Sensory Organs of Fishes
Part 1 - Mechanoreception
By Koaw - January, 2017
Fishes live in water, for the most part. What does this mean for how they perceive the world around them? It means that they are surrounded by constant and intense stimuli. After hundreds of millions of years of evolution, sensory organs in fishes have become highly modified and specialized—this means there exists a lot of diversity in sensory mechanisms.
Water is a Unique Habitat:
The density of water allows for vibrations to carry farther, meaning sounds and movements are more readily detectable than in air. Dissolved salts and other impurities in water enable conductive properties, providing an environment favorable to the transfer of electric impulses, and most fishes can detect the bioelectric currents emitted from other organisms. And fishes are surrounded by molecules in solution, meaning that they can have taste and smell receptors on different locations of the body, depending on the species.
“Sensory organs are basically accessories to the nervous system that act as transducers. They capture specific types of signals, such as light, sound, molecular shapes, or electricity, and convert them into changes in action potentials, which are then carried by sensory neurons to the brain where the information is interpreted.”[i]
There are five main categories of sensory organs: mechanoreception (lateral line & hearing), electroreception, vision, chemoreception, & magnetic reception.
Fishes, and most all aquatic organisms, rely greatly on the ability to detect vibrations in water, or mechanoreception. This is how they hear, maintain and orient themselves in currents (rheotaxis) and within schools (groups of fish swimming in synchronicity), detect their prey and avoid being preyed upon, and converse with one another via inter- and intraspecific communication; (interspecific means with other species and intraspecific means within the same species). The density of water allows vibrations to carry great distances—much farther than in our atmosphere where the abundant molecules of nitrogen and oxygen are more dispersed compared to water.
Whales (although not fishes) are probably the most notorious ‘loud-mouths’ in the oceans, able to communicate over great distances. Other cetaceans, such as dolphins, not only are highly communicative but also use echolocation, or blasts of sound, to locate prey such as fishes. Certain snapping shrimp living atop the benthic (sea floor) can also be extremely noisy, as certain populations will fumble the radar readings on sonar equipment, such as what is used in military submarines[ii]. With so much life and so much movement within water, including the introduced motorized vehicles of mankind, aquatic ecosystems can be very noisy and excited places. Check out this website that has recordings of all sorts of aquatic sounds from animals to weather to wartime noises. (http://www.maritime.org/sound/)
There are two general types of mechanoreception in fishes: (1) The lateral line system & (2) the inner ear, both utilizing sensory hair cells.
The Lateral Line System:
There are two subdivisions of the lateral line system: (1) superficial neuromasts – these are free-standing on the dermal (skin) tissue & (2) canal neuromasts – these are embedded within channels both along the body and in dermal bones of the head.
A neuromast is a collection of hair cells, often covered in a protective gelatinous capula, that are connected to support cells and a sensory nerve. These detect disturbances in the water, allowing a fish to detect the nearby movement of prey and predator, and other obstacles.
Nifty Fact: Fishes that live in no light and minimal light conditions often rely heavily on their mechanoreception to detect prey, instead of using their vision, such as Lake Trout (Salvelinus namaycush) where this species is able to follow hydrodynamic trails of other fish in complete darkness as the hairs in neuromasts detect the vibrational movements within water[iii].
Equilibrium and Balance:
The pars superior within the inner ear of fishes is responsible for equilibrium and balance. Jawed fishes have three fluid-filled semicircular canals and a chamber called a utricle within the inner ear that use hair cells to detect changes in direction. The semicircular canals detect lateral movements as the fish moves as the acceleration changes will cause the fluid within the inner ear to move against the hair cells. (Think of a cup of water in a car or airplane; as the vehicle accelerates, the liquid appears to be uneven compared to the surface rim of the cup). The utricle is responsible for detecting changes in vertical orientation. The utricle has an otolith, a solid deposit made of calcium carbonate, also known as an “ear stone”, specifically labeled as the lapillus, resting atop sensory cells. Fishes also use their dorsal light reflex, the detection of light from above, in coordination with their utricle for vertical orientation.
Most fishes do not detect sound above 500 Hz, but some clupeids can detect much higher frequencies, such as the American shad (Alosa sapidissima), able to detect ultrasonic signals up to 180 kHz[iv]. The inner ear is mostly responsible for hearing in fishes. In teleosts (bony fishes) the utricle chamber of the pars superior, in combination with two chambers of the pars inferior, the saccule and lagena, both also containing otoliths (those solid deposits), called the sagitta and astericus, respectively, are responsible for detection of higher frequencies and spatial coordination.
Fish tissue and water share a relatively similar density and so the tissue is transparent to sound. This means that the otoliths, comprised of calcium carbonate, will resonate differently than surrounding tissues, including the hair cells within the otoliths’ chambers, triggering action potentials in the sensory neurons[v].
Fishes will hear differently according to their environment. Noisy environments, like coastal waters and turbulent rivers, have fishes with higher sound thresholds and smaller ranges of frequency detection as opposed to fishes living in quieter, calmer habitats.
Hearing Specialists & Generalists:
Hearing specialists are fishes that are capable of using the gas bladder to communicate sound to the otoliths organs (similar in function to a hearing-aid). The gas within the gas bladder resonates differently than the surrounding tissues, sending more information to the inner ear, making hearing specialists sharper at detecting vibrations than hearing generalists that do not possess a connection from the inner ear to the gas bladder, or simply not possessing a gas bladder at all. Notable specialists include cods (gadidae), squirrelfish (holocentridae) and herring and sardines (Clupeidae).
The most notable of hearing specialists may be the otophysan (also ostariophysians) fishes, which consist of more than 60% of freshwater fishes; these include the carps, minnows, milkfishes, characins like piranha and tetras, and catfishes. These fishes have a unique adaptation called the Weberian apparatus which consists of small bones connecting the gas bladder and inner ear. The Weberian ossicles, vibrating in tandem with the gas bladder, make the otophysans the most sensitive hearers with the largest frequency range among all fishes.
Bony fishes do not have any exterior opening to the inner ear. Sharks and rays may have an endolymphatic duct connecting the inner ear to a small exterior opening.
Main Reference for this article (Helfman, Collette, Facey, & Bowen, 2009)
[i] (Helfman, Collette, Facey, & Bowen, 2009)
[ii] (American Association for the Advancement of Science, 2000)
[iii] (Montgomery, MacDonald, Baker, & Carton, 2002)
[iv] (Platcha & Popper, 2003)
[v] (Schellart & Wubbels, 1998)