Olfactory

 

STRUCTURE AND FUNCTION OF CHEMOSENSORY ORGANS

Theolfactory organs ornares (singular, naris)of nearly all fishesare paired bilateralstructures (unpaired and medialin position in hagfishes and lampreys) that consist of olfactory chamberslined with various numbers of folds of olfactory epitheliacalled lamellae.

The presence and structure of these lamellaedramatically increase the surface area of the olfactory chambers. This surface area provided by the olfactory lamellaeincreases with growth over the life of the fish both through an increase in size of individual lamellae as well as by addition of more lamellae.

The shape of the olfactorychamberand the number of lamellaecontained within varies enormously among fishes. In general, a greater number of lamellae is associated with greater olfactory requirementsand sensitivityto odors.

For example, the threespinestickleback (Gasterosteusaculeatus), which relies most heavily on visionto make its way in the world, has small rounded olfactory chambers, containing very few lamellae.

Whereas, most eels(order Anguilliformes), which rely heavily on olfactory cues, have large, elongate olfactory chamberswith numerous (>60) lamellae. Species with intermediate sensitivities to chemical odors (which make up the vast majority of fishes) have oval-shaped chamberswith an intermediate number of lamellae.

 

The olfactory organs of terrestrial vertebratespass through the roof of the snout and open into the pharynx.

We say they have internal nares. This passageway allows air and air-born chemical

odors to be pulled from the external environment, over the olfactory sensory nerve cells that

line the nasal passages, and into the throat and lungs, through the process of respiration.

In contrast, the olfactory organs of nearly all fishes are completely closed to the pharynx. We say they have external nares, anterior and posterior openings, called anteriorand posteriornares.

Water, along with water-born chemical odors, is drawn into the anterior naris, passed over the sensory receptors of the olfactory lamellae, and then forced back out into the external environment by way of the posterior naris.

 

The arrangement of tubular anterior and posterior naresof an eel. Water flows into the anterior naris, across, around, and between the numerous olfactory lamellae, and back out through the posterior naris. Among fishes, exceptions to this general rule of closed nares include the hagfishes(Myxini), a very few teleosts(some uranoscopidsand the bathydraconidgenus Gymnodraco), and, of course, the lungfishes(Dipnoi). Representatives of these few groups have internal narescomparable to those of terrestrial vertebrates.

 

A stargazer, genusIchthyscopus

 

Water is pushed through the naresby various mechanisms. In sharks and rays, the nares are on the underside of the snout, directly in front of the mouth. Water is forced or pulled across the naresby the actions of a branchialpump.

In the case of teleosts, most of which have naressituated on the sides or on top of the snout, currentsthat flow across the head and snout produced by swimmingare often enough to cause a steady flow of water into and out of the nares. But, in most cases, ciliaryactionwithin the olfactory chamber helps to move water across the olfactory lamellae.

Olfactory sensitivity: All available evidence points to a great acuityof the sense of smell in many species of fishes, both with respect to the (1) perceptionof very low concentrations of odors and to (2) discriminationof two or more odors in a mixture.

Early research on thresholdsinvolved behavioral experiments

in which individuals trained by reward or punishment were conditionedto select or avoid water that held the chosen chemical and then were tested on more and more diluted concentrations. More modern methods involve conditioned heart rate, electroencephalograms, recordings from nerve tracts, and other electrophysiological methods. Various published sources for a number of fish species and chemical substances show wide variation.

You can see that there is considerable variation but, in general, the olfactory sensitivityof fishes is comparable to that of mammals. For example, the rainbow trout(Oncorhynchusmykiss) has been shown to detect phenyl-ethyl alcoholat concentrations of 1 x 10-9molar, which is on the same order of sensitivity as humans.

The European eel (Anguilla anguilla), with its highly developed olfactory chambers and numerous olfactory lamellae, can detect phenyl-ethyl alcoholat concentrations of 3 x 10-20molar or about the same concentration detectable by dogs.

Someone figured out that this olfactory acuitycorresponds to 1 ml of alcohol dissolved in a lake of a volume 58 times as great as that of Lake Constance(bounded by Germany, Austria, and Switzerland; 207 square miles), which equates to oneor two molecules of alcohol in the olfactory chamberat any one time.

 

Gustation, the sense of taste, is important to fishes primarily in the location and identification of possible food sources. Depending on the ecology of the species, it can be a very highly developed sense.

For example, in certain minnows(family Cyprinidae), the sensitivity to various sugarsand saltshas been found to be 512 and 184 times greater, respectively, than that of humans.

 

Unlike olfactory sense receptors, taste receptorsare not restricted to a single location. In addition to positions throughout the mouth, gill arches, and skin, catfishes (Siluriformes) have well-developed barbels(sometimes longer than the fish itself) that bear dense concentrations of taste receptors.

These barbelsare under muscular control and often stiffened and can be used as “sweepers” to cover a wide search-area for these often-nocturnal species. Similarly, many species of codsand their allies (family Gadidae) have taste buds on specialized chin barbels.

 

Barbelsof various fishes: (A) a sciaenid or drum (genus Pogonias); (B) a catfish (genus Clarias); (C) a mullet (genus Mullus); and (D) a cod (genus Gadus).

 

A searobin(genus Aspitrigla), with specialized pectoral-fin raysused as “feelers” to search

for food on the bottom, but also, lined with taste buds, the rays are used to taste potential

prey.

 

 
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