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]LEFT]Internal Anatomy
of Shark


Upon incision of the belly from the pelvic fins to the pectoral fins the
first organ encountered is the liver. The liver of sharks occupies
most of the body cavity. This large, soft and oily organ can comprise
up to 25% of the total body weight. It serves two functions within the
shark. The first is as an energy store since all fatty reserves are
stored here. The second function of the liver is to serve as a
hydrostatic organ. Oils that are lighter than water are stored in the
liver. This decreases the density of the body providing buoyancy to
counteract the sinking tendency of sharks.





Aside from the liver, the stomach
can be seen within the body cavity. Often found within the stomach are
the contents of the sharks last meal. The stomach itself terminates at a
constriction known as the pylorus, which leads to the duodenum and then
to the spiral valve intestine. The spiral valve intestine is an
internally coiled organ that increases the surface area across which
nutrients can be absorbed. The spiral valve intestine empties into the
rectum and anus which in turn empties into the cloaca. The claoca is
the chamber where the digestive, urinary and genital tracts all open to
the outside.






Some of the organs mentioned can be
seen in this photograph of a mature male porbeagle shark. Also seen
here is the epididymis, part of the male reproductive tract.





Also easily found within the body cavity is the pancreas. The pancreas
is a digestive gland with two pink lobes. Secretions pass from this
organ to the duodenum from the ventral lobe through a small duct.

There are two other organs that are visible but do not belong to the
digestive system. The first is the spleen, which is a dark organ near
the stomach that belongs to the lymphatic system. The second is the
rectal gland, a small organ that opens by a duct into the rectum. It
acts as a salt gland, removing excess sodium chloride (salt) from the
blood. The secretion is a colourlesss solution of salt that is twice
the concentration found within the blood plasma and higher than that of
the surrounding saltwater.



Upon removal of the digestive
organs the reproductive organs can be viewed. For details about the
reproductive anatomy of sharks



Sharks are sexually dimorphic.
That is, there are visual differences between males and females. Males,
like the one seen in this picture, have pelvic claspers which are
modified pelvic fins used for sperm delivery.


Claspers are rolls of cartilage that become stiffened with
calcium in the adult. The presence or absence of these claspers make it
very easy to differentiate males from females. Males also have paired
testes, however the right one is always more developed than the left,
which may be smaller or absent altogether


The testes are internal and are located in the anterior end of the body
cavity within the epigonal organ. The urinary and reproductive tracts
join together to form the urogenital sinus. From there the sperm is
eventually released into the groove of the claspers and is then
delivered to the female during copulation.



Females do not have claspers.
Instead they have a cloacal opening (as do the males) between the pelvic
fins. The internal ovaries are found anteriorly in the body cavity and
are paired, but as was the case with the male testes the left side is
often reduced. Indeed, the left ovary often releases very few or no
eggs.

mature female blue shark.

Once eggs are released and
fertilized, a horny shell or membrane is secreted around each one as
they pass through the oviducal gland or shell gland. Some sharks
produce a shell that is tough and can protect the young. In other
species the membrane is slight or vestigial and the young develop and
hatch within the uterus of the female. The eggs and egg cases produced
by different species are highly variable. Eggs can be up to 60 or 70 mm
in diameter and encased in shells up to 300 mm long. Shapes vary from
spindle to purse-like with tendrils and hooks.





During copulation sharks meet face
to face. As seen in this picture the male inserts one of his claspers
into the cloaca of the female. Sperm contained within sperm packets
called spermatophores are delivered to the female via a groove in the
clasper. The spermatophores are forcefully ejected by contracting
organs known as siphon sacs which use seawater currents to carry the
spermatophores




Another marked difference between the male and female sharks of some
species is the thickness of their skin. The skin on a female blue shark
is nearly twice as thick as that of males. It is believed that this is
because of the viciousness of mating. Males will often bite females
during face to face copulation leaving them with wounds. Without the
extra thickness of the skin females could be severely injured.





Mating behaviour in sharks can be very complex and evidence suggests
that there is some degree of mate selection within certain species.
Sexes often segregate, and only come together to mate. Occasionally
congregations form in mating behaviour. For instance, basking sharks
have been seen forming mating circles like the one seen in this picture.
The exact purpose of this behaviour is not known however it is clear
that it related to mating.

Development

There are three modes of
reproduction within the sharks. In general most sharks bear live young,
however there are some sharks who lay eggs. The most advanced form of
development is called viviparity. This is when the female provides
nourishment for the embryos as they develop inside of her. The
nourishment can be delivered as a secretion called uterine milk or
through a placental connection.

The hammerhead shown in this picture is an example of viviparous
development. The placental connection to this fetus would have been
connected to the uterine wall of the mother.




The most common form of development is referred to as ovoviviparity.
This is similar to viviparity because the eggs are fertilized, hatch and
develop within the body of the female. However the embryos do not
receive any direct nourishment from the mother other than the initial
investment of the egg production. In some species, the young can
receive nutrients by devouring newly ovulated eggs or smaller, less
developed embryos or siblings. This is known as oophagy.



Examples of ovoviviparous developmen. In this example of a portuguese
shark, the developing ova can be clearly seen within the ovaries. The
eggs will eventually move into the uteri where they will hatch,
developing into an embryo nourished by large amounts of yolk. Further
examples of various embryo stages of ovoviviparous development can be
seen in the spiny dogfish and porbeagle sharks.



In this example of a porbeagle embryo, note the yolk stomach which looks like a distended belly under the embryo.



Black dogfish embryos (Courtesy of Einar Ásgeirsson and Ingibjörg Jónsdóttir, Iceland)



The final type of development is
known as oviparity. In oviparous reproduction, sharks lay eggs in the
form of egg cases, which they attach to algae or corals. The egg cases
of at least one species are designed to be pushed into the sediment.
Once the eggs are secure the female then leaves and the eggs receive no
subsequent protection or nourishment.



A number of small dark eggs can be
seen in the upper parts of the coral in this picture. Small tendrils
attached to the egg case are used to fasten it to corals and algae.
Without the protection of the mother,the embryo is then able to develop
and hatch with some level of protection from predators.

The egg case of a deepsea cat shark (as seen above).

The embryo of a deepsea cat shark
within the egg case. Notice the large yolk sac which nourishes it while
developing inside the egg case.




God bless you all








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