[Ebook] Encyclopedia of Aquarium & Pond Fish – Introduction to FISHKEEPING – What is a fish?

Introduction to FISHKEEPING

What is a fish?

Defining a fish is harder than it seems. To most people, it is an animal that lives and breathes in water. Some fish, however, emerge onto land, breathe air, and use their fins like legs. And many other animals, including amphibians, mammals, and invertebrates, live in water. To add further confusion, some creatures called fish—starfish, jellyfish, and cuttlefish—are not really fish at all.

Fish are vertebrates, which means that—like humans, but unlike starfish, jellyfish, and cuttlefish—they possess a backbone. Most are cold-blooded (ectothermic), so they cannot raise their body temperature above that of their environment, unlike mammals and birds; this limits the effective range of fish to warmer waters (though there are many exceptions). All fish have gills, which they rely upon to varying degrees to obtain oxygen, and most species have two pairs of fins in place of arms and legs, as well as several other fins on the body. The majority of fish are covered in scales, which are thin, overlapping outgrowths of the outer skin that protect the body. They secrete a slimy coating that protects them from parasites and bacteria and helps them slip through the water. The presence of scales helps to distinguish fish from amphibians—another group of water-dwelling vertebrates.

Body and fins

There are many alternative body plans for fish. A “typical” fish is designed to be streamlined so that it can cut through the water with the least effort. It has a spindle-shaped profile, though it is somewhat wider to the front of the midline, and its head joins the body without a neck. Its eyes are flush with the head, and only the fins extend beyond the body; even the fins can be pressed flat against the sides to minimize water resistance.

Fish rely on their fins for locomotion, though they may have more specialized uses in some species. Swimming through water, which is much more dense than air, requires considerable muscular effort, and the main thrust for swimming is provided by the caudal fin at the rear of the body; this starts the rippling movement that spreads through the fish’s body. The dorsal fin, which runs down the center of the back, helps to keep the fish moving in a straight line and is counterbalanced on the underside of the body by the anal fin.

Paired ventral (or pelvic) fins are set on either side of the midline in front of the anal fin on the underside of the body. They act like stabilizers, keeping the fish upright, and in some species, such as corydoras catfish, they are used to hold the eggs during spawning. The pectoral fins, located farther forward, close to the gills on each side of the body, also help the fish to maneuver. In bottom-dwelling species, these fins may be adapted for use as props, or legs on which the fish can support themselves or even walk around.

Sometimes the pectoral fins are equipped with spines for defense. In gouramis, the pectoral fins may be transformed into hairlike structures that help the fish locate food by detecting scents in the water.

Some fish, notably characoids, have an additional smaller fin behind the dorsal fin. This is known as the adipose fin, and as its name suggests, it acts as a store of adipose (fatty) tissue and has only a minor role in locomotion.

BODY COVERINGS

The skin of a fish is usually covered by protective scales or bony plates. A fish has the same number of scales throughout its life; if some are lost, they will be replaced, but new scales are not added. Several basic types of body covering are recognized.

• Ctenoid scales, such as those of a Queen Angelfish, have a comblike rear edge. Ctenoid and cycloid scales are found in the vast majority of bony fish.
• Cycloid scales, such as those of a goldfish, have a smooth rear edge. Like ctenoid scales, they have a hard surface layer over a deeper fibrous layer made of collagen.
• Bony plates, as seen in this catfish, offer better protection than scales but restrict mobility. They start as folds in the skin of fry; the folds harden and develop into bony plates.

MOUTH SHAPES

The shape, size, and position of a fish’s mouth give a good insight into its feeding habits. Predators tend to have much larger mouths than omnivores. Some species have obvious canine-shaped teeth; in others, teeth are absent or less clearly visible.

• Surface feeders, such as hatchetfish, have a relatively short upper jaw, which enables them to grab invertebrates at the water’s surface easily
• Mid-water feeders, such as tetras, have slightly protrusive jaws so they can catch passing food particles. This is the most common mouth design.
• Bottom-feeding fish, like the plecos, have suckerlike mouths for scraping food and algae from rocks, and scavenging from the bottom of the tank.

