page_title: Porbeagle
section_title: Thermoregulation
hierarchical_section_title: Porbeagle / Biology and ecology / Thermoregulation
caption_attribution_description: English: Cross-section through the trunk of a porbeagle (orientation is belly-up); note the central red muscles. Français&#160;: Maraîche coupée transversalement dans une poissonnerie au Québec. Lamna nasus coupé transversalement dans une poissonnerie. Photo: Sylvie Leblanc.
context_page_description: The porbeagle 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. The porbeagle typically reaches 2.5 m in length and a weight of 135 kg; 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.
context_section_description: Like other members of its family, the porbeagle is endothermic; metabolic heat generated by its red muscles is conserved within the body by specialized systems of blood vessels called retia mirabilia (Latin for "wonderful nets"; singular rete mirabile), that act as highly efficient countercurrent heat exchangers. The porbeagle has several rete mirabile systems: the orbital retia accessing its brain and eyes, the lateral cutaneous retia accessing its swimming muscles, the suprahepatic rete accessing its viscera, and the kidney rete.
  Among sharks, the porbeagle's capacity for elevating body temperature is second only to the salmon shark's. Its red muscles are located deep within the body, adjacent to the spine, and its lateral rete is composed of over 4,000 small arteries arranged in bands. It has one of the highest core temperatures within its family, 8–10 °C (14–18 °F) warmer than that of the surrounding water. Being warm-bodied may allow this shark to maintain higher cruising speeds, hunt in deep water for extended periods of time, and/or enter higher latitudes during winter to exploit food resources not available to other sharks. The orbital retia of the porbeagle can raise the temperature of its brain and eyes by 3–6 °C (5–11 °F), and likely serve to buffer those sensitive organs against the large temperature shifts that accompany changes in depth; potential benefits of this include increased visual acuity and reduced response times.
caption: Cross-section through the trunk of a porbeagle (orientation is belly-up); note the central red muscles.
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