External Morphology: Body forms, shapes, coverings, appendages and openings Chapter - 3 The body form of a fish can give a quick assessment of the fish's way of life. Different shapes allow some fish to be fast or slow, bottom dwellers or live in the pelagic zone, and others to survive the boundaries of the ocean. Body form is a good indicator of how a fish moves and where it lives. There is great diversity in the size, shape and details of fishes. Some fish are string-like, like the eel, or globe-shaped like the puffer or greatly flattened, like the flounder. Some fish lack eyes, and others lack some of the features by which fish are recognized, including gills, fins, and scales. Their appearance is greatly influenced by their environment. The body shape is one of the best indicators in determining the fish's environment. Surface dwelling fish have an upturned mouth, a flattened back. Tall bodied, laterally compressed species are adapted to life in slow-moving waters. Slender, torpedo shaped fish are better adapted to moving waters. Bottom-dwelling fish have flattened bellies and inferior mouths.
Streamlined or fusiformed: The typical fish body is streamlined and spindle-shaped. It’s cross-section is oval or fusiform. Streamlined body shape helps them to lower the frictional resistance in water body and hence they are fast swimmers and usually live in open water. They can move constantly including long migrations. Tuna, Mackerels etc. are the examples of streamlined fish.
Compressed: They are flattened from side to side. This body shape is well designed for making quick turns and quick bursts of speed over short distances. Compressed body form fish commonly live in quiet water habitats of relatively dense cover. Many typical of this group live in weed beds in ponds, lakes and river waters and even in coral reefs. They sometimes school together in shallow open waters. Tilapia, Hilsa, Tautog, Bass and Sunfish are examples of this group.
Depressed: Here the body is depressed from top to bottom. They are flat in shape and bottom-dwellers in water-adapted to lying on or below surface of sand. Their fins up and down to swim through the water in the same way a bird flaps its wings. Flounders, halibut, rays and skates are the examples of this group.
Elongated: Fish of this group are long and skinny or filiform eel-like. They can slighter through the water like a snake. Spiny eel, pipe fish etc. are examples of this group.
Table 1: Principal types of fish body shapes with examples
Body Shapes
Fusiform (Streamlined)
Laterally Compressed (Flattened from side to side)
Depressed (Flattened from top to bottom)
Eel-Like (Body shaped like a snake)
Fig. 1: Different types of fish body shape and their characteristics External Morphology: Body forms, shapes, coverings, appendages and openings The mouth is an important clue to food source for The caudal, or tail, fin is responsible for bony fish. propulsion in most bony fish.
Fish with continuous Large caudal fins (dorsal, caudal, and anal fins For eating whole fish attached) are able to or chunks of fish swim in and around cracks and crevices.
Fish with lunate Small caudal fins tend to be the fastest fishes and For nibbling on plants maintain a rapid speed and small animals for long durations.
Dorsal Many continuously swimming fish have For eating near the forked caudal fins. surface
Anterior Fish with truncate caudal fins are usually For eating in the water strong, but slow, column swimmers.
Ventral Fish with rounded For eating on the caudal fins are usually bottom strong, but slow, swimmers.
Body covering An ordinary fish is covered with a relatively tough skin. The skin is continuous with the lining of all the body openings and is transparent as it runs over the surface of the eye. Much of the diverse coloration of fishes is due to its color cells, and the slimy coating is due to its mucous cells. Most fish are covered with scales, which protect the body. Some fish such as catfish have bony plates which serve the same purpose. Other species have very small scales or no scales at all. Fishes have two layers of skin: a thin outer layer known as the epidermis, and a thicker inner layer known as the dermis. In most cases, the fish's body is encased in a scaly exterior. These overlapping plates, which grow out of the skin, provide streamlining and protection against injury. The scales are covered in a thin mucus layer that protects against parasites and gives "slip-ability". External Morphology: Body forms, shapes, coverings, appendages and openings Morphology: Scale type Scales have evolved over time and are of major importance in classifying fishes. Most scales are deeply buried in the fish's epidermis, or outer skin layer, with only part of them showing. Below the pictures of scales are examples of how the scales would look on the fish's body.
There are four common types of scales in fishes. They are shown in the figure below. Below each scale type is an illustration of what they would look like on the fish.
