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 

  Iziko Museums of Cape Town, 2008

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