Fin variants

The shape and position of the fins vary between species and provide valuable clues to their lifestyle. For example, shoaling fish that live in areas of open water have forked caudal fins, which provide them with good propulsive power. The open V-shaped structure gives little drag but does not have the power to provide rapid propulsion from a stationary position. Sit-and-wait predators that catch their prey in a sudden burst of speed tend to have rounded, paddle-shaped caudal fins—ideal for fast acceleration. To move quickly from a standing start, some fish rapidly expel a stream of water from their gills in an aquatic form of jet propulsion.

Water position and buoyancy

All fish rely on their fins—especially the pectoral and ventral fins—to control their position in the water and prevent them from being swept away by currents. In fish that live in fastflowing mountain streams, the fins can be fused together; the result is a suction cup that anchors the fish in place. This adaption is seen in the hillstream loaches of the family Balitoridae. Position in the water is also influenced by the swim bladder—an elongated gas-filled organ situated beneath the vertebral column. To achieve neutral buoyancy (when the fish neither rises nor sinks), the swim bladder must occupy about 8 percent of the fish’s body volume. The amount of gas in the bladder can be adjusted in two ways: the fish can gulp down air, which enters the swim bladder via the foregut, or gas can be released into the bladder from blood vessels.

Digestion and respiration

The digestive system of a fish is typical of vertebrates; as with terrestrial species, herbivorous fish tend to have longer intestines than carnivores, because plant matter is tough, fibrous, and difficult to break down. The respiratory system, however, is unique to fish. Most fish extract oxygen from the water, rather than the air, using gills, which are located on the sides of the head behind the eyes, hidden under flaps known as opercula.

Gills are bony rods to which are attached fleshy filaments, rich in thin-walled blood capillaries. Water enters the fish’s open mouth, which then closes.The water is forced over the filaments and out again through the opercula. Oxygen dissolved in the water is taken up into the bloodstream through the filaments, which usually have fine secondary flaps (or lamellae) to maximize the surface area available for gas exchange. Astonishingly, the total surface area of the gills can be more than 10 times the fish’s outer body area. Within the mouth, straining devices called gill rakers prevent food and debris from passing over and damaging the gills.

Some fish also gulp atmospheric oxygen using the swim bladder as a basic “lung.” These species are usually the natural inhabitants of muddy pools, where dissolved oxygen may be in short supply Color and pattern Almost all fish use color to aid camouflage or to attract mates. Some are colored with inconspicuous browns and greens to blend in with the background and escape the attention of predators; others—such as the flatfish—change their pattern to match their background. The brilliant colors displayed by many tropical species are also a form of camouflage; bold vertical stripes, for example, break up the outline of a body and make it hard to see. And dark stripes through the eyes often continue through the iris, making the eye almost invisible. Some fish have “false” eyespots (also called ocelli) on their tails; predators will attack what they believe to be the head, giving the prey a few moments to escape.

Fish have many organs—such as the brain, stomach, liver, and kidneys—in common with humans. Others, like the gills and swim bladder, are not present in our bodies.

Fish that have a solid, dark color tend to have lots of pigment in their skin, while species that appear silvery have little skin pigment but rely more on the iridescence of the scales. This reflectiveness is caused by the presence of the chemical guanine—a waste product from the blood. Many fish have transparent bodies as fry and develop color only with age.

SENSES AND COMMUNICATION

Although fish brains are poorly developed compared to those of mammals, fish possess acute and often highly specialized senses and means of communication

• Barbels—structures on the lips that resemble elongated whiskers— are common in fish that live in water where visibility is poor. Barbels may contain touch and taste organs that help the fish navigate and find food
• The lateral line runs down each side of the fish’s body. It comprises a row of pores opening into a channel that leads to the head. The channel is filled with a viscous jelly, which detects vibrations in the water.
• Fish eyes are similar to those of other vertebrates and can see colors. Vision is particularly sharp in fish that live close to the surface. The Four-Eyes (see pp.156–157) can see in both air and water at the same time.
• Electrical fields produced by mormyrids (see p.186), like this Peter’s Elephant-Nose, enable fish to sense their environment. Some experiments suggest that the electrical signals may also be used in communication.
• Some species use sound to communicate with each other, such as the Croaking Gourami (see p.110). Their “drumrolls” are produced by the action of muscles beating against the swim bladder.