Ganoid Placoid Cycloid Ctenoid
Ganoid scales are hard Sharks have placoid Many fishes with which we Highly evolved fishes often and smooth, and may scales: tiny, tooth-like are most familiar have have ctenoid scales, which take the form of only a structures that are cycloid scales, which are are much like cycloid scales few scales (or scutes, as partially embedded in the the thin, round, almost except that they have tiny, in the sturgeon and skin. These tiny, pointed transparent scales that we comb-like projections stickleback), partial scales, made of the same find when we are cleaning (ctenii) on their posterior plating, or overall body materials as their (and trout, salmon, or herring. edges (the edges that show, plating. Sturgeons, our) teeth, make their Minnows also have cycloid and are not buried in skin). gars, and sticklebacks skin feel like sandpaper. scales. These scales are The colors of brightly have ganoid scales (or mostly buried in the colored fishes also show on scutes). epidermis, allowing only these posterior edges. the small posterior margin Cichlids have ctenoid to show. scales.
Lateral Line
The lateral line organ is a series of fluid-filled ducts located just under the scales. The lateral line system picks vibrations in the water. Thus fish are able to detect predators, find food, and navigate more efficiently. Many fish species can navigate without vision in darkness or muddy water. The Blind Cave Fish relies entirely on its lateral line system.
Body Appendages
Appendages of fishes comprise the fins and the cirrhi (flaps of flesh) and the barbels. The fins are used for movement, stability, nest-building, spawning, and as tactile organs. Fins can be single or paired. External Morphology: Body forms, shapes, coverings, appendages and openings The caudal or tail fin is used for propulsion. Fish that have forked caudal fins are regular fast-swimmers. Fish that have rounded caudal fins are fish capable of quick action like predators. Large, elongated caudal fins are often used to attract mates.
The single anal fin is located on the underside of the body just forward of the caudal fin. The anal fin serves to stabilize the fish while it is swimming. Long anal fins that are moved in an undulating (rising and falling) manner are used for propulsion.
The paired pelvic or ventral fins are located forward of the anal fin. Ventral fins are used to provide further stability in swimming. Sometimes these fins are modified as long, thread-like fins used as a tactile organ.
The paired pectoral fins are located near the gill cover and are used for manoeuvring the fish. These fins have been adapted, in the case of some bottom-dwelling species, so fish can support themselves up or even walk around above or below water. Sometimes the pectoral fins are equipped with spines for defense.
The single dorsal fin is located on the back of the fish and serves to help balance the fish while swimming. The rays of this fin are often sharp, and a spine is often present. The adipose fin is a tiny fin found between the dorsal and caudal fins on some fish.
Fig. External Morphology of a bony fish.
Body openings:
The mouth, the gill apertures and the vent (anus) are the principal openings connected with the alimentary canal tract in the fish body.
Mouth: the fish mouth is situated anteriorly in the head, in terminal position, but adaptively its position may be superior (opening dorsally) or inferior.
Gill apertures: in many fishes with gill covers there is a single opening on each side of the head. In Hagfishes, the coverings vary from 5 to 14 pairs. All lampreys have 7 pairs. Nearly all sharks have 5 pairs, only a few, 6 or 7; all rays have 5 pairs.
Anus: the anus or vent of a fish is on the mid-ventral line of the body. Most commonly, it is just behind the bases of the pelvic fins and just in front of the anal fin.
Coloration in fishes
Fish display a wide variety of colors and color patterns. Skin coloration can have many functions. Many fish have color patterns that help them blend in with their environment. This may allow the fish to avoid External Morphology: Body forms, shapes, coverings, appendages and openings being seen by a predator. Some fish, such as the flat fishes (Pleuronectiformes), can change their skin coloration to match the surrounding habit. Coloration in fishes is primarily due to skin pigments. Free- swimming, open-water fishes including the early planktonic stages of many bottom fishes are mostly of simple coloration grading from a whitish belly, through silvery lower sides, to upper sides and back that are iridescent blue or green. Bottom dwellers and weed-bed occupants are often very strongly and intricately marked above and pale beneath. Three types predominate in the color of oceanic fishes: silver in the upper zones; red in the middle range; and violet or black in the greater depths.
Thayer’s Principle
The common ground of coloration in fishes is the prevalent lightness on the ventral body surface, darkness on the back, and gradual shading on the sides from light below to dark on the back. This plan illustrates the primary principle of camouflage by obliterative countershading; which is called Thayer’s principle.
Sources of color
Coloration in fishes is due to schemachromes (colors due to physical configuration) and biochromes or true pigments. White schemachromes are seen in the skeleton, gas bladder, scales, and testes; tyndall blues and violets are in the iris; and iridescent colors are in the scales, eyes and intestinal membranes. Biochromes include carotenoids (yellow, red, and other hues), chromolipids (yellow to brown), indigoids (blue, red and green), melanins (mostly black or brown), porphyrins and bile pigments (red, yellow, green, blue, and brown), purines (white or silvery), and pterins (white, yellow, red, and orange).
