The following points highlight the top five devices required for protection of fishes from their enemies. The devices are: 1. Scales 2. Electric Organs 3. Poison Glands in Fishes 4. Luminous Organs 5. Colouration.
Device # 1. Scales:
The body of all fishes (except the members of the family Siluridae and some bottom- dwellers) is covered by an exoskeleton in the form of scales. The mode of development of scales in fishes is different. In elasmobranchs, the scales originate from the dermal papillae with contribution from the epidermis, while in other fishes these are derived exclusively from the dermis.
The size and shape of the scales differ greatly in different forms. In eels the scales are minute and lie deeply embedded in the dermis. The scales are usually present throughout the body surface, but in some fishes, Chimaeras, Polyodon and Acipenser the scales are present in some localised areas. The primitive fossil fishes possess exoskeleton in the form of plates and scales which consist of three distinct layers.
The layers are:
(a) Innermost layer of isopedine,
(b) Median spongy vascular layer and
(c) Outermost layer of dentine.
Structurally five types of scales are recorded in fishes. These are:
This type of scale is universally found in elasmobranch fishes with a few exceptions. The placoid scales appeared first in the ancestral shark of the upper Devonian time. In Chimaeras, these develop during embryonic life but soon disappear excepting in certain scattered regions.
Each scale consists of a rounded or rhomboid bony plate which remains embedded in the dermis. A spine projects outward from the basal plate. The spine projects outward from the epidermis and points posteriorly. The structure of placoid scales is already dealt in detail in the biology of Scoliodon.
Ganoid or rhomboid scales:
This type of scale shows considerable diversities. Usually these are rhomboidal plate-like structures and remain closely fitted together at their margins.
The ganoid scales exist in two forms:
(a) Paleoniscoid ganoid scale present in Polypterus and
(b) Lepisosteoid scale present in garpike (Fig. 6.95).
In paleoniscoid ganoid scale, the deepest layer is composed of compact bonelike isopedine, the next is made up of hard dentine (Cosmine) and the outer side is composed of hard translucent mesodermal material, called ganoin.
The usual spongy-bone layer next to isopedine is absent and the cosmine layer is reduced. In the second category of scales the spongy-bone and cosmine layers are totally eliminated and the ganoin lies upon the innermost isopedine layer.
This type of scale is the characteristic feature of the primitive members of the crossopterygians and dipnoans (Fig. 6.96). These have the same structural plan as that of the bony armours in ostracoderms and in many placoderms.
This type of scale has four distinct layers:
(i) Innermost layer is called the isopedine layer which resembles compact bone;
(ii) The next layer contains spongy bone with many vascular lacunae;
(iii) The next layer is composed of cosmine resembling hard dentine and
(iv) The outermost layer has an enamel-like consistency.
The cosmoid and ganoid types of scales are fundamentally similar and are usually described by many authors under one category. The ganoid type is regarded to have derived from the cosmoid forms in course of evolution.
This type of scale has rounded form. The centre of the scale is thicker and gradually thins out towards the margin. Each scale has concentric lines of growth. The scales are situated in the dermal pockets. This type of scale is present in teleost and in surviving dipnoans.
This type of scale has rounded shape but the free margin possesses many comb-like projections. This type of scale is present in most teleosts. These are also embedded in small dermal pockets. The basal end of this type of scale is usually wavy.
The cycloid and the ctenoid scales are basically built on the same fundamental plan. The origin and development of these two types of scales are also similar. The cycloid scales are regarded to be phylogenetically more primitive, because cycloid scales are recorded in Cretaceous period.
Transitional stage between the cycloid and ctenoid scales is found in many fishes which prove that ctenoid scales have evolved from cycloid forms. In some flounders, the upper side is provided with ctenoid scales and the underside is lined by cycloid scales. Such a combination of two types of scales is quite common in some other fishes also.
Modification of scales:
The scales in fishes have undergone modifications for specialised functions. The teeth in sharks are modified placoid scales and transitional stages in the process of transformation are encountered in these forms. The lateral rows of blade on the ‘saw’ of the saw-fish are modified placoid scales.
