Reproduction and Development 123
Egg-laying selachians produce intermediate numbers. The 10 to 50
eggs of heterodontid Horn Sharks are spread over a one- to two-month
laying season. Scyliorhinid catsharks lay pairs of eggs at varying intervals
and numbers.
Rajiforms are egg-layers and fall into the intermediate production cat-
egory. Little Skates spawn twice a year and produce up to 35 eggs annu-
ally. Blue Skates, the world’s largest skate at 285 cm (9.5 ft) long and 100
kg (220 lb), lay only 40 eggs annually. An exception to these numbers is
the Mottled Skate (Raja pulchra), one of the few species that encases more
than one young in an egg case. Females produce an average of 240 fertil-
ized eggs per year, but the number can reach 550. In contrast, live-bearing
batoids such as guitarfishes and stingrays produce only about six young
per year, following the low-output pattern seen in live-bearing versus egg-
laying selachians.
Another interesting aspect of shark reproduction is the state of develop-
ment of young in a female. In many species, a mother shark may be carry-
ing young of varying developmental stages, from undeveloped eggs right
through to pups ready to be born. Some embryos inside Nurse Sharks may
be 50 to 100 mm (2–4 in) long and still in egg cases. Other, larger embryos
(215–290 mm, 9–12 in long) are free from their egg cases, the smaller still
attached to yolk sacs and the larger ones having absorbed all their yolk, hav-
ing only yolk sac scars. Whale Shark mothers may also be carrying young in
various stages of development, including 42-cm-long (17-in) embryos still in
egg cases and 64-cm-long (26-in) mobile young close to being born.
To the best of our knowledge, live-bearers release all their young at about
the same time. However, many egg-laying species deposit a few eggs at a
time, spreading egg laying out over several days. Horn Sharks and skates de-
posit a few eggs every few days over several months, with a total egg output
Scalloped Hammerheads produce
intermediate numbers of young
compared to most other shark and
ray species. Shown here is a female
Scalloped Hammerhead and her 13
fully developed pups that were re-
moved from her uterus after capture.
Each pup still has its umbilicus at-
tached. Photo by Charles Stillwell, National
Marine Fisheries Service, National Oceanic and
Atmospheric Administration