Editing Porbeagle (section)

==Biology and ecology==
[[File:Porbeagle head.jpg|thumb|alt=View of the front half of a shark with large black eyes and open mouth showing many rows of sharp teeth, lying on a pier|The streamlined shape and long gill slits of the porbeagle are adaptations for a fast, active lifestyle.]]
Fast and energetic, the porbeagle can be found singly or in groups.<ref name="compagno"/> Its fusiform body, narrow caudal peduncle with lateral keels, and crescent-shaped tail are [[adaptation]]s for efficiently sustaining speed, which have also been [[convergent evolution|independently evolved]] by [[tuna]]s, [[billfish]]es, 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 metabolism|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.<ref name="martin2"/><ref name="carlson et al"/>

Porbeagles are among the few fishes that exhibit apparent [[play (animal behavior)|play]] behavior.<ref name="martin"/> Reports, principally off the [[Cornwall|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.<ref name="martin"/><ref name="claybourne"/> 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 [[fishing float|balloon floats]] used by anglers.<ref name="martin"/><ref name="claybourne"/><ref name="housby"/>

[[Great white shark]]s (''Carcharodon carcharias'') and [[killer whale]]s (''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.<ref name="compagno"/> Known [[parasite]]s of this species include the [[tapeworm]]s ''Dinobothrium septaria'' and ''Hepatoxylon trichiuri'',<ref name="rocka"/><ref name="waterman and sin"/> and the [[copepod]]s ''Dinemoura producta'',<ref name="henderson et al"/> ''Laminifera doello-juradoi'',<ref name="carli and bruzzone"/> and ''Pandarus floridanus''.<ref name="cressey"/> Natural annual [[mortality rate|mortality]] is low, estimated to be 10% for juveniles, 15% for adult males, and 20% for adult females in the western North Atlantic.<ref name="francis et al"/>

===Feeding===
[[File:Maraiche (dents).jpg|thumb|right|upright=1.2|alt=Two arrowhead-shaped shark teeth|Porbeagle teeth are suited for grasping fish, with a long central cusp and a tiny cusplet on either side.]]
The porbeagle is an active [[predator]] that predominantly ingests small to medium-sized [[bony fish]]es. It chases down [[pelagic fish]]es such as [[lancet fish]], [[mackerel]], [[pilchard]]s, [[herring]], and [[saury|sauries]], and forages near the bottom for [[groundfish]]es such as [[Cusk (fish)|cusk]], [[haddock]], [[redfish]] (i.e., ''[[Sebastes]]'', ''[[Lutjanus]]'', [[Trachichthyidae]], and [[Berycidae]]),<ref>{{cite web | url=https://animaldiversity.org/accounts/Lamna_nasus/ | title=Lamna nasus (Blue dog) | website=[[Animal Diversity Web]] }}</ref> [[Gadidae|cod]], [[Merluccidae|hake]], [[Channichthyidae|icefish]], [[Zeidae|dories]], [[sand lance]]s, [[lumpsucker]]s, and [[flatfish]]. [[Cephalopod]]s, particularly [[squid]], also form an important component of its diet, while smaller sharks such as [[spiny dogfish]] (''Squalus acanthias'') and [[tope shark]]s (''Galeorhinus galeus'') are rarely taken. Examinations of porbeagle stomach contents have also found small shelled [[mollusc]]s, [[crustacean]]s, [[echinoderms]], and other [[invertebrate]]s, which were likely ingested incidentally, as well as inedible debris such as small stones, [[feather]]s, and garbage fragments.<ref name="roman"/><ref name="martin"/><ref name="joyce et al"/>

In the western North Atlantic, porbeagles feed mainly on pelagic fishes and squid in spring, and on groundfishes in the fall; this pattern corresponds to the spring-fall migration of these sharks from deeper to shallower waters, and the most available prey types in those respective habitats. Therefore, the porbeagle seems to be an opportunistic predator without strong diet specificity.<ref name="joyce et al"/> During spring and summer in the [[Celtic Sea]] and on the outer Nova [[Scotian Shelf]], porbeagles congregate at [[tide|tidally]] induced thermal fronts to feed on fish that have been drawn by high concentrations of [[zooplankton]].<ref name="padea et al"/><ref name="campana and joyce"/> Hunting porbeagles regularly dive from the surface all the way to the bottom, cycling back every few hours; this vertical movement may aid in the detection of [[olfaction|olfactory]] cues.<ref name="padea et al"/> A one-year-old porbeagle {{convert|1|m|ft|abbr=on}} long, was reported to have had fed on [[krill]] and [[polychaete worm]]s.<ref name="henderson et al"/>

