Shark Talk
Background:
The first sharks lived about 400
million years ago. That is about 200
million years before the dinosaurs existed.
The earliest sharks whose relatives are still alive today evolved around
200 million years ago. These were the
Bullhead, Cat, and Cow sharks (Six and Seven gill sharks). Contrary to popular belief, sharks are not
more “primitive” than fish. The bony
and cartilaginous fishes evolved around the same time.
Sharks can range in size from 40 feet
(Whale shark) to 10 inches (Dwarf shark), although over one half of all shark
species are less than 3.3 feet.
The age of a shark is determined by
the growth rings on the shark’s vertebrae.
These are made on a yearly basis.
However, in many shark species the accuracy is assumed due to the lack
of research in shark ageing.
Biology:
A shark’s entire skeleton is
cartilaginous, but some areas of it show varying degrees of calcification. The vertebrae in particular are
calcified. There are three types of calcification
found in the vertebrae:
1.
Cyclospondylous- A ring of calcification
2.
Tectospondylous- Two rings of calcification
3.
Asterospondylous- Star shaped zones of calcification
A shark’s skull is one continuous cartilaginous piece, unlike bony fish where the skull is made up of plates of bone. Attached to the skull by a muscular ligament are the jaws. The fact that it is not fused to the skull allows a wider range of motion. A shark can thrust its jaws out and bit upwards. Which helps it to take chunks from its prey. The teeth are lightly attached to the jaw by a collegen sheath of connecting tissue. In a lemon shark it was shown that the lower teeth lasted on average 8 days before being replaced, while the upper teeth only lasted 7.
The basking shark does not use teeth for catching prey, they use gill rakers, but like all other sharks, they shed the gill rakers. Unlike the other sharks, the basking shark tends to shed their gill rakers all at one time, in the winter when there it little food. In the spring they grow the gill rakers back.
An interesting theory… The Canadian Shark Research Laboratory states that the spiracle is actually the vestigial first gill slit. Paired with the theory that the jaw in all jawed fishes arose from the first gill arch, one can see how the spherical may have evolved.
A shark’s heart is a two-chambered tube, having one atrium and one ventricle, unlike ours, which is paired. Most sharks are poikilotherms, meaning they cannot control their body temperature. However, Great whites, Makos, Threasher, and Porbeagle sharks have a body temperature, which can be as high as 10 degrees warmer than the surrounding water. They use the heated blood from the muscle to warm the cold blood coming from the gills. The advantages of having warmer body, is that the shark can move quicker than would otherwise be possible.
Of all the internal organs,
the liver is the easiest to identify.
The liver of a shark can account from 5 to 15% of a sharks total body
weight. The liver is used to help the shark
maintain buoyancy, but it is also the only place where fat is stored in a
shark. Unlike most animals, they do not
store fat in the muscle, but only in the liver. The liver can be used to tide over a shark in lean times. Mating males and pupping females may not eat,
and depend on the stored fat in the liver for energy.
When digesting food the
stomach produces very strong enzymes, which reduces nearly everything to a thin
pulp. However, bones and other
indigestible things are prevented from entering the intestine by the smallness
of the pyloric opening. They are later
regurgitated.
With all animals that live in the sea, sharks have evolved a very
unique way of dealing with salt. Sharks
are actually saltier than the sea.
Because of this, the water from the sea will flow into the shark’s
gills. Sharks maintain this high level
of salt by retaining their urea. Some
sharks, like the bull can lower their salt concentrations to such a degree that
they can enter fresh water, but most cannot.
The extra salt is excreted through the rectal gland.
Senses:
Most sharks can see quite
well even in murky waters. It has been
suggested by the shape of a shark’s eye, that they are far sighted. They posses rods and cones like humans, but
also posses a Tapetum lucidum. This is
a layer of pigment located at the back of the eye, which reflects light back
onto the retina. It allows a shark to
make use of whatever light they have.
(To put it in perspective, cats also have a tapetum lucidum. It is what makes their eyes appear to
shine.) To protect their eyes, sharks
adapt a wide variety of defenses. Some
posses a nictitating membrane that covers the entire eye, in others it only
covers part of the eye and the sharks roll their eyes back into their
head. Some sharks also retract their
eyes as well.