In Tetrodon (globefish) and Diodon (Porcupine fish) the scales become transformed into protective spines. In Acanthurus (Sturgeon) the scales at the base of the tail have been transformed into sharp cutting blade. In Cetorhinus (Basking shark) the gill-rakers are modified scales. In Hippocampus (Seahorse) and Syngnathus (Pipe-fish) the scales become fused to form protective bony rings round the body.
Scales and age of fish:
The cycloid and ctenoid scales help in calculating the age of fishes. The scales grow as the fish grows. The fishes inhabiting specially the temperate zones leave distinctive record of age and growth rate on the scales. The scales appear first at a very young stage. During summer when the nutritional source is abundant, the fish grows very fast.
Simultaneously with the rapid growth of the fish, the scales also grow. In winter the growth rate is slow and the difference is distinctly marked by concentric ‘rings’. By counting the annual rings (or lines of growth) the age of fish can be determined with a degree of accuracy.
Device # 2. Electric Organs:
Many fishes possess specialised organs by which an electric current is generated. This unique property of generating electric current is nowhere observed in the animal kingdom. About two hundred fifty species of fishes, both elasmobranchs and teleosts, are recorded to possess electric organs.
Presence of electric organ in different fishes is difficult to explain from the phylogenetic point of view but these organs have possibly evolved independently within the fishes to meet certain piscine needs. Many fishes possess electric organs and it is difficult to give a roll call of all the species.
However, the following fishes are notable for having well-defined electric rays (Torpedo and Narcine) and the skates (Raja, Fig. 6.97A) possess well-developed electric organs. Members of a few families of teleosts have developed the electric organs.
Members of a few families of teleosts have developed the electric organs. Cnathonemus (Fig. 6.97B), Gymnarchus (Fig. 6.97C) and Mormyrus are found in the rivers of Africa. Electrophorus electricus, Gymnotus (Fig. 6.97D) and Sternarchus (Fig. 6.97E) are living in the rivers of South and Central America.
Malapterusus is the typical African catfish. The stargazzers, the American Astroscopus (Fig. 6.97F) and the Mediterranean Uranoscopus are some of the best known electric fishes.
Structure and origin of electric organs:
The size, shape and location of electric organs vary greatly in different fishes. However, the electric organs are built on the same and identical structural plan. Typically an electric organ is composed of disc-like cells, called electroplates or electroplaxes.
These are embedded in a jelly-like extracellular matrix and are bounded together by connective tissue to form compartment or elongated tube like structure. The jelly gets blood supply and one end of the electroplate is connected with the nerve fibres.
Each electroplate is a multinucleate syncytial cell with a clear cytoplasm and exhibits characteristic surface convolutions. The nervous end of each electroplate is generally smooth while the other or the non-nervous end bears large papillae.
The electroplate cells are formed from the striated muscle cells together with the jelly-like masses. The development of the electric organ has been studied in Torpedo. In this genus, the whole of the electric organ is developed from the branchial muscles which were originally meant for moving the gill-arches.
In Electrophorus, Skates and Mormyrids the electric organs are modified muscles and are derived from the muscles of the tail. In Astroscopus, the electric organs are transformed eye muscles.
Electric organs in torpedo:
In Torpedo the electric organs are best developed amongst the elasmobranchs. The electric organs are paired and are situated on either side of the head between the anterior margin of the pectoralfin and the head (Fig. 6.98A).
Each organ forms a large flat kidney-shaped mass running through the entire thickness of the body. Each electric organ is composed of about 46 vertical columns which are separated from each other by fibrous tissues. Each column is composed of numerous electroplates. The electric organs are innervated by numerous nerve fibres deriving mainly from four nerves.
These nerves originate from an electric lobe of the medulla oblongata and a nerve from the fifth cranial nerve. The innervated end of the electroplate is electrically negative and the other side is electrically positive. In this species, the electric current passes from the dorsal (positive) to the ventral side (negative).
Torpedo generates electric current and the strength of the electric discharge generated by this fish varies from 30-50 volts. In other rays, the electric organs are comparatively smaller and situated at the lateral sides of the base of the tail. Narcine produces 37 volts, and a skate, Raja clavata generates about 14 volts of electric current.
