| Skeleton
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The internal
skeleton of skates and rays (or endoskeleton)
– as with sharks – lacks true bone, and is instead made entirely of
cartilage. Cartilage is a
strong and durable material that is lighter and more flexible than bone,
enabling elasmobranchs (which lack a swim bladder) to stay afloat and turn
in a tighter radius than other fish. However,
parts of the elasmobranch skeleton – such as the skull, the vertebrae,
and dermal spines – are often strengthened by the deposition of calcium
and salts, a process called calcification. When the
skeletal structures of elasmobranchs become sufficiently calcified, they
can resemble bone, and have similar strength characteristics to bone,
without the added weight. The
process of calcification varies greatly from one species to the next, with
some species - especially deep-sea and primitive forms - having little
calcification while in others, such as pelagic sharks, calcification is
extensive.
The vertebral centra of elasmobranches are cylindrical and biconcave in shape and are generally divided into two primary types: precaudal and caudal vertebrae. The number of vertebrae a given individual will contain throughout its entire life is set during embryonic development, a period which also marks the beginning of vertebral calcification. Precaudal vertebrae generally begin forming before caudal vertebrae, and all vertebrae appear to grow throughout the lifespan of individual sharks, skates, and rays. This enables the use of vertebral centra for ageing. Age is determined in elasmobranchs by counting vertebral rings that are deposited annually, not unlike those of trees. This method is analogous to the use of otoliths (or earstones) to determine age in bony fishes. For more information on how skates are aged, so to the Skate Research section of this website.
In batoids, the spinal column is quite elaborate compared with that of sharks. Specifically, posterior to the neurocranium (or the elasmobranch skull) at the anterior end of the vertebral column, the precaudal vertebrae are fused into a tube called a cervicothoracic synarcual, which can include as few as 4 and as many as 45 vertebrae, depending on the species (Fig. 1). This rigid structure is much less flexible than the rest of the vertebral column, and functions to support the greatly expanded pectoral fins. Some species of myliobatoids even have a 2nd synarcual! The liver of
elasmobranchs is a large, soft and very oily organ which occupies most of
the body cavity and can comprise as much as 25% of the body weight.
It is the first organ encountered when an incision is made
ventrally from the axils of the pelvic fins to the anterior end of the
body cavity. It consists of
two large, pointed lobes that are greenish-gray to dark reddish-brown in
colour. The function of the
liver in sharks, skates, and rays is two-fold.
First, as in all animals, the liver concentrates the fatty
reserves, and therefore provides for energy storage.
Secondly, the liver of elasmobranchs acts as a hydrostatic organ by
storing lighter-than-water (or low density) oils.
These oils counteract the sinking tendency of the elasmobranch by
decreasing the density and increasing the buoyancy of the animal on a
whole, such that the animal becomes neutrally
buoyant. Without such a
large liver, sharks, skates and rays would have difficulty staying off the
bottom, as they lack the swim
bladder characteristic of bony fishes.
The second
most noticeable structure in the body cavity of sharks, skates, and rays
is the digestive tract which consists of two contiguous organs: the
esophagus and the stomach.
The anterior end of the stomach (also known as the cardiac
stomach) is J-shaped and saclike, and tapers into the posterior part
of the stomach known as the pyloric
stomach, which bends anteriorly.
The pyloric stomach terminates at a constriction called the pylorus,
which leads to the short duodenum
and then to the larger spiral valve
intestine, which is highly coiled and twisted internally.
The function of the spiral valve intestine is to increase the
surface area for digestion and absorption of food, while also conserving
space in the body cavity for the large liver and for the development of
embryos (in the case of rays) or egg capsules (in the case of skates).
The spiral valve, in turn, leads to the rectum
and the anus, which opens into
the cloaca, a cavity where the
digestive, urinary, and genital glands open to the outside. The pancreas
is a gland that aids in digestion by secreting digestive enzymes into the
duodenum. It consists of two
connected lobes: a ventral lobe, which contains a duct from which
pancreatic secretions enter the duodenum, and a dorsal lobe.
Both are usually pinkish in colour. The spleen
is a dark brownish organ, triangular or slightly elongate in shape, which
lies against the stomach. However,
it does not play a role in the digestive process. Instead, it is part of
the lymphatic system, a system which is a major component of the immune
system. The rectal
gland is a small, finger-like organ that concentrates large quantities of
excess salt (or sodium chloride) from the bloodstream for eventual
excretion via the anus. Specifically,
it secretes a colourless solution with about twice the concentration of
sodium chloride found in the blood plasma into the rectum via a small
duct. This organ is very
important to elasmobranchs, whose livers produce large amounts of urea,
thereby making these marine fishes slightly hyperosmotic
to seawater (i.e. their internal solutes are greater than the
surrounding seawater, so they are constantly gaining water and salt). The kidneys
are part of the urogenital tract, and are involved in the manufacture and
transport of urine as well as in the regulation of plasma urea
concentrations. In skates and
rays, they are either semi-lunar shaped or ribbon-like, dorsoventrally
flattened, dark red organs that are highly lobed and lie dorsally on
either side of the spinal column outside of the body cavity.
A tough membrane, called the peritoneum,
separates the kidneys from the rest of the body cavity.
The kidneys are drained into the cloaca by the ureters. Return to TOP |
