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. In addition, porbeagles within a group have been seen chasing each other, and they will reportedly "play with anything floating on the water"; individuals have been observed prodding, tossing, or biting natural and artificial objects, including pieces of driftwood and balloon floats used by anglers. Great white sharks (Carcharodon carcharias) and killer whales (Orcinus orca) are plausible, albeit undocumented, predators of the porbeagle. In one record, a small individual caught off Argentina bore bite marks from a copper shark (Carcharhinus brachyurus) or similar species, but whether the porbeagle was the target of attempted predation or if the two were simply involved in interspecific aggression is uncertain. Known parasites of this species include the tapeworms Dinobothrium septaria and Hepatoxylon trichiuri, and the copepods Dinemoura producta, Laminifera doello-juradoi, and Pandarus floridanus. Natural annual mortality is low, estimated to be 10% for juveniles, 15% for adult males, and 20% for adult females in the western North Atlantic. Feeding The porbeagle is an active predator that predominantly ingests small to medium-sized bony fishes. It chases down pelagic fishes such as lancet fish, mackerel, pilchards, herring, and sauries, and forages near the bottom for groundfishes such as cusk, haddock, redfish (i.e., Sebastes, Lutjanus, Trachichthyidae, and Berycidae), cod, hake, icefish, dories, sand lances, lumpsuckers, and flatfish. Cephalopods, particularly squid, also form an important component of its diet, while smaller sharks such as spiny dogfish (Squalus acanthias) and tope sharks (Galeorhinus galeus) are rarely taken. Examinations of porbeagle stomach contents have also found small shelled molluscs,