Electric organs in electrophorus electricus:
The electric organs in electric ‘Eel’ (Electrophorus electricus) of Brazil and Venezuela are highly developed amongst the fishes and are capable of generating about 270-550 volts (Fig. 6.98B). On land, Electrophorus can generate about 550 volts, but in water, the voltages drop to about 270 volts partly due to short- circuit.
Electrophorus (= Gymnotus) is a blind eel-like fish measuring about 2.5 metres in length and inhabits the shallow muddy waters of the Amazon and many other South American rivers. The electric organs are paired structures situated one on each side of the body (Pig. 6.98C). These two organs are unequal in size and one is larger than the other.
Each electric organ is composed of about 70 longitudinally arranged columns, each containing a series of about 600 dislike electroplates. The sum total of the tissues responsible for production of electric current comprises half of the total weight of the body. The electric organs are innervated by the nerves from the spinal cord. The polarity of the electric current is from the head to the tail end.
Electric organs in malapterurus:
In African cat-fish, Malapterurus electricus, the electric organs are located in the skin and thus form a thick covering over the body (Fig. 6.98D, E). The electric organ in this particular fish is a dermal derivative and is divided into a number of compartments by connective tissue partitions.
Each compartment is composed of numerous electroplates with their nervous ends directed towards the tail. The electric catfish is recorded to generate about 350-450 volts of electric current and the polarity is from the tail to the head end.
Electric organs in other teleosts:
Uranoscopus and Astroscopus produce comparatively weak current by the electric organs. The electric organs are small in size and are innervated by the oculomotor nerves. These are modified muscle tissue and are restricted as two oval patches behind the eyes.
In Steatogenys elegans, the electric organs exist in a groove below the dermis running from the anterior margin of the lower jaw to the basal end of the pectoral fins. Mormyrus produces weak electric shocks by the modified tail musculatures.
Mormyrus kanume is capable of discharging a continuous stream of feeble electric current of variable frequencies. The discharge of electric current is lowest when the fish remains stationary, while during locomotion the rate is rapidly increased. The cerebellum and acousticolateralis regions of the brain become well-developed in Mormyrus for the coordination of the impulses.
Functions of the electric organs:
The electric organs are capable of generating electric current of variable voltages. The primary function of the electric organs is defensive, but secondarily it sub-serves many other functions.
The fishes with highly developed electric organs use these offensively in their hunt for food and defensively against their enemies. Electrophorus, Torpedo and Narcine are seen to paralyze fishes prior to feeding. The fishes with feeble electric organs utilise the electric field as the warning device.
Mormyrus is seen to create an electric field around itself and if any object enters the field, the change is perceived by the fish. As Mormyrus possesses ill-developed eyes, the electric field works as a warning device. The highly developed cerebellum and the special cells at the base of the dorsal fin possibly help in the detection of the break in its electric field.
Device # 3. Poison Glands in Fishes:
Many fishes have developed poison glands in association with their spines. Both these structures act as the organs of offence and defence. The poison glands are modified skin glands which have evolved for protection from the enemies.
Amongst ‘he elasmobranchs, particularly in sting rays, the serrated spines on the tail can inflict painful and fatal wounds. The spines are not connected with any special poison gland, but the secretory products of the mucous glands of the skin are possibly poisonous in nature. But amongst teleosts, well-formed multicellular poison glands have been developed in association with the spines.
In Synanceia, the poison glands have been developed in association with the spines (Fig. 6.99C). In Synanceia, the poison glands are highly developed. Each dorsal spine has a groove on both the sides and a poison gland is placed at the base of each groove (Fig. 6.99C).
In Trachinus, the opercular spines are covered by this skin except at the tips. Each spine bears a groove and a poison gland opens at the base of the groove (Fig. 5.99A).
The poison gland secretes a toxic poisonous fluid which is forcibly introduced into the body of the victim by the spines. In many species, the opercular spines are perforated with a complete canal like that of a hypodermic needle. The basal end of each canal is connected with a poison gland which discharges its contents into the canal.
In common cat-fishes (Siluridae) as exemplified by Heterdpneustes fossilis, each pectoral fin bears a spine with a poison gland at its base. In other members of the family Siluridae, there are deeply serrated dorsal and pectoral spines associated with basal poison glands.
