| The electric ray
possesses a unique organ capable of delivering an electric shock in excess
of 200 volts. Like other rays, they are bottom dwellers and are able to
bury themselves in mud or sand. |
In his log for
1770 Captain James Cook recorded taking several rays weighing more than
100 kg (2201b) in 'Stingray Bay', later renamed Botany Bay.
Rays are
broad, flattened fishes found in waters over most of the world. There are
about fifty Australian species. They share some of the physical features
of sharks: both rays and sharks have open gill slits, spiracles or water
inhalers, internal fertilization capacity, and no bony skeleton, scales
or swim bladder. Both are cartilaginous. There are even more points of
difference: the rays' five gill slits are situated on the ventral, or belly,
side behind the mouth, and those of sharks on the sides in front of the
pectorals; the rays' spiracles are behind their eyes, and those of sharks
on the sides of the head in front of the gill slits. The positioning of
the spiracles is nature's way of allowing the rays, benthic
animals living on or near the bottom, to breathe without opening
their mouths and taking in quantities of sand and mud. Such neat compartmentalization
is not always possible. Some rays such as the pelagic Mobulidae are not
bottom dwellers and they draw in water through their mouth and breath as
the sharks do.
Other points
of physical difference between rays and sharks are in the teeth, the fins
and the tail. Rays have blunt teeth, suitable for crushing the shells of
the molluscs on which they feed, and those of sharks are sharp and pointed.
The rays' pectoral fins, which are joined to the head, are developed to
the point of distortion, while those of sharks are in proportion to their
other fins. The rays' tail is usually slender and may or may not have a
tail fin, whereas in sharks the upper lobe of the tail fin is always very
prominent.
Even in
rays themselves there are variations in tail shapes and proportions. Most
rays have narrow whip like tails, hardly more than a filament, and they
swim with an undulating
movement of their pectorals, which gives
them the appearance of prehistoric underwater birds. This is not true of
species like the shovel noses, which have well developed tails which they
use with their dorsal fins for swimming.
|
The manta, or devil
ray takes it's name from a pair of fins, or horns on either side of the
mouth.
Rays are frequently
taken by anglers fishing from the rocks, particularly if there is an adjoining
beach. They feed on molluscs present in these areas and often take a beach
fisherman's pipi intended as bream bait. |
|
Ray families:
Representatives
of all the ray families are found in Australian waters. They are the Torpedinidae
(electric rays); Rajidae (skates); Dasyatidae (stingrays);
Myliobatidae (eagle rays and bull rays); Mobulidae (devil fishes and manta
rays);
The electric
rays, also called cramp fish, numb fish and torpedo rays, have flat, almost
round bodies, large ventral fins and a thick tail. They are sluggish creatures;
some dwell at great depths and are blind but most bury themselves in the
mud or sand in shallow water. An electric centre behind the eyes allows
them to deliver electric charges as powerful as 220 volts, and to repeat
the charges fifty times within ten minutes. They use this power
to paralyse the small creatures they need for food and to defend themselves
against predators, not always successfully as large sharks often manage
to feed on them. They are immune to their own power.
Gilbert Whitley in his book on the sharks of Australia
indicates the similarities and dissimilarities between rays and sharks
by the whimsical analogy of a mangle used by Mrs Neptune below
the sea: a shark passed through the rollers of a mangle would, no doubt,
emerge in a very flattened condition, with its gill slits resting beneath
its body, and its side fins considerably widened.
Members of the various ray families live in
both salt and freshwater, and they tolerate a variety of temperatures from
tropical to the cooler range of tropical waters. |
Skates are kite
shaped fish, measuring 30-45cm (1-1.5ft) across, and having a pointed snout
and a well formed tail. They deviate from the ray norm in not being viviparous.
Instead, the females produce tough egg cases about 30 cm (12 in) long,
which are known as mermaid's or sailor's purses. The cases have attachments
on all four corners to keep them on the bottom but they are, nevertheless,
often washed ashore. Skates also have an electric centre in the tail, but
this is weak and used for communication, not defence.
Members
of the stingaree family, Urolophidae, and of the stingray family, Dasyatidae,
are found in Australia. The Urolophidae, of which there are only a few
species, favour the Queensland estuaries where they are barely visible
against bottoms matched by their own sandy or muddy coloration. Their bodies
are broader than they are long, and they have a single venomous spine at
the base of their tail. Fortunately, they tend to avoid contact with humans.
The Dasyatidae
are more diverse, but all species have the same long, whip like tails con-
tain one or more venomous spines. They include: the attractively
marked blue spotted rays whose tail has a blue stripe on each side; the
giant coach whip rays which may be 150 cm (5 ft) broad and weigh up to
100 kg (2201b), and which have a black tail twice as long as the body,
containing one large spine up to 38 cm (15 in) long; and cow tail or fantail
rays, also giants measuring more than 180 cm (6 ft) across. |
