Most of the more than three hundred living species
of the order Batoidea, the rays and skates,
are adapted for living on the bottom. In body
form they are strongly depressed (flattened), with
enlarged pectoral fins extending forward to the
head region. Their teeth are usually
pavementlike, for crushing their
hard-shelled invertebrate prey. Most
species give birth to live young, except
the skates (family Rajidae), in
which the eggs develop in a leathery
egg case (the "mermaid's purse" that
beach visitors often find in the sand).
Several ray families include members
with a venomous spine on the
tail, including the Dasyatidae (stingrays),
the Potamotrygonidae (river
stingrays), and the Myliobatidae (eagle
rays). The largest species among
rays and skates is the giant manta ray
or devilfish (Manta birostis), which
may attain a width of over six meters
between the tips of its pectoral fins
and a weight in excess of 1,300 kilograms.
Like two other cartilaginous
fish giants mentioned earlier (the
whale shark and basking shark), the
giant manta ray is a plankton feeder. It
directs plankton into its mouth as it
swims by means of large scooplike
extensions on its head-the "horns"
responsible for the name "devilfish."
It then filters out the plankton with its
comblike gill rakers.
Rays and skates swim by means
of flapping movements of their
winglike pectoral fins. Some species,
including eagle rays and manta rays,
can make spectacular leaps from the water.
Among the more remarkable rays are the electric
rays (family Torpedinidae). These sluggish fishes
use electrical discharges of up to two hundred
volts, produced by a pair of disk-shaped electric
organs on the sides of the head, to stun their prey
and perhaps to repel predators. Another specialized
group among the rays is the sawfish family
(Pristidae). Asawfish resembles a somewhat flattened
shark in body form but has a long, flat,
toothed extension (the "saw") on the end of its
snout. This is used to slash through a school of
The Place of Rays in Evolution
Cartilaginous fish represent an early line in the evolution of vertebrates. Understanding their interrelationships is crucial to an understanding of the ancestry of other fishes and of tetrapods (amphibians, reptiles, birds, and mammals) and thus, ultimately, of humans. Even though humankind's ancestors split from the ancestors of sharks and rays more than 400 million years ago, many anatomical and physiological features are shared. A prime example is the eye, which is extraordinarily similar in all vertebrates. The same system of eye movement, involving six muscles innervated by the same three cranial nerves, has remained unchanged throughout vertebrate evolutionary history. Thus, the study of shark eyes, or any other aspect of shark biology, deepens the understanding of the evolution of higher animals. Much remains to be learned about rays and other cartilaginous fishes. Studying their ecology, behavior, and evolutionary relationships is important for further understanding of their basic biological nature. It is also essential for maximizing the benefit of commercially important species and minimizing the risk posed by dangerous species.
Subclass: Elasmobranchii (sharks, skates, and rays)
Geographical location: Worldwide
Habitat: Bottom of the oceans, seas and freshwater reservoirs
Gestational period: Unknown
Life span: Unknown
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