Reproduction and Development 115
fin against his forehead while he inserts a multi-lobed pelvic fin clasper into
the female.
We know very little about the cues that male elasmobranchs use to de-
termine if a female is ready to mate. Chemical attractants produced by fe-
males are a likely candidate, given how sensitive sharks are to odors and
the way that males follow closely behind females. It’s also possible that the
electric sense comes into play, at least in batoids like the Round Stingray.
Round Stingray males can locate buried females by detecting the electric
field produced by a female. Skates in particular seem to use their electricity
in male-female interactions. Some female skates, such as Little Skates, emit
weak electric pulses at a frequency of 2 to 3 Hz, which is the frequency to
which the males are most sensitive.
Do sharks lay eggs or do they give birth to live young?
Both. Once fertilization happens, young may be deposited (laid) out-
side, protected by a leathery, shell-like structure. Or they may develop in
the mother, either inside a shell-like structure for a while or exposed the
entire time while developing in the mother’s uterus. Most sharks and all
rays give birth to live young, whereas rajid skates and chimaeras lay eggs.
Among the few egg-laying selachian sharks are heterodontid horn sharks,
scyliorhinid cat sharks, hemiscyliid bamboo sharks, and the stegostomatid
Zebra Shark. All these egg-layers are small (less than 1 m, or 3 ft, long) and
swim near or lie on the bottom to begin with. They lay their eggs on the
bottom or attach them to vegetation or other objects. A few shark groups
that lie on the bottom, including orectolobid carpet sharks, give birth to
live young, with eggs developing inside the mother first.
The skin of female Blue Sharks
(right) is thicker than the skin of
males (left), probably to prevent
muscle damage during courtship,
when males bite females. (The ruler
length is 15 cm, or 6 in.) Photo by H. L.
Pratt, from H. L. Pratt, Jr., “Reproduction in the
Blue Shark, Prionace glauca,” U.S. Fishery Bul-
letin 77 ( 1979 ): 445 – 70