In Scorpion fishes, the dorsal spines bear deep grooves and the duct from the basal poison gland is prolonged up to the tips to open there. In the stargazer (Uranoscopus) poisonous spines are present on the operculum and as well as on the dorsal fin.
Device # 4. Luminous Organs:
Many fishes, specially the deep-sea forms, possess light producing organs called photophores or luminous organs. Few elasmobranchs inhabiting the surface zone bear luminous organs. These organs are actually modified integumentary glands and lie in the skin. Considerable variation in the number and disposition of the luminous organs are observed in different fishes.
In majority of the cases, the luminous organs are arranged in one or two rows on the ventro-lateral sides extending from the head to tail as in Scopelus (Fig. 6.100A) and Halosauropsis. Pachystomias microdon (Fig. 6.100B), Opostomias micripnus and Scopelus benoitii bear a single or a pair of large luminous cent organs which are present under the eyes.
In Porichthyes (Fig. 6.100C) numerous complex photophores are present along the lateral line of the body. Diaphus, a member of lantern fishes (Myctophidae) has a large luminous organ at the end of the snout rather like head-light. Harpodon is a genus of the Indian ocean which possesses phosphorescent organs all over the body. In photostylus (Alepocephalidae), the photophores are nodular, not stalks.
The photophores produce light by two ways in fishes. These organs house photogenic symbiotic bacteria in specialised areas, but most of the fishes are self-luminous, i.e., the light is emitted by glandular cells backed by pigmented reflector. The self-luminous photophores exhibit great structural evolution.
In simple cases, these photophores are represented by radially arranged glandular tubules. These tubules get nerve supply from the peripheral nerves. In a complex photophore, the dioptic parts are developed.
In Porichthyes, the photophores after originating from the stratum germinativum of epidermis, become differentiated into two layers, the upper layer forms the lens while the lower layer constitutes the glandular part.
The glandular part emits phosphorescent light and the other structures help to transmit the light (Fig. 6.101). In Pachystomias each photophore assumes a cup-like form with its wall made up of many concentric layers.
Numerous glandular cells are lodged in the cup and the cup is externally enveloped by pigmented layers. A lens-like body is present in the mouth of the cup and the overlying integument serves as an iris. The light reflecting layer is thick and highly developed. It is composed of reflecting spicules. These organs are innervated by a branch from the fifth cranial nerve.
In the photophores where light is produced by the symbiotic photogenic bacteria, the bacteria remain in the glandular tubules. This category of photophores is capable of producing light for a considerable period of time.
The provision for off-and-on of the light production in the photophores is developed in many forms. In Photoblepharon, the light can be cut off by drawing up a fold of darkly pigmented fold like an eyelid. In Anomalous the light is shut off by drawing the photophore downwards.
The controlling mechanism of production of light by the photophores in fishes is not fully known. The production of light is stimulated by injecting adrenalin and also by exposure to electric shock. The activities of the photogenic bacteria are directly under the nervous control.
In the self-luminous photophores the production of light is under the ultimate control of sympathetic nerves. So the role played both by the nervous and endocrine systems governing the physiological activities of the luminous organs is quite suggestive.
The significance of light production in fishes is quite diverse.
The functions are:
(a) In deep-sea forms, the light produced by luminous organs helps to search the prey in dark environment.
(b) Most deep-sea angler fishes use the light to lure the prey near the mouth.
(c) In Stomias and related forms, the photophores are sensory in function and help to detect the whereabouts of wandering prey.
(d) The orientational pattern of photophores on the body helps to recognise individuals of the same species.
(e) The luminous organs also serve as the defensive organs because quick flashing of light may frighten the enemies.
(f) The sexual role of the photophores is also recognised in some instances where it acts as the luminescent lures.
Device # 5. Colouration:
Fishes exhibit wide range of colour patterns which enable them to conceal themselves from their predators. The general colouration with characteristic markings, in the forms of stripes, bars or spots are usually constant in the individuals of a particular species. The colour in fishes is produced by the cells present in the skin.
(a) Pigment cells or chromatophores and
(b) Reflecting cells or iridocytes.
