How Fearsome Are Shark Embryos?

If you think you had it bad growing up with a mean sister or brother, spare a thought for sand tiger sharks.

Sand tiger shark embryos are known to cannibalize one another in the womb.
Sand tiger shark embryos are known to cannibalize one another in the womb.

Scientists have learned that sibling rivalry among sand tiger sharks is fatally vicious -- before they are even born. During the course of the nearly yearlong gestation period, the shark embryos engage in a battle for survival, with the largest embryo usually devouring all but one of the others.

Although this process might seem unnecessarily hostile, researchers believe that it accomplishes two basic things: ensuring that only one father is responsible for the offspring (a female sand tiger shark may be impregnated by more than one male) and giving the baby sharks the survival advantage of being much larger than the newborns of other shark species. A sand tiger shark can be up to 3 feet (.9 m) long at birth, or about a third of the size it will reach in adulthood.

Although scientists have yet to discern what makes one father's offspring dominant in utero, they do see a common theme in the findings. "Sexual selection is very much like an evolutionary arms race, and the males and females are basically one-upping each other," James J. Gelsleichter, a marine biologist at the University of North Florida, told Live Science.

More surprises from the sand tiger shark:

  • Sand tiger sharks gulp air, which allows them to float just under the surface without moving.

  • Witnesses have seen sand tiger sharks working in unison to attack prey, including snapping their tails like whips to frighten and surround kingfish.

  • Unlike other shark species, the sand tiger shark adapts well to captivity, with many living for decades in manmade enclosures.

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