The Physical Fish
Before you can even have any real success keeping fish, you need a basic understanding of what makes these interesting creatures work.
There are over 20,000 known species of fish in the world today, with more being discovered by scientists in remote areas of the earth. With this many species, it is not strange that there are many physical and behavioural differences in fish.
COMMON CHARACTERISTICS:
All fish are vertebrates. That is, they have a spinal column. They are the oldest known vertebrates, ancestors of all other vertebrates on earth, including Man. The evolution of fish to amphibians, reptiles, birds, and then to mammals is well documented. Their many forms and species have developed and become adapted to various kinds of water habitats.
Despite the differences necessitated by habitat-such as the kind and depth of water, currents and temperature, underwater topography, plant life, and availability of food – there are some general characteristics that are possessed by all species of fish.
Almost all species have become adapted to living exclusively in water. (Although there are a few fish that can breathe air, they will not be included in this article.) A fish obtains oxygen and necessary minerals from water which is taken through its gills. It must have the proper balance of oxygen and the proper minerals in the water it lives in to survive.
Like reptiles, all fish are cold-blooded. That means that they cannot regulate their own blood temperature as mammals can. A fish’s internal body temperature is always the same as that of the medium-water-around it. But, unlike most reptiles which are amphibious, a fish cannot climb out of water that is too chilly and bask in the sun to warm up. A fish in an aquarium is virtually a prisoner-totally dependent on the “keeper” to maintain properly balanced water at the right temperature.
The External Fish
Although all fish are made up of basically the same parts, they may differ greatly in external appearance. Color, body, shape, fin and mouth placement, and tail size and shape all reflect adaptations made by a particular type of fish to its environment. Even the most uninformed observer can quickly learn to notice certain salient characteristics that will identify a particular type of fish.
BODY SHAPE
Most fish bodies are bilaterally symmetrical-that is, both sides are alike. Traditionally, a fish’s body is a streamlined, torpedo shape, tapering gently outward tapering down to meet the joining of the tail. This shape, along with fins that can be easily folded flat against the body, forms a smooth, nonresistant package that can glide quickly and easily through the water. Torpedo-shaped fish are very fast swimmers. They are good at catching food and escaping from predators.
Slight adaptations in this torpedo shape can be seen in some fish. A fish with a flat-bottomed body, such as a catfish, is able to stay on the bottom and feed, while one with a straight, flat back is adapted to swimming near the top of the water without breaking the surface. Even more extreme in variation, some fish, such as the triangularly shaped hatchet fish, have flat backs and extremely well-developed body muscles that enable it to skim just below (and sometimes above) the water’s surface.
Fish that have other body shapes cannot swim as quickly as the torpedo-shaped species, but they have adapted to their habitats and developed other ways of protecting themselves against predators. Laterally compressed, thin-bodied, disc-shaped fish such as some of the cichlid family angelfish and discus fish, for example-are adept at slipping in between the clumps of weeds and grasses that are found in the shallow river edges they inhabit for cover. Disc-shaped saltwater fish use the cracks in coral for protection.
Bottom-living fish with flat, depressed pancakelike body forms, such as flounders, are usually found in ocean waters, and can readily hide beneath sandy bottoms.
Steamer-type fish with long cylindrical bodies are able to slither through the rushes and reeds found in rivers and lakes with muddy bottoms, and can easily hide in small holes in rocks. Eels and pipefish fall into this category.
The most obvious outward form of protection is that of the armored fish, such as some catfish and slow-moving marine boxfishes and spiny brackish or freshwater pufferfish.
FINS
Most fish have seven fins, two pairs and three single, midline fish. These fins can be very different in size and placement and are supported by rays that are either hard and spiny or soft. Spiny-rayed fins are generally in one piece, while soft-rayed fins are separated or branched. Fins serve to balance, steer, and propel a fish through water and can be raised and lowered by special small muscles.
The tail, or caudal fin, varies considerably in different fish species. It acts as a rudder and, along with the accompanying wavelike motion of the fish’ body, helps to give the final push to propel it through the water. Most fast swimming fish have largely forked or crescent-shaped tails. In some species, males have an extended caudal fin. This long, pointed extension, seen in swordtails, seem to be merely ornamental.
Extra long, lacy caudal fins have been carefully developed through aquarium-breeding in some species such as fancy goldfish and Siamese fighting fish. These fish are usually slow swimmers. The area where a fish’s body narrows to join the tail (sometimes referred to as the tail stalk) is called the caudal peduncle.
A fish’s dorsal fin is in the center of the black and acts as a stabilizer, keeping the fish upright in the water. It is often carried erect, like a sail, and can convey aggression or serve to attract a female. Sometimes, it is modified to become a defensive weapon. The lionfish’s dorsal fin, for example, has long hollow spines that contain venom. Dorsal fins vary a great deal in size and shape and may be single, divided into front and rear fins, or in some cases into several small fins. In some species, such as characins, there is also a small, fleshy, rayless fin called an adipose fin located on the back near the tail.