The chromatophores are large and branched cells contain pigment in the cytoplasm. Each chromatophore has a much branched sac-like cell body with thin wall. The contained pigment granules may be concentrated into a minute dot by contraction. Expansion of the pigment granules may result dispersion to form irregular colour pattern.
The pigment granules deposited in each of the chromatophore may be either black, red, orange or yellow in colour. Other intermediate shades are produced by the blending effect of two or more of the primary colours. The iridocytes are made up of opaque crystals of guanin. The iridocytes are able to reflect light and are present either outside or inside the scales.
The iridocytes, located outside the scales, give an iridescent silvery appearance while those situated inside the scales produce a reflecting layer (argenteum). A combination of the chromatophores and iridocytes produces interference effect so that a wide range of colour is produced in fishes.
The relative abundance of the chormatophores with their particular kind of pigment granules as well as the iridescence and the reflecting capacity of the iridocytes play their role in determining the colour of a particular fish.
The significance of colour in fishes is threefold:
(a) Concealing colouration (cryptic),
(b) Warning colouration (sematic) and
(c) Sex colouration (epigamic).
The cryptic colouration is achieved by two ways:
(i) By assimilation with the background and
(ii) By breaking up the outline of the fish.
Lack of pigmentation in the pelagic fishes is a typical example of assimilation. Many fishes living among weeds (e.g., Hippocampus, Syngnathus, Lophius and many others) resemble the sea weeds in colour.
Extreme cases are observed in Phycodurus and Histrihistrio (frog-fish) which, besides their matching colouration with the weeds, develop ‘leaf-like’ processes. Many reef-dwelling fishes show an extra-ordinary variety of colour markings.
The significance of such markings is to break up the outline of the fishes. Many bottom-living fishes have a darker upper side and the underside is lighter. In surface feeders, like Hilsa, Catla, the dorsal side is blackish and the colour becomes gradually lighter towards the lateral sides. In the shore-dwelling fishes inhabiting the sea-bottom, the colour markings of the body imitate the background.
Orectolobus (Carpet shark) imitates a well-covered rocky background in which it lives. The colour pattern is usually constant in a particular group of fishes, but different species of the genus, Ostraction (Trunkfish) show a variety of colour depending on the environment they live. Such variation in colour is also observed among the individuals of the same species of Brown Trout (Salmo trutta).
Warning or sematic colouration involves the adoption of special colour pattern which acts as danger signals to the predaceous fishes. The fishes of this category have either poisonous flesh or possess venomous glands in association with the spines and the like. Trichinus (Weever-fish) provides an excellent example of warning colouration.
The pectoral fin of Solea mimics the aggressively coloured dorsal fin of Weever and thus escapes the eyes of the predators, because the predators see it as a member of Weever-fish. Torpedo ocellata has conspicuous coloured spots.
Two sexes of many fishes show colour difference specially during the breeding season. Such a phenomenon becomes conspicuous in fishes which exhibit some sort of courtship. Colour difference appears during the breeding season and after the season is over the colours vanish, and two sexes become more or less similar.
Usually the male individuals exhibit colouration which depends on genetic factors lodged in the Y chromosome. In Lebistes, there are many ‘races’ of distinctive coloured males while the females are typically coloured. The significance of such colour change is not fully understood.
Evidences have shown that such changes are associated with the sexual activities. The characteristic colour pattern of fish depends on the relationship between the chromatophores and the iridocytes. Majority of the fishes have the ability of either slow or instantaneous change of colour.
Such a change is primarily due to the action of the chromatophores by:
(i) Contracting or expanding the contained pigment granules,
(ii) An increase or decrease in the number of chromatophores and
(iii) Altering the distributional pattern of the chromatophores in the skin.
The physiological mechanism of the movement of the pigment granules within the chromatophores is not known. It is apparent that the initial stimulus of change of colour is received through the eyes and the direct action of light on the production of pigments is certain. Because absence of light (as exemplified by the cave-dwelling fishes) leads to the disappearance of pigmentation.
It is, however, suggested that both nervous and endocrine systems control the process. Slow changes of colour are under the control of hormones, while fast or instantaneous changes are under nervous control. The ability of colour-change enables the fishes to take the same hue of the surroundings they inhabit and help the fishes to survive in hostile environment.