The special cells that give color to fishes are of two kinds, chromatophores and iridocytes. Chromatophores are located in the dermis of skin, either outside or beneath the scales. Chromatophores are assorted in hue and impart true color.
Chromatophores, the pigment granules are the actual source of the color. The basic chromatophores according to the colors of their pigment granules are red and orange (erythrophores), yellow (xanthophores) and black (melanophores). Iridocytes could be called mirror cells, because they contain reflecting materials that mirror colors outside the fish.
Table 1: Principal Types of Chromatophores and their Properties
Chromatophores Pigment Type Colors Melanophore Melanins Color Black, Brown Xanthophore Pteridines and Color Red, Orange Carotenoids Erythrophore Pteridines and Color Orange, Yellow Carotenoids Iridophore Guanine and other Reflective White, Silver, Blue, Others Purines Table 2: Fish organs having different pigments
Name of pigments Organs of fish where it is found
Carotenoids, melanins, Found in fish skin flavines and purines
Carotenoids Liver, eggs and eyes
Melanins Occurs in the endoderm and scales External Morphology: Body forms, shapes, coverings, appendages and openings Porphyrins Muscles and blood
Flavines Widespread in blood, muscle, spleen, gills, heart, kidneys, eggs, liver and eyes
Purines Found in the scales and eyes
Pterins Eyes, blood, liver, kidneys and stomach
Significance of coloration in fishes
The function of coloration in fishes has been hard to determine. However, this has not hindered naturalists from assigning many roles to color in the lives of fishes. The principal functions of coloration have been grouped under three headings- Concealment, Disguise and Advertisement. The various kinds of concealing color are suggested as being:
General color resemblance Variable color resemblance Obliterative shading Disruptive (troublesome) coloration and Coincident disruptive coloration
Color resemblance:
General color resemblance between fish and the background is the basic characteristic of fishes to resemble the shades and hues of the habitats which they frequent. Variable color resemblance is the ability of a fish to change color, gradually or rapidly in order to match its background more perfectly.
Obliterative shading:
The optical principle upon which this type of concealing coloration depends is that of countershading. Light and shade give an observer the third dimension of objects seen.
Disruptive coloration:
Another means of concealment which has evolved in fishes is disruptive coloration, a further means of camouflage. The function of camouflage for a fish, may be thought to be prevent, or to delay as long as possible, recognition on sight.
Fish can also have disruptive markings to hide body parts. Species such as the jackknife fish (Equetus lanceolatus), high-hat (Equetus acuminatus) and some angel fishes (Pomacanthidae), have dark lines that run through the eyes. These lines may serve to hide the eyes so that other animals can not tell where the fish is looking or even if it is a fish. Also horizontal lines may be a sight-line for aiming attacks on prey. Some fishes, like butterflyfishes (Chaetodontidae), have spots on their body that resembles eyes. This may serve to confuse prey and predators alike. In addition to coloration, some fish, like the sea dragon (Phyllopteryx), have body shapes that can further mimic their habitat.
Camouflage
This is the most widespread and important type of coloration. This really makes sense when you consider the importance of not being seen to avoid being eaten. Camouflage coloration is accomplished in a few ways. a) The most common type of coloration effect occurs in most, but particularly pelagic fishes, counter- shading. This is the matching of reflectivity from below and above with ambient reflection. Melanophores External Morphology: Body forms, shapes, coverings, appendages and openings cover the topside of the fish, making it darker, and iridophores start about halfway down their sides of the body, increasing in density until almost solid underneath the fish. Tunas, jacks swimming in the open sea, or herrings or anchovies show this type of coloration. b) Deepwater fishes have peculiar and unusual coloration. Most are black or red, with the red usually being very bright. c) Cavefish of a few species live in worlds without light and have lost their pigment through evolution. Most have lost their eyes as well and use other senses to navigate. The blind Cave Tetra (Astyanax jordani) is a fish sold in the aquarium trade that is almost devoid of pigment and totally blind. d) Matching camouflage is extremely widespread, especially in the tropics. There are several excellent examples of fishes that have active color patterns that match their immediate environment. George Barlow (1981) recognized up to eleven such color patterns in Badis badis. Most of these patterns were found to match different surroundings.
