4 Main Sense Organs in Fishes | Phylum Chordata

The following points highlight the four main sense organs in fishes. The sense organs are: 1. Eye 2. Ear 3. Lateral-Line System 4. Hoagland.

Sense Organ # 1. Eye:

The eyes of fishes are the photorecep­tors to see things under water. The eyes are typically built on the principle of a photo­graphic camera. The eye ball is composed of three layers—the outer is the sclera, the medi­an is the choroid layer and the innermost pho­tosensitive layer is called the retina. The lens is mostly globular and the cornea is flat.

In the elasmobranchs, the eyes are very large and in the deep-sea holocephalans the eyes are the largest. The eyes in most elasmo­branchs are held in position inside the orbit by a cartilaginous stalk or optic pedicel. In some sharks and rays, the pedicel is slender, elastic and helps in the protrusion of the eye ball.

Most conspicuous feature in the eye of elasmo­branchs is the presence of cartilage in the scle­ra, absence of intrinsic muscles in the ciliary body, presence of a fold of conjunctiva recall­ing the nictitating membrane of higher forms (in a few sharks) and the presence of large rounded lens.

In most elasmobranchs, a light- reflecting layer, called tapetum lucidum (composed of guanin), is exceptionally well- developed. The tapetum lies in the choroid.

In Basking shark, Cetorhinus and in an abyssal shark, Laemargus, the tapetum is wanting. In most teleosts, a silvery layer, argenteum is present between the sclerotic and the choroid layers. The cells composing the layer contain fine crystals. Absence of choroid process is a notable feature in the eyes of teleosts.

A choroid gland is present which is not glandular in nature but is composed of a compli­cated network of vessels (rete mirabilis). The retinal layer lacks the cones in the elasmo­branchs except Mustelus, Myliobatis and a few other forms. In these forms, the cones have been derived independently from the rods. In other fishes, the eyes exhibit great variations.

The cartilate in the sclera may be replaced by bony plates. The cones are present in teleosts. Accommodation of the eyes in caused by change of position of the lens. This is accom­plished by the falciform process in conjunction with campanula Halleri in teleosts. The falci­form process ends in a muscular knob called campanula Halleri or retractor lentis.

But in elasmobranchs, accommodation is brought about by the activities of the smooth muscle fibres. These fibres bring the lens towards the cornea. The deep-sea teleosts inhabiting the total darkness possess enormous eyes. In these forms the pupil and lens are exceptionally large. The rods are well-developed but the cones are totally lacking.

The eye ball is tubular in some deep-sea forms. Amongst the elasmobranchs, in a deep-sea electric ray, Benthobates moresbyi, the eyes are degenera­ted. Amongst the deep-sea teleosts, a few are blind and in others the eyes are degenerated. In Ipnops murrayi, the eyes have altogether disappeared. Certain cave-dwelling fishes have minute functionless eyes.

In some teleosts, the eyes are adapted for vision in water as well as in air. The typical examples are Periophthalmus and Boleophthalmus. The eyes in these two forms are prominent and each eye can be rotated in all directions. In four-eye fish, Anableps of Central America, each eye has two pupils in adults. The eye is divided into two parts by a horizontal band.

The upper half usu­ally remains out of water and is used in aerial vision. In another fish, Dialommus two pupils are present in each eye which are actually circular transparent areas in an opaque and pigmented cornea.

The fishes possess monocular vision, i.e., the two eyes cannot be focused on the same object. But certain deep-sea fishes have tele­scopic vision. The eyes are mostly present on the lateral sides of the head, but in flat fishes both the eyes are placed on the dorsal side.

Sense Organ # 2. Ear:

The ear in fishes is represented only by the internal ear or membranous labyrinth. The internal ear is completely enclosed by the otic bones. The membranous labyrinth shows many structural peculiarities in elasmo­branchs. The internal ear is composed of three semicircular canals and three other parts (the utriculus, sacculus and lagena).

The last three chambers contain otoliths (ear stones), namely the lapillus, sagitta and asteriscus. In many elasmobranchs, a small tube arising from the dorsal side of the sacculus opens to the surface of the head. This duct is termed the invagination canal which is homologous to the endo-lymphatic duct of the forms. In these forms, the cavity of the mem­branous labyrinth is filled with the sea-water, rather than endolymph.

In other fishes, a new endo-lymphatic duct appears and the invagina­tion canal disappears altogether. In some teleosts, the presence of Weberian ossicles correlates the function of the swim-bladder with equilibrating and audi­tory functions of the internal ear. In elasmo­branchs, the experimental evidences have established that the ear may function as a vibration receptor.

Olfactory sense organs:

In fishes, the olfac­tory organs are paired and show an evolution­ary advancement. In many fishes these organs may be just blind pits with olfactory epitheli­um or Scheiderian membrane. The olfactory epithelium may produce numerous Scheide­rian folds to increase the area in fishes.

In bony fishes, the nostrils are generally located on the dorsal side of the head, but in elasmobranchs these are situated on the ventral side. Excepting the Choanichthyes, the nasal cavi­ties are not in communication with the mouth in other fishes, although an oronasal groove in some elasmobranchs foreshadows the condi­tion in Choanichthyes.

