Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
Senses and brain of cartilaginous fish
Cartilaginous fishes have well-developed sense
organs. Their eyes are usually large and well-developed,
particularly in many deep-sea sharks. A few deep-water
electric rays
have degenerate eyes and are blind. Some sharks have a binocular
field of vision, but the eyes of most cartilaginous fishes have
virtually independent fields. Many cartilaginous fishes have vision
adapted for low light, nocturnal activity or deep-water conditions,
and poor color definition, and have retinas densely packed with rods
and few cones. Some day-active sharks, including the
Great white
shark (Carcharodon carcharias), have numerous cones as well as rods,
and may have good color vision and high visual acuity. Many
deepwater cartilaginous fishes have a prominent layer of reflective
material (the tapetum lucidum) behind their retinas, which serves to
reflect light passing into the eye back into the retina, and hence
increases its sensitivity. The eyes of these species glow bright
green or yellowish when caught, from reflected light.
The sense of smell (olfaction) is
well-developed in cartilaginous fishes, which have large nostrils
and olfactory organs. Some sharks can detect attractive substances
at over one part per million parts of sea water, and are able to
follow scent trails directionally and from great distances. Such
sharks will swim against a current, tracking the scent trail to its
source. Olfactory cues may play some role in orientation of
cartilaginous fishes, perhaps in finding receptive mates or other
members of their species, or locating specific areas, but this needs
further research. Taste buds are well-represented in the mouths of
cartilaginous fishes; some sharks are very selective on potential
food items, swallowing or disgorging them after taking them into
their mouths.
The inner ears of cartilaginous fishes have
large semicircular canals for maintaining equilibrium, but their
sound-detecting apparatus is of limited range and complexity
compared to birds and mammals. Sharks respond best to the lower
sound frequencies, below 1000 hz, corresponding to many natural
underwater sounds. Some low sound frequencies are attractive to
sharks and may draw them from considerable distances. No
cartilaginous fishes are known to produce underwater sounds, unlike
many bony fishes.
Related to sound reception is the lateral line
canal system of cartilaginous fishes and other aquatic vertebrates,
a network of tubes below the skin, with several branches on the head
but with a single line, the lateral line proper, running along the
body on both sides and extending to the caudal fin. These canals
have short tubes with external pores opening at frequent intervals
to the outside, and have sensory cells that are responsive to
low-frequency, close-range water vibrations. These can aid in
avoidance of obstacles, location of prey, and detection of
low-frequency sound. A similar function has been suggested for the
pit organs, blind subdermal pockets with similar sensory cells as in
lateral line canals, and with pores connecting to the surface. Pit
organs are scattered along the body and may be very numerous in some
large sharks such as
hammerheads.
Cartilaginous fishes are equipped with the
ampullae of Lorenzini, clusters of elongated, blind tubes with
sensory cells on the blind end, a gelatinous filling, and openings
to the outside; these form conspicuous groups of pores on the head
and snout. These ampullae are sensitive to electrical fields, and
provide a means for cartilaginous fishes to locate potential prey or
one another by sensing the electrical fields produced by muscles and
nerves. The ampullae may also function in navigation; cartilaginous
fishes, moving through the water, produce electric fields that vary
directionally according to the position of the cartilaginous fish
relative to the Earth's electromagnetic field. These localized
electrical fields can be detected by the ampullae and may provide
directional information for long-distance navigation without visual,
olfactory, or other sensory cues.
The input of the various sensory organs of
cartilaginous fishes are integrated by their brains, and provide the
individual a multifaceted `picture' of its environment. Pioneer
researchers on sense organs, preoccupied with shark attack and ways
to prevent it by studying how shark sense organs worked,
concentrated on each sense organ independently rather than looking
at all of them as an integrated unit. As with most living organisms,
cartilaginous fishes do not rely on a single sense to interpret
their environment but use all their senses to locate prey, other
members of their species, or enemies. Cartilaginous fishes have
fairly simple brains compared to those of large, advanced mammals,
with the forebrain usually not greatly enlarged. Surprisingly these
fishes have large brains proportional to their body size, and
overlap birds and more primitive mammals in their ratios of
brainweight to bodyweight. The devilrays (Family Mobulidae) may have
the largest brains of any cartilaginous fishes.
Electric organs
Some rays have electric organs, organic
`batteries' formed from modified muscle tissue. The cells of these
organs form tiny stacks of batteries in series to increase the
voltage, while adjacent stacks produce a parallel effect to increase
the amperage or current flow.
Skates have elongated, spindle-shaped
electric organs in their tails, which may serve in self-defense to
ward off potential predators.
Electric rays have large,
kidney-shaped electric organs in their pectoral disks, which may be
primarily defensive in small-mouthed species that eat tiny bottom
invertebrates. Some large electric rays also use these organs
offensively, to stun pelagic prey.
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale
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