Reproduction and Development 127
Can sharks reproduce asexually?
Normal reproduction in chondrichthyan fishes involves males and fe-
males, with the male’s sperm fertilizing the female’s eggs. Asexual repro-
duction, also called “parthenogenesis,” or virgin birth, involves a female
producing eggs that develop into embryos without being fertilized by a
male.
We have yet to find a chondrichthyan in the wild that appears to have
been produced via asexual reproduction. However, captive female sharks
have given birth to young without mating with a male; genetic testing ruled
out the possibility that the female had mated in the wild and stored sperm.
At least four examples of parthenogenesis are known in sharks (but not in
rays or chimaeras). In 2001, a Bonnethead Shark was born without a father
at the Henry Doorly Zoo in Omaha, Nebraska. The mother had not been
in contact with a male for over three years, and genetic testing confirmed
that no male genes were present in the newborn (which was unfortunately
killed by a stingray in the same tank shortly after it was born). In 2002, a fe-
male White-spotted Bamboo Shark at the Belle Isle Aquarium in Detroit,
Michigan, laid fertile eggs from which two babies hatched. The mother
had been in captivity for six years and had never been in the presence of a
male. In 2008, a female Blacktip Shark died during a routine physical exam
and when opened was found to contain a single, well-developed pup that
was ready to be born. Genetic testing again confirmed no genetic material
from a father. And between 2007 and 2011, a captive Zebra Shark at the
Burj Al Arab Hotel (“the World’s Most Luxurious Hotel”) in Dubai, gave
birth to 15 pups from eggs laid in the aquarium. DNA analysis again con-
firmed that the female had not mated with a male.
It is tantalizing to speculate about these multiple events. Does parthe-
nogenesis occur in the wild? Or are these virgin births somehow a result of
Horn Sharks, Heterodontus francisci,
produce complex leathery egg cases
that have a conical spiral shape. The
mother shark literally screws the
egg case into cracks in the sea floor,
making it more difficult for predators
to remove. Photo by devra, Wikimedia Com-
mons, http://en.wikipedia.org/wiki/File:Horn
_shark_egg_morro_bay.jpg