The vertical bands of freshwater angelfishes (Pterophyllum) also match vertical bands of plants, and lighten and darken with the presence of such physical objects. e) Transparency. One of the more clever forms of concealment is to be transparent. Indian Glassfish (Parambassis ranga) and Glass Catfish (Kryptopterus bicirrhis) are two good examples of fishes that use this scheme that are common in the ornamental fish trade. This is obviously a good strategy for avoiding predators that rely on sight. f) Fish coloration can also be useful in catching prey. Many sharks exhibit coloration known as counter shading. Sharks that have counter shading are dark on the dorsal (upper) side and light on the ventral (lower) side. With this color scheme any prey looking down on the shark will see a dark shark against a dark sea bottom, making it hard to detect the shark. Conversely, any prey looking up at the shark, will see the light belly of the shark on the light background of the ocean surface water lit by the sun or moon.
Advertising:
Advertising is another function of color and color changes in fishes. Some forms of coloration of fishes appear to advertise or to reveal rather than conceal their presence. In such large and sometimes dark, always relatively dense environments as the waters of the world advertising ones presence, species identity and possibly sex are critical. Coloration can also be used to advertise. Fishes like the darters (Percidae) and sticklebacks (Gasterosteus), may use color to attract and recognize potential mates. a) Recognition is a very important function of coloration. Both pattern and color are often used in different types of recognition.
Sex recognition. Coloration of this type may be of significance for sexual recognition. Though not obvious to most of us, there are often morphological and/or coloration differences between the sexes of fishes. Often this brightened condition is disadvantageous during non-breeding seasons; hence fishes show their most heightened coloration only while attracting mates and breeding.
A third type of recognition is warning coloration, within or between species. c) Another example of advertising involves color and cleaning symbiosis. Cleaner fishes (e.g. Labroides wrasses) are brightly colored so they will attract larger fishes that want to be cleaned. This "signal releaser" coloration also advertises that the cleaner is not to be ingested as prey. Some other species of fishes utilize this protection (as mimics) like Aspidontus, the saber-tooth blenny.
Disguise: Mimicry External Morphology: Body forms, shapes, coverings, appendages and openings Disguise is also accomplished by various conspicuous localized characters, which simply tend to reduce the resemblance of the fish to itself. Deflective and directive marks are important here. A simple illustration is the dark spot on the tail of the bowfin (Amia); another is in the young of certain angelfishes. Prey fishes move into the region of the obscured mouth, perhaps directed or attracted there by fleshy flaps that resemble food and are engulfed. The luring of prey to the mouth region is another function of directive marks, usually a combination of structure and color.
Other Uses and Responses of Coloration
There are several "secondary" apparent uses, results of coloration and its change:
1) Thermoregulation. Some fishes are notably darker in the early morning hours, lightening up with increased temperature, even with loss or constant illumination, and darkening on one side if the water is warmed there.
2) Environmental Responses. Most fishes have the capacity to adapt to some extent to changes in the shade, color or pattern of their environment. a) Response to background has been widely observed. The best example here are some of the flatfishes (Pleuronectiforms) that can adjust their body markings, even to checkerboard, to match the substrate. A curious observation is that "practice" shortens the span of time for these matching. b) Response to darkness. This is a varying of coloration based on direct illumination on the animal.
3) "Psychic Response". Reactions by fishes from being handled or darkening when angry (think of a red devil cichlid (Amphilophus) or triggerfish (Balistidae) by what it observes outside.)
SKELETON
In biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata.
The skeletons of most fish consist mainly of (1) skull, (2) a backbone, (3) ribs, (4) fin rays and (5) supports for fin rays or fins.
The Skeletal System serves many important functions; it provides the shape and form for our bodies in addition to supporting, protecting, allowing bodily movement, producing blood for the body, and storing minerals.
Functions
Skeletons form a rigid framework to which the softer tissues and organs of the body are attached. Vital organs are protected by the skeletal system. The brain is protected by the surrounding skull as the heart and lungs are encased by the sternum and rib cage. Bodily movement is carried out by the interaction of the muscular and skeletal systems. Blood cells are produced by the marrow located in some bones. Bones serve as a storage area for minerals such as calcium and phosphorus.
The skeletal systems of fishes have three main components:
The Vertebral Column
The Skull
The Appendicular (appendage bearing) skeleton External Morphology: Body forms, shapes, coverings, appendages and openings
Fig. A typical skeleton of bony fish
Divisions of the Skeleton
The skeleton is divided into two distinct parts:
1. The axial skeleton consists of bones that form the axis of the body; and support and protect the organs of the head, neck, and trunk. 2. The appendicular skeleton is composed of bones that anchor the appendages to the axial skeleton.
