Open main menu
Home
Random
Recent changes
Special pages
Community portal
Preferences
About Wikipedia
Disclaimers
Incubator escapee wiki
Search
User menu
Talk
Dark mode
Contributions
Create account
Log in
Editing
Porbeagle
(section)
Warning:
You are not logged in. Your IP address will be publicly visible if you make any edits. If you
log in
or
create an account
, your edits will be attributed to your username, along with other benefits.
Anti-spam check. Do
not
fill this in!
===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 8β9 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 6β11 years, and females at a fork length of {{convert|2.0|-|2.2|m|ft|abbr=on}} and an age of 12β18 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 8β11 years, and females at a fork length of {{convert|1.7|-|1.8|m|ft|abbr=on}} and an age of 15β18 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 30β40 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"/>
Edit summary
(Briefly describe your changes)
By publishing changes, you agree to the
Terms of Use
, and you irrevocably agree to release your contribution under the
CC BY-SA 4.0 License
and the
GFDL
. You agree that a hyperlink or URL is sufficient attribution under the Creative Commons license.
Cancel
Editing help
(opens in new window)