The other single fin is the anal fin, located on the underside of a fish, behind the ventral. It can be long or fin-shaped and act as a rudder, or it may help to propel a fish through water. In the case of livebearing fish, the male’s anal fin has modified rays and is a tube-like organ that helps direct sperm into the female’s vent during spawning. Some males in the characin family have tiny barbs or hooks on their anal fins the enable them to hold the female close while spawning.
There are two sets of paired fins, the pectoral fins, located just behind the gills, and the pelvis, or ventral, fins, which are beneath the midsection of the fish in front of the anal fin. Both of these pairs of fins help a fish to steer and stabilize itself while moving in the water. Pectoral fins are often seen moving very quickly in a rotating fashion, just like oars or paddles, helping a fish to change direction or turn in the water. They also serve as brakes when held at right angles to the body. It is these fins that evolved into flippers and the legs, enable some fish eventually to become amphibians. Some fish, like hatchet fish, have large winglike pectorals that help them to glide through the air.
Pelvic fins have sometimes been referred to as the ‘hind legs’ of fish. Catfish females use their pelvic fins to carry fertilized eggs, while freshwater angelfish have well developed pelvic fins that can be used as defensive weapons. In some species, the pelvic fins also serve as sensory organs and help to locate food.
SKINS, SCALES, AND COLOR
The thick, tough inner layer (dermis) of a fish’s skin covers the entire body, including the eyes, and contains color pigments. It also houses various nerves which act as sensory receptors and is usually covered by a thinner outer layer of skin called the epidermis. The barbels of catfish, barbs, and loaches originate in the skin.
The scales seen on most aquarium fish except catfish originate in the thick inner skin. There are several types of scales, which overlap to form a streamlined, protective body coating. The skin also contains mucus, or slime, glands that produce a thin, slippery, parasite-repelling layer that coats the scales. In the cichlid family, this mucus also serves as food for newly hatched fry.
Fish can control the shade or intensity of color that they display. Color can be used to camouflage a fish by allowing it to blend in with its surroundings or to confuse by means of disruptive coloration to conceal the fish’s body shape. Fear, ill health, or simply general unhappiness with the surroundings can cause a usually brilliantly colored fish to become pale. Some fish, such as pencil fish, normally change color at night, when it becomes dark. Color is also used to identify a fish to other fish and to attract a mate. Much male fish become intensely colored during the mating season, and some females have high color after breeding so that their fry can easily see and identify them.
There are two kinds of color-producing cells in a fish’s skin: chromatophores and iridocytes. Chromatophores contain granules of pigment, while iridocytes reflect light and colors outside the fish, producing the familiar silvery-white iridescence seen on most fish. It is the chromatophores that can change a fish’s hue. The intensity of color produced by the chromatophore is controlled by a fish’s reaction to its environment.
MOUTH
Most fish have mouths that seem big in comparison with their body size, The shape and position of a fish’s mouth is a good indicator of its diet and mode of eating.
Some fish have teeth in their mouths to help them in grasping and tearing food. Others have no teeth in their mouths but do have bony plates (or extra teeth) in their pharynxes to break food down.
Bottom-feeding fish that such in their food, like most loaches, usually have underslung mouths with elongated upper lips, with the opening located underneath their bodies. Their mouths are often fringed with barbels that help them locate food. Midwater algae-grazers also may have barbels attached to underslung mouths.
A surface-feeders mouth is located at the top of the head and is upturned. There is a shovellike bottom lip so that the fish can scoop up insects that are floating on the top of the water.
Fish that swim and eat in the middle part of the water usually have straight mouths placed at the tip of the snout,IMAGES.
enabling them to grab food as it floats by. As mentioned above, some midwater fish that eat algae have underslung mouths that aid them in gathering algae from flat surfaces.
The Internal Fish
Every fish has a complex nervous system, a heart, liver, kidneys, stomach, reproductive organs, and intestines. Many of these organs and systems work in the same way as those of other animals, but fish do have a few special characteristics that enable them to survive in their watery environment.
DIGESTIVE WASTE
The digestive system of fish differs greatly according to diet, and there is still a lot about the way fish digest and utilize their food that is not fully known. Most digestive wastes are disposed of out of the vent, which is located just in front of the anal fin. The vent is the opening for an internal cavity called the cloaca, into which intestinal, urinary, and genital tracts all open.
In addition, some fish deposit waste products just underneath their own skin. These wastes, known as guano, make up the iridocytes that reflect light.
BODY FLUIDS
Because fish are surrounded by water, a process known as osmosis has a direct effect on their bodies. Osmosis is a phenomenon in which water seeks to equalize its density and will travel through a permeable or semipermeable membrane from a solution that is less concentrated to one that is more highly concentrated.