Sense Organ # 3. Lateral-Line System:

The lateral line system is an integral part of the acousticolateralis system specially in fishes. Besides the lateral-line sense organs this system includes the ear also. The lateral-lines sense organs consists of a peculiar system of epidermal lateral-line sense organs and found in cyclostomes, fishes and a few larval amphibians.

This system is best developed in fishes where the presence of lateral line has been recorded from the fossil records of Silurian strata. The whole system comprises of the following organs.

Lateral line proper:

The lateral line lying on either side of the body of a fish consists of groups of sensory cells, the neuromasts housed in the epithelial cells of the outer layer of the epidermis. They are surrounded by many supporting cells. The sensory cells of the neuromasts bear a hair-like process. The hairs of the whole group of sensory cells remain sur­rounded by a gelatinous mass, called cupula, secreted by the cells.

The neuromasts are commonly placed over the paths of nerves. They usually sink down into the skin in grooves which become closed over to form the lateral-line canal. The canal becomes filled up with mucus secreted by mucous cells. In Holocephali, however, the grooves remain open. In other fishes where the groove is closed, the lateral-line canal opens to the surface of the skin through small pores (Fig. 6.102A).

This system arises from skin placodes on special thickening connected by strands with underlines rudiments of the cranial nerves. In embryonic and larval conditions one dorsal longitudinal, one ventral longitudinal canal and one lateral longitudinal canal extend the length of body on either side. In course of development the dorsal and ventral canals disappear in the trunk region but persist only in head region.

In full-grown fishes, usually in the head region a supraorbital canal runs forward above the eyes, an infraorbital canal courses forward below the eye, one supra-temporal canal runs across the posterior part of the head and con­nects lateral-line canals of one side with those of the other side of the body.

Besides there may be mandibular, hyomandibular, opercular canals which are named according to their position. Usually all the canals remain conti­nuous with one another but this is not univer­sal (Fig. 6.102B).

The canals of the the head region are sup­plied by branches of the facial nerve. The canals of the lateral sides of body are supplied by the branches of the vagus nerve. However, different groups of fishes may show slight variations in structure of the lateral-line organ.

The body canal is extreme­ly reduced in Rhodeus, may be secondarily reduced in several fishes inhabiting the deep- sea. In Mugil there are many nearly parallel lateral lines. In many ganoids and dipnoans the canal walls become ossified, particularly in the head region.

Sense Organ # 4. Hoagland:

Hoagland has shown that this organ can receive sound waves. It acts as a temperature receptor also. It has also been reported to be a pressure receptor. Moreover, this organ informs the fish of localised disturbances arising from mechanical vibrations in water, provided the vibrations are of the value lesser than 100 cycles per second.

But the most important function is ‘distant touch orienta­tion’, i.e., the location of distant objects which reflect the water movement, is brought about by the fish itself.

Due to the similarities in the structure of the sensory cells of both internal ear and lateral line and due to the fact that both can receive sound waves and both develop from skin placodes, the two are held to be only different components of the same ‘Acousticolateralis system’.

Ampulla of loranzini:

Many pores on the dorsal and ventral sides of the head in many elasmobranch fishes lead into peculiar senso­ry organs, called ampulla of Lorenzini. It con­sists of a long tube-like structure terminating into 8 to 9 ampullary sac radially disposed around a central core, the centrum. This tube and these ampullary sacs contain a gelatinous matrix. The sacs bear sensory as well as glan­dular cells.

The ampullae are supplied by the branches of the 7th cranial nerve. Sand showed these organs to be thermo receptors. Besides, these organs are recorded to respond to the changes in the salinity of surrounding water. Murray has shown that these organs may also respond to changes in hydrostatic pressure. Besides, they are sensitive to mechanical and weak electrical stimuli.

Sensory crypts or pits:

Many elasmobranch fishes have in their head and trunk regions a group of sense organs sunk in pits and arranged in lines. Each sense organ consists of sensory cells and supporting cells (Fig. 6.103). They are supplied by branches of 5th, 7th cranial and the lateral is nerves. Recently they are thought to be taste receptors rather than a part of the lateral-line sense organ system.

Vesicles of savi:

In the ventral region of the snout of electric rays are found a peculiar type of sense organs, called vesicles of Savi, which consist of gelatin-filled follicles enclosing grey amorphous materials. They are supplied by the branches of the 5th cranial nerve. The function of the vesicle of Savi is not yet known.

Organs of fahrenholz:

They are pits in the head of larval dipnoi and bear haired sensory cells. The function is unknown.

Spiracular organs:

The spiracles of Chondrichthyes, Holostei and Chondrostei show peculiar types of sense organs of unknown functions. They are of sensory struc­tures of basic lateral-line type.

Cutaneous sense cells:

Cutaneous sense cells of the Mormyrids and Gymonotid eels are also held to be a component of the lateral line sense organ system. The function of these cells is probably electro sensitive.

The importance of the lateral-line sense organ system in relation to the aquatic life of fishes is emphasized by the fact that they are totally absent in any terrestrial animal. However, why this system needs a peculiar apparatus of canals is not very clear.