116 Sharks: The Animal Answer Guide
Live-bearing sharks and rays fall into one of two groups depending on
how the embryo gets its nutrition before birth. In the more common group,
the young use up a yolk supply to which they are attached from the start,
with no additional nutrition provided by the mother. In the other group,
the developing embryo receives nutrients other than yolk from the mother.
All live-bearing sharks except some of the carcharhiniform requiem
sharks and the lamniforms (White Shark and relatives) use yolk sack nutri-
tion. Some female squaliform dogfish and triakid houndsharks supplement
yolk nutrition by secreting mucous from their uterus walls that is ingested by
the embryo. In some stingrays, the mother secretes fatty substances, called
uterine “milk,” that the embryo eats. Another type of mother-provided
nutrition is called oophagy, literally “egg-eating.” Here, the mother keeps
on producing unfertilized eggs, which are eaten by the developing pups.
Oophagy occurs in lamniforms (White, makos, and others) and pseudo-
triakid false cat sharks. One lamniform, the Sand Tiger, carries oophagy
one step farther: the first embryo to develop eats not only eggs but also its
later-developing siblings. Sand Tigers give birth to only two 1-m-long (3-
ft) young, one from each uterus. The surviving embryos already have well-
developed, functional recurved teeth in both jaws when only 45 mm (1.5
in) long. Cownose Ray pups also eat unfertilized eggs and their siblings,
the result being that a female gives birth to only one pup a year.
One group of live-bearers, composed of five families of carcharhini-
forms, has evolved a direct connection between mother and pup, similar to
the placenta and umbilical cord that connects a developing mammal (dog,
giraffe, human) to its mother. The umbilical cord in these sharks develops
from the tube that connected the developing shark to its yolk sack. The tube
has many small outgrowths and is attached to the uterine wall; nutrition
passes from mother to young along the tube’s outgrowths once the nutri-
tion in the yolk sack is used up. Young sharks in this group are born with a
placental scar on their chests (so, yes, many baby sharks have a belly button).
The pups produced by live-bearing sharks and rays are relatively large
replicas of their parents, able to feed on their own and defend themselves
from predators. However, large size and good defenses create problems
during the birth process. These problems are solved by special adaptations.
Hammerhead shark pups would have a difficult time getting their expanded
heads through the birth canal, so the lobes of the hammer are soft and pli-
able at birth and then stiffen soon after. Squalid dogfish sharks have stiff,
sharp spines in front of their dorsal fins. The spines of embryonic dog-
fish are covered with pads of tissue until after the young are born. Mother
sawfish are similarly protected from their developing and emerging young.
The saws of fetal sawfish are at first soft and enclosed in a rubbery enve-
lope. When Bat Rays are born, the pectoral wings are folded over the body
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