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Saturday, June 04, 2005

Intrauterine Cannibalism in Sharks

MS NBC website, 3 Jun 05
Scientists to breed 'test tube' sharks
Plan hopes to boost critically endangered gray nurse species

The Associated Press

SYDNEY, Australia - Australian scientists will attempt to breed gray nurse sharks in artificial wombs under a plan to boost the critically endangered species' numbers, a state fisheries minister said Friday.

Embryos harvested from female sharks in the wild will be reared separately in artificial wombs designed to stop the ravenous fish from devouring each other before birth in what is known as "intrauterine cannibalism." "This is literally survival of the fittest at work, but unfortunately it means that, in the wild, each female gray nurse shark produces only two pups every two years — not enough to increase species numbers," New South Wales state Fisheries Minister Ian Macdonald said in a statement.

Scientists believe only about 460 gray nurse sharks remain in eastern Australian waters and fear they could vanish from the region altogether within 20 years.

The gray nurse shark, which feeds primarily on other fish and is not considered dangerous to humans, was decimated by overfishing and hunting until 1984, when it became the first shark species to be protected by the Australian government.

However, the species has been slow to recover, and some scientists believe the local gray nurse shark population is already too low to regenerate itself naturally.

Macdonald said scientists would begin by collecting reproductive and biological information to construct the artificial shark wombs, but that experiments would be carried out first on non-endangered sharks to avoid any risk to the gray nurse population.

He said scientists are still in the process of developing techniques for harvesting embryos from the wild sharks.
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Intrauterine Cannibalism in Sharks


Sandtiger Shark (Carcharias taurus)

Two forms of within-the-womb cannibalism are known in sharks. The most extreme form of intrauterine cannibalism - in which the largest and strongest embryo actually consumes its lesser womb-mates - is termed "embryophagy" or, more colorfully, "adelphophagy" - literally "eating one's brother". It was discovered accidentally in 1948, when a researcher probing the uteri of a late-term Sandtiger Shark (Carcharias taurus) was startled by a bite on the hand. To date, adelphophagy is known only in the Sandtiger. The less extreme and by far more common form of intrauterine cannibalism - in which developing embryos feed on a steady supply of tiny, unfertilized eggs - is termed "oophagy" (sometimes called "oviphagy") - meaning "egg-eating". The earliest documented case of oophagy dates back to 1907, in the Porbeagle (Lamna nasus). Both forms of intrauterine cannibalism continue throughout embryonic and fetal development, so that at birth each pups often has aa conspicuously swollen abdomen known as a "yolk stomach".

Until quite recently, intrauterine cannibalism was thought to be restricted to lamnoid sharks. This grisly form of within-the-womb nutrition is now known from two carcharhinoids and even one orectoloboid. Following is a list of all sharks in which intrauterine cannibalism has been documented, or for which exists strong circumstantial evidence:

Order Orectolobiformes

Family Ginglymostomatidae

Tawny Nurse Shark (Nebrius ferrigineus)

Order Lamniformes

Family Carchariidae

Sandtiger Shark (Carcharias taurus)

Family Pseudocarchariidae

Crocodile Shark (Pseudocarcharias kamoharai)

Family Alopiidae

Pelagic Thresher Shark (Alopias pelagicus)
Bigeye Thresher Shark (Alopias suprciliosus)
Common Thresher Shark (Alopias vulpinus)

Family Cetorhinidae

Basking Shark (Cetorhinus maximus)

Family Lamnidae

Salmon Shark (Lamna ditropis)
Porbeagle (Lamna nasus)
White Shark (Carcharodon carcharias)
Shortfin Mako (Isurus oxyrinchus)
Longfin Mako (Isurus paucus)

Order Carcharhiniformes

Family Pseudotriakidae

Slender Smoothhound Shark (Gollum attenuatus)
False Catshark (Pseudotriakis microdon)

 
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Did You Know?
Cannibalism within the womb (intrauterine cannibalism) is known to occur in some fish species, such as the spotted ragged-tooth shark (see page 17 of “Oceans of Plenty: South Africa’s Teeming Seas” ). The most common form of this type of cannibalism is oophagy, which occurs when an embryo eats unfertilized eggs. Less common, but far more extreme, is embryophagy, when the strongest embryo eats weaker ones within the same uterus. Also known as adelphophagy, or “eating one’s brother,” embryophagy is only known to occur in sand tiger sharks.

While intrauterine cannibalism is rare, cannibalism in general is not uncommon in the animal kingdom. Scorpions, spiders, ants, otters, muskrats, and even pigs and bears are known to have cannibalistic tendencies. Animals may commit cannibalistic acts to ensure reproductive supremacy by eliminating rivals or simply to ensure survival in times of famine. According to the Humane Society of the United States, minks raised in cages for fur production have been driven to cannibalistic acts by the severe conditions in which they live.

—Elizabeth Connell

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