===Life history===
{{Multiple image|direction=vertical|align=right|image1=Shark male reproductive system.jpg|image2=Shark female reproductive system.jpg|width=220|caption2=The male (top) and female (bottom) reproductive systems of the porbeagle (click to enlarge).|alt1=The dissected reproductive system of an adult male shark; the paired testes are connected by the tubular epididymis to the base of a pair of cylindrical claspers. Inset close-ups show the base, or head, of the epididymis, and the ampulla, which comprises the lower section of the epididymis. Another label indicates the epigonal organ, located near the lower end of the testes|alt2=The dissected reproductive system of an adult female shark: there is a single large, round ovary, which leads into a junction that splits off into the lower uterus, which leads to the vagina and then the cloaca, and a pair of anterior uteri, each connected to an oviducal gland by a narrow tract or isthmus}}
The timing of the porbeagle's reproductive cycle is unusual in that it is largely similar in both hemispheres, rather than being offset by six months. This suggests that its reproduction is not significantly affected by temperature or day length, perhaps owing to its endothermic physiology.<ref name="francis and stevens"/> Mating takes place mainly between September and November, though females with fresh mating scars have been reported as late as January off the [[Shetland Islands]]. The male bites at the female's pectoral fins, gill region, and flanks while courting and to hold on for [[animal sexual behavior|copulation]].<ref name="jensen et al"/> Two mating grounds are known for western North Atlantic porbeagles, one off [[Newfoundland (island)|Newfoundland]] and the other on [[Georges Bank]] in the [[Gulf of Maine]].<ref name="jensen et al"/><ref name="marshall"/> Adult females have a single functional [[ovary]], on the right, and two functional uteri. They probably reproduce every year. The litter size is typically four, with two embryos oriented in opposing directions sharing each uterus; on rare occasions, a litter may contain as few as one or as many as five pups.<ref name="francis and stevens"/> The [[gestation]] period is 8–9 months.<ref name="compagno"/><ref name="jensen et al"/>

Like other members of its family, the porbeagle is [[aplacental viviparous]] with [[oophagy]], i.e. the main source of embryonic nutrition are unfertilized eggs. During the first half of pregnancy, the mother [[ovulation|ovulates]] enormous numbers of tiny [[ovum|ova]], packed into [[egg case (Chondrichthyes)|capsules]] up to {{convert|7.5|cm|in|abbr=on}} long, into her uteri. A newly conceived embryo is sustained by a [[yolk sac]] and emerges from its egg capsule at {{convert|3.2|-|4.2|cm|in|abbr=on}} long. At this time, the embryo has well-developed [[external gills]] and a [[spiral valve]] [[intestine]]. When the embryo is {{convert|4.2|-|9.2|cm|in|abbr=on}} long, it has resorbed its external gills and most of its yolk sac, but cannot yet feed, as it lacks the means to open egg capsules. At a length of {{convert|10|-|12|cm|in|abbr=on}}, the embryo grows two massive, recurved "fangs" in the lower jaw for tearing open capsules, as well as two much smaller teeth in the upper jaw. It begins to feed voraciously on yolk, acquiring an enormously distended stomach; to accommodate this, the muscles on the belly split down the middle and the skin on the abdomen stretches greatly.<ref name="francis and stevens"/><ref name="jensen et al"/>

At {{convert|20|-|21|cm|in|abbr=on}} long, the embryo appears pink because it lacks [[pigment]] except in its eyes, and its head and gill regions are laterally enlarged and gelatinous. The yolk stomach can comprise up to 81% of the embryo's total weight when it is {{convert|30|-|42|cm|in|abbr=on}} long. The embryo gains pigment and sheds its fangs at a length of {{convert|34|-|38|cm|in|abbr=on}}. Around this time, the mother stops producing ova. From then on, the embryo relies mainly on the yolk stored in its stomach, though it may continue to feed on remaining eggs by squishing the capsules between its jaws or swallowing them whole. It begins to transfer its energy stores from its stomach to its liver, causing the former to shrink and the latter to grow exponentially. The embryo is essentially fully pigmented by a length of {{convert|40|cm|in|abbr=on}}, and has assumed its newborn appearance by a length of {{convert|58|cm|in|abbr=on}}. By then, its stomach has shrunk enough for the abdominal muscles to close, leaving what has been termed an "umbilical scar" or "yolk sac scar" (neither is accurate). Several series of single-cusped teeth grow in both jaws, though they lie flat and remain nonfunctional until birth.<ref name="francis and stevens"/><ref name="jensen et al"/>

