the water column, including the bottom. Most commonly found over food-rich banks on the outer continental shelf, it makes occasional forays both close to shore and into the open ocean to a depth of 1,360 m (4,460 ft). It also conducts long-distance seasonal migrations, generally shifting between shallower and deeper water. The porbeagle is fast and highly active, with physiological adaptations that enable it to maintain a higher body temperature than the surrounding water. It can be solitary or gregarious, and has been known to perform seemingly playful behavior. This shark is aplacental viviparous with oophagy, developing embryos being retained within the mother's uterus and subsisting on non-viable eggs. Females typically bear four pups every year. Only a few shark attacks of uncertain provenance have been attributed to the porbeagle. It is well regarded as a game fish by recreational anglers. The meat and fins of the porbeagle are highly valued, which has led to a long history of intense human exploitation. However, this species cannot sustain heavy fishing pressure due to its low reproductive capacity. Direct commercial fishing for the porbeagle, principally by Norwegian longliners, led to stock collapses in the eastern North Atlantic in the 1950s, and the western North Atlantic in the 1960s. The porbeagle continues to be caught throughout its range, both intentionally and as bycatch, with varying degrees of monitoring and management. The International Union for Conservation of Nature (IUCN) has assessed the porbeagle as vulnerable worldwide, and as either endangered or critically endangered in different parts of its northern range. The etymology of the word 'porbeagle' is obscure. A common suggestion is that it combines 'porpoise' and 'beagle', referencing this shark's shape and tenacious hunting habits. Another is that it is derived from the Cornish porth, meaning 'harbor', and bugel, meaning 'shepherd'. The Oxford English Dictionary states that the word was either borrowed from Cornish or formed from a Cornish first element with the English 'beagle'. The Dictionary also notes that no evidence exists for a connection to the French porc, meaning 'swine', or to porpoise, as has been proposed. Other common names for the porbeagle include Atlantic mackerel shark, Beaumaris shark, bottle-nosed shark, and blue dog. The first scientific description of the porbeagle was authored by French naturalist Pierre Joseph Bonnaterre in the 1788 Tableau encyclopédique et methodique des trois règnes de la nature, and based on an earlier 1769 account by Welsh naturalist Thomas Pennant. Bonnaterre named the shark Squalus nasus, the specific epithet nasus being Latin for 'nose'. In 1816, French naturalist Georges Cuvier placed the porbeagle into its own subgenus, Lamna, which later authors elevated to the rank of full genus. Several phylogenetic studies, based on morphological characters and mitochondrial DNA sequences, have established the sister species relationship between the porbeagle and the salmon shark (L. ditropis), which occurs in place of it in the North Pacific. The genus Lamna evolved 65–45 Mya. When its two extant species diverged from each other is uncertain, though the precipitating event was likely the formation of the ice cap over the Arctic Ocean, which would have isolated sharks in the North Pacific from those in the North Atlantic. Fossilized porbeagle remains are known from Late Miocene epoch (about 7.2 Mya) deposits in Belgium and the Netherlands, Pliocene epoch (5.3–2.6 Mya) deposits in Belgium, Spain, and Chile, and Pleistocene epoch (2.6 Mya to 12,000 BP) deposits in the Netherlands. However, Lamna teeth that closely resemble those of the porbeagle have been found in the La Meseta Formation on Seymour Island off the Antarctic Peninsula, which date to the middle to late Eocene epoch (50–34 Mya). Much taxonomic confusion remains regarding Lamna in the fossil record due to the high degree of variability in adult tooth morphology within species. The porbeagle has an almost global amphitemperate distribution, i.e. it is absent from the tropics; in the North Pacific, its niche is assumed by the salmon shark. It is found mostly within 30–70°N and 30–50°S latitudes. In the North Atlantic, the northern limit of its range extends from the Newfoundland Grand Banks off Canada, through southern Greenland, to Scandinavia and Russia; the southern limit of its range extends from New Jersey and Bermuda, through the Azores and Madeira, to Morocco. It is found in the Mediterranean Sea, but not the Black Sea. Normally, North Atlantic sharks only stray as far south as South Carolina and the Gulf of Guinea, but pregnant females from the western North Atlantic population are known to range into the Sargasso Sea, almost as far as Hispaniola, to give birth. In the Southern Hemisphere, the porbeagle apparently occupies a continuous band bound in the south by the Antarctic Convergence, and extending as far north as Chile and Brazil, the Western Cape province of South Africa, Australia to southern Western Australia and southern Queensland, and New Zealand. In New Zealand it is most common south of the Cook Strait. The porbeagle is thought to have colonized the Southern Hemisphere during the Quaternary glaciation (beginning around 2.6 Mya), when the tropical climate zone was much narrower than it is today.