Touch is the primary and most immediate of senses and it is as important to fish as it is to any other group of animals.
In fish, the sense of touch is most important in those species that live in close association with objects: the sea floor, coral reefs, aquatic plants and in some cases other fish. For many fish the sense of touch is also important during mating.
As in most animals, the skin of a fish is an important sensory organ and it is endowed with a mass of sensory nerves. Many of these relate to the fish’s sense of touch, but others sense water conditions such as temperature and salinity.
A Sense of Temperature
Scientific research has shown that fish are highly sensitive to variations in water temperature and that they can detect changes in ambient water temperature as small as 0.003 to 0.007 degrees. This is obviously useful to a fish that is trying to move to a warmer or cooler area.
Fish detect water temperature in one, or two different ways. In most teleost fish it is the lateral line that contains the temperature senses. However cartilaginous fish and some others posses specialized organs called ‘Ampullae of Lorenzini’. These are a form of electroreceptor, and in cartilaginous fish, they form a network of jelly-filled pores across the fishes head.
A Sense of Salinity
Marine fish, as might be expected, are also highly competent at sensing the salinity (salt content) of the water around them. In scientific experiments, fish have been shown to be able to discriminate between salinity levels varying by as little as 0.5 parts per thousand.
How fish detect changes in salinity (the saltiness of water) is fascinating but chemically complex. In simple words fish have special calcium based receptors that detect changes in density of cations in the water around them.
In more detail these receptors are called Calcium polyvalent cation-sensing receptor (CaRs). They are G protein-coupled receptors that detect alterations in divalent and polyvalent cations present in a surrounding liquid. CaRs proteins are present in many organisms including human beings where they work to regulate Ca2+ ions in extracellular fluids.
In fish they also work to allow the fish to sense and respond to alterations in water salinity based on changes in Ca2+, Mg2+, and Na+ concentrations.
A Sense of Pressure Pressure
Fish are also known to be able to detect small changes in pressure. In fish with swim bladders the swim-bladder plays an important role in this ability. But fish without swim-bladders have also been shown to be sensitive to small changes in water pressure.
In fish of the superorder Ostariophysi, pressure sensitivity works through a group of small bones connected to the swim bladder. Together these are know as Webarian Apparatus.
The three most important bones in the Weberian Apparatus are the ‘claustrum’, the ‘scaphium’ and the ‘tripus’, these are called the ‘Weberian Ossicles’. All the bones in the Weberian Apparatus are modified vertebrae.
They work by detecting and amplifying changes in the shape of the swim bladder and then transferring this information physically to the inner ear, to which they are physically connected. In doing this they magnify minute changes in the shape of the swim bladder, which itself is responding to pressure changes, some of which are caused by sound waves.
You can learn more about other fish senses from the links here: about fish anatomy.
Image Credits: Ampullae of Lorenzini by Chris huh – Public Domain