Some fish membranes are permeable and allow water and some salts to go through them. Thus, because of osmosis, a freshwater fish, whose blood contains a higher concentration of salts than the water surrounding it, is in constant danger of swelling up and bursting. It is a semipermeable bag of saltwater stuck in a big pot of fresh water, and the water is constantly coming into its body through the gills and all of the membranes in the fish’s intestine. So, the fish must continuously pump large amounts of fluids out of its body through urine and its gills. Naturally, freshwater fish drink very little, if any, water.
Saltwater fish, on the other hand, is in constant danger of drying out because their bodies contain less salt than the water surrounding them and they are constantly losing fluids. A marine fish is like dilute bag stuck in a big pot of salt water. Therefore, it needs to drink a lot of water, urinate little, and excrete excess salt in an effort to maintain its body fluid levels.
The process by which a fish compensates for the excess or lack of fluid in its body due to osmosis is known as “osmoregulation.” If a fish’s scales are damaged in any way it will allow more fluids to leave or enter its body and their delicate balance will be affected.
RESPIRATION
Fish take in water through their mouth and pass it out through their gills. The exchange of gasses takes place in a fish’s gills, which extract oxygen from the water. The oxygen is then circulated through a fish’s body in its bloodstream. Some fish are able to rise to the surface of the water and gulp air which is then processed in a special pouchlike organ. Fish with suckermouths, such as catfish and loaches, have divided gills or extra gill stills so that they can take in water while still sucking food in which their mouths.
Since water contains less available oxygen than air, and the warmer the water the less oxygen in solution, tropical fish must process a great deal of water in order to obtain enough oxygen. Ammonia, a waste product of a fish’s metabolism, is passed out from a fish’s gills along with carbon dioxide. This ammonia must be removed from the water because bacterial action will change it into poisonous nitrates.
SWIM-BLADDERS
Almost all aquarium fish except some bottom dwellers have a gas bladder, or swim-bladder, that expands and contracts in response to the pressure of the water outside of the fish and allows it to maintain its equilibrium and stay at any level in the water that it desires. The gas is secreted from nearby blood vessels and reabsorbed by a different set of blood vessels. Some fish their swim bladders with air that they gulp at the water’s surface. In some cases, the swim-bladder is linked by small bones to the inner ear and acts as a sound amplifier. These bones are called the Weberian ossicles.
Fish Senses
Like all living creatures, fish have an elaborate sensory system that enables them to capture message from their environment and send them to their brain via nervous impulses, where they are interpreted and then acted on.
VISION
Fish usually have large eyes with no eyelids. The lenses are big, thick, and round in order to capture as much light as possible because light does not travel far in water. IN most aquarium species, the eyes are located on the sides of the head. This means that fish have monocular peripheral vision. That is, they can see in all directions on both sides of their bodies but lack binocular vision or depth perception. In most instances, each eye can move independently, which adds to the range of the fish’s vision.
Although their vision is acute and they can see light and motion well, most fish cannot focus clearly on objects that are more than about two feet away. Interestingly, their lenses do not change shape in order to focus an image on the retinal surface; instead, the entire lens moves in and out. All but deep-sea fishes are able to perceive colors.
SMELL
Two or more nostrils are located just above a fish’s mouth. They are not used for breathing but are directly connected to a small organ. Fish have a keen sense of smell which they use primarily to locate food but it also helps them to find other fish while schooling and to recognize familiar locations.
HEARING
Fish have no visible outer ears. Their inner ears are divided into two parts that control equilibrium and hearing. Unlike light, sound travels far in water and is very easily detected by fish. Fish are extremely sensitive to vibrations in the water, due to their good hearing and to their lateral-line sense organ. As mentioned, some species have particularly acute hearing due to the amplification of sound in their swim-bladders.
LATERAL LINES
Fish have additional sense organs, called lateral lines. IN most aquarium fish, these lines, or nerve and fluid-filled canals, can be seen extending along each side of a fish’s body, from just behind the gills to the caudal peduncle. They look like a raised welt that might have been made by scratching the fish’s skin with a fingernail. IN some fish, they are branched or curved, and may even extend to the snout.
With the aid of this system, fish can detect water currents, vibrations, and sound waves, enabling them to navigate and to locate other members of their species in the murkiest of waters. It is thought that these vibrations are amplified in the swim-bladders of some species. Blind fish are able to swim through the most complicated mazes by means of this sensory system.
ELECTRICITY
To a greater or lesser extent, all fish can generate and discharge electricity. Some, like electric eels, rays, and catfish, are able to give severe shocks, which are used as a protective mechanism and to stun prey. Most fish, however, simply emit low-voltage currents that form an electrical field around their bodies. When this field is broken it alerts a fish to the presence of obstacles or other fishes in the immediate vicinity.
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