Reproduction
Mating activity
Few people have witnessed the mating activity of sharks.
With larger, more rigid species the male orients himself parallel
and head-to-head with the female, while with smaller, more flexible
species the male coils around the female.
Females often bear scars or marks. Upon examination, these marks show
they have been made by upper jaw teeth, as during mating, males of many
species bite females on the pectoral fins or the middle of the back to hold
onto them.
Thus males of some elasmobranchs have longer, narrower teeth than females,
while some female sharks, such as the Blue shark (Prionace glauca),
have the skin on the back and flanks more than twice as thick than the males.
Shark and batoid eggs are fertilized internally, as opposed
to external fertilization in many bony fishes. Internal
fertilization is a key adaptation for energy-intensive
reproduction.
Whether born or hatched, young sharks are fully formed and physically able to
fend for themselves, and as these independent shark youngs have a better chance for
survival, the number of sharks produced in a litter is
rarely over 100. The majority of the species bear far
fewer pups.
Claspers are modified inner edges of the pelvic fins of
male sharks and rays. During copulation, the erectile claspers
are bent forward. The male inserts one I in cartilaginous clasper
at a time into the female. In some species, claspers conta' hooks
and spurs that dig into the walls of the female oviduct,
anchoring the clasper. Muscles force seminal fluid down a groove
in the clasper and into the female oviduct.
Embryonic development
There are three types of embryonic development: oviparous,
ovoviviparous, and viviparous.
In oviparous sharks, a gland secretes a shell, or case,
around the egg as it passes through the oviduct, protecting the
shark until it hatches. The mother deposits the egg cases in the
sea. When an egg case is first laid, it is soft and pale;
the case hardens and darkens in a few hours.
The
developing embryo receives nutrients from a yolk formed
prior to fertilization.
Oviparous sharks include Horn sharks and Swell
sharks (Cephaloscyllium ventriosum).
In ovoviviparous sharks, the shell is often just a thin
membrane. Sometimes there is more than one egg in a membrane;
this group of eggs is called a candle. The mother retains the
egg, and the embryo soon sheds the membrane and develops in the
mother's uterus.
Theoretically, all the embryo's nutrients come from
the yolk. In some species, however, the lining of the
uterus probably secretes nutritive fluids that are
absorbed by embryos.
In other species, embryos
continue to obtain nutrients after their yolk is absorbed
by swallowing eggs and smaller embryos in the uterus.
This is termed "intrauterine cannibalism" or
ovophagy. In these sharks, usually only one embryo
survives in each uterus. (Females have two uteri.)
Ovoviviparous sharks include mako sharks (Isurus
spp.) and sandtiger sharks (family Odontaspididae).
In viviparous sharks, the yolk stalk that connects the
embryo to the yolk grows long in the uterus. Where the small yolk
sac comes in contact with the mother's uterus, it changes into a
yolk sac placenta.
The embryo receives all its nutrients from the mother
in one of two ways:
- Tissues of the embryo and the mother are in
intimate contact and nutrients are passed
directly from the tissues of the mother to the
tissues of the developing embryo.
- The
uterine lining secretes "uterine milk,"
which bathes the developing embryo. The branched
yolk stalk absorbs this fluid.
Viviparous sharks include Hammerhead sharks.
Gestation
Gestation periods vary among species and between individuals
within a species. Since sharks and batoids are cold-blooded,
there is no precise gestation time. The rate at which the embryo
develops depends on the water temperature. In general, most
embryos develop somewhere in the range of two months (for some
rays) to two years (for some spiny dogfish).
Location
Sharks generally bear their young or lay their eggs in coastal
nurseries where other large sharks are usually absent.
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