The paired fins support (paired fins are supported by girdles) The unpaired fins support
A fish's skeleton provides a framework for the head, trunk, tail, and fins. The central framework for the trunk and tail is the backbone. It consists of many separate segments of bone or cartilage called vertebrae. In bony fish, each vertebra has a spine at the top, and each tail vertebra also has a spine at the bottom. Ribs are attached to the vertebrae. The skull consists chiefly of the brain case and supports for the mouth and gills.
The pectoral fins of most fish are attached to the back of the skull by a structure called a pectoral girdle. The pelvic fins are supported by a structure called a pelvic girdle, which is attached to the pectoral girdle or supported by muscular tissue in the abdomen. The dorsal fins are supported by structures of bone or cartilage, which are rooted in tissue above the backbone. The caudal fin is supported by the tail and the anal fin by structures of bone or cartilage below the backbone.
Axial Firm Skeleton: The Skull The Vertebral Column The Ribs External Morphology: Body forms, shapes, coverings, appendages and openings The Intermuscular bones
Skull
The bony fish skull is composed of two distinctive parts, the neurocranium and the branchiocranium.
Fig: A typical skull of bony fish
The neurocranium has two major parts:
a) A series of inner elements that provide the floor to the brain case and protect the olfactory, optic an otic capsules and the anterior part of the notochord.
b) A series of outer dermal bones that roof the brain case, and give form to the face.
The branchiocranium has three regions:
a) Jaws or mandibular
b) Hyal (the jaw supporting hyoid arch and the bones of the gill-covering opercular series)
c) The gill arches
The bones of the neurocranium may be grouped by location into the following regions: olfactory (nasal area), orbital (about the eye), otic (about the ear) and basicranial.
The olfactory region of the neurocranium remains partly cartilaginous in the adult. Dermal bones of this region are the paired prefrontals and nasals, and the unpaired vomer which bears teeth in many fishes.
The orbital region has cartilage-bones, the median orbitosphenoid and the paired alisphenoids. The frontals cover all three of these bones dorsally.
The otic region has several cartilage-bones often difficult to identify.
In the basicranial region, the cartilage-bone is the unpaired basioccipital. The single dermal bone is the median parashpenoid.
The mandibular region of the branchiocranium is composed of the oromandibular and associated bones, all of which are paired elements. The hyoid region is composed of both paired and unpaired bones of both cartilage and dermal types.
Vertebral Column and Ribs: External Morphology: Body forms, shapes, coverings, appendages and openings The backbone of a fish is composed of a series of segments, the vertebrae. Grossly, there is one vertebra per body segment but two may occur as in the tail of some sharks. These vertebrae are modified according to the body region. Through the length of the trunk, the bodies of the vertebrae often have lateral processes that bear ribs. Through the length of the column, the vertebrae aslo form, above the centra, a series of arches that protect the spinal cord. Below each centrum in the tail there is an arch that partly encases main axial blood vessels. The rays of the caudal fin are supported by altered vertebral elements. Intermuscular Bones:
Many bony fishes have small, splint bones of various shapes in the myosepta; groups represented include the herrings (Clupeidae), Pikes (Esocidae), Suckers (Catostomidae), Carps (Cyprinidae) and some Salmons and their relatives (Salmonidae). These are “C” “I” “Y” shaped on the basis of various species of fishes Intermuscular bones can be most annoying in food fishes and sometimes cause real difficulty to man whey they become lodged in the throat during eating.
Appendicular firm skeleton:
The skeletal support of the median and paired fins differs fundamentally in that the pectoral and pelvic fins are supported by girdles whereas the unpaired fins are not. Pectoral fin supports:
The lampreys and hagfishes, lack not only paired fins, but also the girdles that support them. In the sharks and relatives, however, the pectoral girdle is composed basically of a strong coracoscapular cartilage that is broadly U-shaped.
In the bony fishes, the pectoral girdle is composed of both cartilage-bone and dermal bone. The elements present in most bony fishes as cartilage-bones are the paired dermal bones, the posttemporals, supracleithra, cleithra and the postcleithra. Pelvic fin supports:
The pelvic girdle in the sharks and relatives is a simple cartilaginous bar, termed the ischiopubic, that bears the fin-ray supporting radials. In the bony fishes, the pelvic girdle is a pair of cartilage-bones, the basipterygia, separated or variously fused. In the highest of the bony fishes the pelvic radials disappear and the fin rays articulate directly with the basipterygia.