Reproduction and Development 119
months. Elasmobranchs as a group have relatively long gestation periods,
an important factor in understanding their conservation because a slow re-
production rate means that when a population is overfished, individuals are
replaced slowly.
Gestation periods vary greatly among sharks and rays. Some of the
shortest periods for live-bearing elasmobranchs are found in rays such as
the Atlantic Stingray, which carries its young for only 3 or 4 months. Manta
rays, in contrast, have a 12-month gestation period. Nurse Sharks have a
relatively short period of about 5 months. Pacific Angelshark young ges-
tate for 9 to 10 months, which appears to be about average for the family.
Blue Sharks take longer, 9 to 12 months, as do Blacktip, Finetooth, Atlan-
tic Sharpnose (Rhizoprionodon terraenovae), and several other carcharhinids.
Smooth Dogfish have an annual reproductive cycle. Sphyrnid hammer-
heads appear to also have an annual cycle, with young gestating for 8 to 11
months after fertilization.
Size isn’t necessarily a good indicator of gestation period. Spiny Dog-
fish, a species that seldom exceeds a meter or so (i.e., 3–4 ft) in length,
have a two-year gestation period. Basking Sharks are suspected of holding
their young for at least that long, but we don’t know enough about Basking
Shark biology to be able to say for sure.
How long does it take for a baby shark to hatch from an
egg case?Hatching times also vary greatly among species. Data on hatching times
from egg-bearers are more accurate than those from live-bearers because
we can monitor the development of eggs that have been laid. It is much
harder to know when a female live-bearer mated and, because of sperm
storage, just when eggs were fertilized. Fortunately, captive animals will
lay eggs in an aquarium, and the entire developmental process can be ob-
served directly. Hemiscylliid bamboo sharks hatch relatively quickly, in
2 to 6 months. Heterodontid Horn Shark eggs hatch in 7 to 9 months;
other heterodontids, in 5 months to a year. Hatching times in scyliorhinid
catsharks and swellsharks vary greatly because some species lay eggs when
the young are just beginning to develop whereas other species retain the
eggs almost up to the time of hatching. Brown Catshark (Apristurus brun-
neus) eggs were collected near Monterey, California, and brought into the
Monterey Bay Aquarium. Pups hatched 27 months later; data from other
locales suggest at least one year to hatch in nature. Swellshark eggs take 7.5
to 10 months to hatch, depending on water temperature. The South Afri-
can Lined Catshark (Halaelurus lineatus) lays eggs further along in develop-
ment, the young hatching in only 3 to 4 weeks.