The lateral line, located down the side of the shark, consists of a
small canal connected to pores. The
cells underneath these pores have a hair (neuromast), which is used in
detecting vibrations. The movement of
the hair transmits to the nerve ending in the cell. In addition to the lateral line, sharks have pit organs, which
detect vibration as well. The pit
organs also consist or the hair cells but are
scattered through out the body.
The Ampullae of Lorenzini are jelly filled pores, which connect to
the nerves. They are used to detect
electrical signals of their prey. The
Ampullae of Lorenzini can detect electrical currents as weak as .01 microvolts. They may also be used to help the shark
navigate murky waters, by using the magnetic field of the earth. The platypus is one of the few animals that
can also detect electricity.
They also posses taste
(works the best when in direct contact with items, which is why sharks “bite”
things to tell what they are), hearing (located at the top of the skull), and
smell.
Movement:
A shark’s tail produces a lift, which
shoves the shark’s head downwards. To
balance this, the pectoral fins produce a lift, which brings the shark up. The dorsal fins are used for balance to help
keep the shark from rolling over. The
faster swimming sharks posses a more equal lobed tail than others.
The swimming motion of a shark
actually starts right behind the head as a sin wave type motion that becomes
noticeable after the first dorsal fin, unless the shark is agitated.
Reproduction:
Male sharks have claspers, which transfer the sperm to the female. In adult males, the claspers are stiffened by calcification. The claspers also posses modified denticles that act as hooks to hold the female in place. The sharks may place one or both claspers into the female’s cloacal opening. The larger sharks position themselves by grasping the female’s pectoral fin with their teeth. In a shark species in the Caribbean (Apristurus riveri) the males have teeth that are twice as large as the females. Side by side the male angles his claspers to reach the opening. In smaller sharks, the males wrap themselves around the female during copulation.
The sperm in male sharks is contained with in packets called spermatophores. These spermatophores are flushed from the claspers with seawater, drawn into the male shark by siphon sacs. In some species the female may store the sperm in the shell gland, so fertilization may not occur right away. Male sharks have paired testes, but in some species (dogfishes) the left one is greatly reduced. In the females, the left ovary is reduced as well and may not produce eggs.
After fertilization the zygote is carried into the shell gland, where different things can happen depending on how the shark give birth. In some cases, as stated above, fertilization may occur in the shell gland.
In oviparous sharks (like horn and swell sharks) the egg is encased in a horny tough casing within the shell gland. The egg is then laid by the mother and receives no nutrients from her. The embryo receives all its nutrients from a yolk, which is progressively reabsorbed by it. Breaking free from the egg requires the embryo to move around in the shell, and is aided by the secretions of the hatching gland located on the top of the head.
In ovoviviparous sharks (like leopard, spiny dogs, and possibly great whites) the egg is encased in a thin membrane in the shell gland. It is shed in the uterus and the embryo is nourished by the yolk sac. When the yolk sac is finished the embryo may also feed on the undeveloped eggs surrounding it (oophagy). It is hypothesized that the great whites may only give birth to two pups, one on either side of the uterus. The embryos are not given any nourishment by the mother. This may also be called aplacental viviparity.
In
viviparous sharks (brown smoothhounds) the shell gland is greatly reduced. The embryos may develop in separate
compartments with in the oviduct like in the lemon, blue, and bull sharks, or
in the uterus. The yolk may form a
placental connection by attaching to the uterus. Or after the yolk is spent they may receive nourishment from
secretions called “uterus milk” (tiger shark).
When birthed, the umbilical cord is snapped by the pup swimming away
from the mother.
References:
http://enchantedlearning.com/subjects/sharks/anatomy/Repro.shtml Enchanted Learning
http://www.mar.dfo-mpo.gc.ca/science/shark/english/index.htm Canadian Shark Research Laboratory
http://www.seatrek.org/curriculum/reference/anatomy.htm Sea Trek
Paul Bunker, The
life of Sharks, Columbia University Press, 1971
Milton Love, Probably
more than you want to know about the fishes of the Pacific Coast, Really
Big Press, 1996
Miranda MacQuitty, Eyewitness
books: Shark, Dorling Kindersley, 1992
Timothy C. Tricas
(author of the section I used), The Nature Company Guides: Sharks & Rays,
Time-Life Books, 1997, pgs. 90-111