Porbeagle - Biology and Ecology

Biology and Ecology

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. It and the salmon shark 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. There are reports, principally off the Cornish coast, of this species rolling and repeatedly wrapping itself in long kelp fronds near the surface; this activity may have an exploratory or self-stimulatory purpose, though alternately the shark 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. There is a record of a small individual caught off Argentina that bore bite marks from a copper shark (Carcharhinus brachyurus) or similar species, but it is uncertain whether the porbeagle was the target of attempted predation or if the two were simply involved in interspecific aggression. 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.