124 Sharks: The Animal Answer Guide
of 10 to 50 or more. However, some scyliorhinid cat sharks hold onto their
eggs until many are ready to be laid and then deposit them all at once.
Chimaeras, all of which are egg layers, also spread egg laying out over
an extended period. Chimaeras are thought to lay two egg cases at a time,
one from each oviduct, repeating the process several times during the lay-
ing season.
How often do sharks reproduce?
The frequency of reproduction reflects two different aspects of repro-
ductive biology—the gestation period plus the “interbirth interval,” or
the time between bouts of reproduction. Together, they tell us how much
time a female needs to replenish the fat stored in her liver before mat-
ing and producing new young. Many live-bearing sharks reproduce on an
annual cycle and require relatively little time for the females to recover
before becoming pregnant again. Atlantic Sharpnose, Smooth Dogfish,
Scalloped Hammerhead, and Pacific Angelsharks are examples, producing
young each year. They do not appear to have a resting period, mating again
shortly after they give birth. Most rays also reproduce on an annual cycle.
But in many other elasmobranchs, large gaps exist between one repro-
ductive cycle and the next. For example, manta rays give birth to a single pup
after a year-long gestation period, but they seldom bear young in consecutive
years. Field observations suggest that they rest for at least a year and maybe
for as long as six years before reproducing again. White-Spotted Marble
Electric Rays have a one-year gestation period followed by two nonpregnant
years, as do Sandbar and Dusky sharks. Sawfishes are thought to mate once
every two years. If they have a “normal” gestation period of one year or less,
they too rest for a year before producing new young. Even the relatively pro-
lific Nurse Sharks, with their five- to six-month gestation period, produce
litters only every other year, which means they take a year off between litters,
as do Blue Sharks and Broadnose Sevengill Sharks. Blacktip and Finetooth
sharks have a nine-month resting period. We know frustratingly little about
the breeding habits of White Sharks; it is estimated that they have a gestation
period of between 12 and 22 months, with an unknown interbirth interval.
Interbirth interval is another important piece of biological information
for shark conservation because it tells us how quickly individuals are re-
placed in an exploited population. Unfortunately, it is also information that
is difficult to obtain.
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