Porbeagle

The porbeagle is a very stout-bodied shark with a fusiform (spindle-like) shape. The long, conical snout tapers to a sharp point, and is supported by enlarged, highly calcified rostral cartilages. The eyes are large and black, without nictitating membranes (protective third eyelids). The small, S-shaped nostrils are positioned in front of and below the level of the eyes. The mouth is large and strongly curved, with moderately protrusible jaws. North Atlantic sharks have 28–29 upper tooth rows and 26–27 lower tooth rows, while Southern Hemisphere sharks have 30–31 upper tooth rows and 27–29 lower tooth rows. Each tooth has a strongly arched base and a nearly straight, awl-like central cusp, which is flanked by a pair of smaller cusplets in all but the smallest individuals. The five pairs of gill slits are long and precede the pectoral fin bases. The pectoral fins are long and narrow. The first dorsal fin is large and high, with a rounded apex, and originating just behind the pectoral fin bases. The pelvic fins are much smaller than the first dorsal fin. The second dorsal and anal fins are smaller still, and placed about even with each other on narrow bases that allow pivoting from side to side. The sides of the caudal peduncle are expanded into prominent lateral keels. A second, shorter pair of keels are present below the main keels. …

The porbeagle has an almost global amphitemperate distribution, i.e., it is absent from the tropics; in the North Pacific, where its ecological 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. …

Fast and energetic, the porbeagle can be found singly or in groups. Its fusiform body, narrow caudal peduncle with lateral keels, and crescent-shaped tail are adaptations for efficiently sustaining speed, which have also been independently evolved by tunas, billfishes, and several other groups of active fishes. The salmon shark and it are the thickest-bodied members of their family (length-depth ratio approaching 4.5), and consequently have the stiffest swimming style; they oscillate their tails while holding their bodies mostly rigid, which confers propulsive power with high energy efficiency, but at the cost of maneuverability. The large gill surface area of the porbeagle allows more oxygen to be delivered to its tissues. It also has a short band of aerobic "red muscle" along each side, which can contract independently of the regular "white muscle" at a lower energy cost, enhancing the shark's stamina. Porbeagles are among the few fishes that exhibit apparent play behavior. Reports, principally off the Cornish coast, have this species rolling and repeatedly wrapping themselves in long kelp fronds near the surface; this activity may have an exploratory or self-stimulatory purpose, though alternately, the sharks may be attempting to feed on small kelp organisms or scrape off parasites. …