Baby Sharks
Sharks have been around for 400 million years. During this time their reproductive
strategies have evolved to be more advanced than those of bony fish and in some
cases their strategies are as highly developed as those of mammals.
Bony fish versus sharks
Bony fish have a reproductive strategy where they invest energy into producing large
numbers of small eggs. The female fish releases millions of eggs into the water and the
male fish simply casts his sperm into the water near the eggs. Fertilization is left to
chance and many individuals will be eaten but due to the sheer number of eggs and
sperms a few will survive to maturity to reproduce. Scientists call this strategy ‘rselection’1.
Sharks produce fewer, but much larger, eggs and fertilization takes place internally.
This means they put their reproductive energy into producing a few, larger young, that
are likely to survive to maturity and reproduce. This strategy is called ‘k-selection’1 and
is used by other large organisms such as dolphins, whales and dugongs.
Differences between male and female
Male and female sharks are very similar in appearance. Often the females grow to a
larger size than the males but the most obvious difference is that male sharks have
modified pelvic fins called claspers which they use to transfer sperm into the female.
Both male and female sharks have a hole near the pelvic fins called the cloaca which is
used to pass waste, it is also the same opening where sperm is transferred to the
female and where pups born from. Internally, males have testes that produce sperm
and secrete hormones that control the development of male characteristics. Males also
have two muscular sacs (siphon sacs) in their abdominal wall, which they fill with
seawater probably to propel sperm into the female’s reproductive tract5. Females have
a pair of ovaries where eggs are produced; in most species only the right ovary makes
eggs1. The ovaries, like testes in the male, produce hormones that control the
development of female characteristics.
Courtship/finding a mate
Male and female sharks tend to live in different areas of the ocean and the only time
they congregate is to mate. Courtship between sharks is something rarely seen by
humans and as such is barely understood. Likely to involve all of the shark’s senses,
chemicals released into the water by the females stimulate the male sharks. Often
mistaken for fighting, shark’s courtship behaviour can look quite rough and female
sharks often end up with teeth marks or ‘love bites’. Male Whitetip Reef Sharks, for
example, bite the female on her fins or back during mating. This doesn’t hurt the
female shark because her skin can be up to three times thicker than male’s2.
Male catsharks have been seen twisting their body around female sharks before
mating. Larger sharks, like the Whaler, Lemon and Nurse Sharks, have been seen with
the male swimming parallel and head-to-head with the female shark3.
Fertilization
During fertilisation one clasper is inserted into the female cloaca. The claspers have
hooks and spines on them, helping them to stay firmly in the female. Sperm passes
from the sperm sacs and goes down the clasper. Water from the muscular sacs is
washed down the clasper to help the sperm move into the female. The eggs are then
fertilised one at a time in the shell gland of the female shark1.
How do baby sharks start life?
A shark developing in an eggcase.
Image © Marc Dando
Around 40% of sharks, including the catsharks and hornsharks, lay eggs which hatch
outside of the female (oviparity). A yolk sac nourishes the developing shark as
oxygen passes in through the eggcase and body wastes seep out. When the baby
shark (pup) hatches it is a fully formed copy of its parents. The female shark can
spend a long time laying her eggcases as they must be in a securely fixed in a safe
place as it can take between six to ten months for the pups to hatch. Port Jackson
sharks lay large eggs shaped like a screw, which are adapted to wedge firmly
between rocks1, 2, 3. Find out more about shark eggcases at www.eggcase.org.
Other sharks do not lay eggs2. In most the fertilized eggs develop inside the mother until the
pups have absorbed the yolk (called aplacental viviparity or ovoviviparity). The embryos
develop within an eggcase which has a thin membrane-like ‘shell’. The embryos develop in the
same way as oviparous sharks (above) but within the female there is a good supply of oxygen
and little danger from predators meaning a greater chance of survival for the pups. Once the
food supply from the yolk is finished the pup will hatch out of the eggcase and then be born1.
In some species the pups are not born immediately after hatching out of the eggcases.
Instead, they stay in a part of the oviduct called the uterus where they are provided with more
food in the form of infertile eggs (oophagy). In some species, such as the Sandtiger, the pups
do not just eat unfertilised eggs but eat their un-hatched siblings as well. The first pup to hatch
will eat all the other developing pups, this is called intrauterine cannibalism1, 2.
Sharks developing
inside the mother
(ovoviviparity)
Image © Marc Dando
The final, and most advanced, strategy is called placental viviparity. Employed largely by the
larger sharks, such as the Blue and Hammerhead sharks, yolk sacs develop into a placenta
which attaches to the uterus. A placental cord then transfers nutrients from mother to pup and
large litters can be nourished in this way. The pups can then absorb nutrients and oxygen
from the mother and excrete their waste products. This process is similar to that employed
by mammals, the main difference being that sharks do not care for their young after birth 6.
Viviparity
Image © Marc Dando
Birth
When the young shark hatches, or is born, they are miniature versions of the adults and
generally have to fend for themselves. To help their young survive, some sharks give birth or lay their eggs in nursery
areas. There are no adult sharks or other major predators in these areas or there is plenty of refuge for the pups, so
the young are able to grow in comparative safety to a bigger size before facing the real underwater world by
themselves.
Why shark numbers decreasing
Sharks only reproduce a few young. The number of offspring varies from two for the Bigeye Thresher to 135 for the
Blue Shark. The length of pregnancy in sharks is relatively long, averaging between 9-12 months. The longest
pregnancy is 22 months for the Spurdog4. In addition, not all shark species reproduce every year; many have a
resting stage of 1-2 years before reproducing again. Young sharks are slow to grow, and reach maturity late. For
example, Lemon Sharks take 15 years to mature and Spurdog take 20 years. Many sharks are killed before maturity
by other marine predators or man, of those that survive; they only produce a few young that may take another 15-20
years to mature.
What can YOU do?
•
Do not buy any shark products such as fins, liver oil pills.
•
Become a member of the Shark Trust and help shark conservation.
•
Adopt a shark.
•
Visit www.sharktrust.org for more information.
References
1. Parker, S & Parker, J., 2001. The Encyclopaedia of Sharks
2. Gibson, A., 2002. Sharks, a Golden Guide
3. Macquitty, M., 1997. Shark.
4. Tricas, T. C., et al., 1997. Sharks and Rays.
5. Whitney N. M., et al., 2004. Group Courtship, mating behaviour and siphon sac function in the Whitetip Reef Shark,
Triaenodon obesus. Animal Behaviour, 68, 1435 – 1442.
6. Compagno, L., et al. 2005. Sharks of the World. Collins Field Guide. ISBN 0 00 713610 2
www.sharktrust.org
Version 3 - 2007