[[File:Porbeagle two sizes.jpg|thumb|left|upright=1.2|alt=Two sharks lying on a boat deck, the one in front about half the size of the one in back, but otherwise similar in appearance|A juvenile porbeagle alongside an adult]]
Newborn porbeagles measure {{convert|58|-|67|cm|in|abbr=on}} long and do not exceed {{convert|5|kg|lb|abbr=on}}. Up to a tenth of the weight is made up of the liver, though some yolk also remains in its stomach and continues to sustain the pup until it learns to feed.<ref name="francis et al"/><ref name="francis and stevens"/> The overall embryonic growth rate is {{convert|7|-|8|cm|in|abbr=on}} per month.<ref name="francis and stevens"/><ref name="jensen et al"/> Sometimes, one pup in a uterus is much smaller than the other, but otherwise normal. These "runts" may result from a dominant, forward-facing embryo eating most of the eggs as they arrive, and/or the mother being unable to provide an adequate egg supply for all her offspring.<ref name="francis and stevens"/> Birthing occurs from April to September, peaking in April and May (spring-summer) for North Atlantic sharks and June and July (winter) for Southern Hemisphere sharks. In the western North Atlantic, birth occurs well offshore in the Sargasso Sea at depths around {{convert|500|m|ft|abbr=on}}.<ref name="campana et al 2010"/>

Both sexes grow at similar rates until the onset of maturation, with females maturing later and at a larger size than males.<ref name="natanson et al"/> In the first four years of life, the annual growth rate is {{convert|16|-|20|cm|in|abbr=on}} and similar in both hemispheres; thereafter, sharks from the western South Pacific begin to grow slower than those from the North Atlantic.<ref name="francis et al 2007"/> In the North Atlantic, males mature at a fork length of {{convert|1.6|-|1.8|m|ft|abbr=on}} and an age of 6–11 years, and females at a fork length of {{convert|2.0|-|2.2|m|ft|abbr=on}} and an age of 12–18 years.<ref name="jensen et al"/><ref name="natanson et al"/> In the Southwest Pacific, males mature at a fork length of {{convert|1.4|-|1.5|m|ft|abbr=on}} and an age of 8–11 years, and females at a fork length of {{convert|1.7|-|1.8|m|ft|abbr=on}} and an age of 15–18 years.<ref name="francis and stevens"/><ref name="francis et al 2007"/><ref name="francis and duffy"/> The oldest porbeagle on record was 26 years of age and measured {{convert|2.5|m|ft|abbr=on}} long.<ref name="campana et al 2002"/> The [[maximum lifespan]] of this species appears to be 30–40 years in the Atlantic,<ref name="campana et al 2002"/> but could be as much as 65 years in the South Pacific.<ref name="francis et al 2007"/>

===Thermoregulation===
[[File:Lamna nasus with fishmonger.jpg|thumb|alt=A shark lying belly up and sliced transversely through the middle, showing the body cavity and the spinal column beneath it, surrounded by pink muscle with two obviously darker muscle blocks flanking the spine|Cross-section through the trunk of a porbeagle (orientation is belly-up); note the central red muscles.]]
Like other members of its family, the porbeagle is [[endothermy|endothermic]]; [[Metabolism|metabolic]] heat generated by its red muscles is conserved within the body by specialized systems of [[blood vessel]]s called ''retia mirabilia'' ([[Latin]] for "wonderful nets"; singular ''rete mirabile''), that act as highly efficient [[countercurrent exchange|countercurrent]] [[heat exchanger]]s. 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''.<ref name="carlson et al"/>

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 [[vertebral column|spine]], and its lateral ''rete'' is composed of over 4,000 small [[artery|arteries]] arranged in bands.<ref name="carey et al"/> It has one of the highest core temperatures within its family, {{convert|8|–|10|C-change}} warmer than that of the surrounding water.<ref name="carey and teal"/> 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.<ref name="carlson et al"/><ref name="henderson et al"/> The orbital ''retia'' of the porbeagle can raise the temperature of its brain and eyes by {{convert|3|–|6|C-change}}, 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 [[reaction time|response times]].<ref name="block and carey"/>