1. Introduction to Fish

2. What are Fish? Fish are aquatic vertebrates (animals with backbones) with fins for appendages. They "breathe" by means of gills. Fish make up the largest of the vertebrate groups with well over 20,000 species. Most taxonomists recognize 45 orders and 435 families of bony fishes. They can be found in a great variety of habitats in lakes, streams, oceans and estuaries.

3. Classification of Fish Fish are classified according to the material which makes up their skeletons. Sharks, skates and rays belong to a group called cartilaginous fish because their skeletons are composed entirely of cartilage. Tropical fish are called bony fish (scientific name Osteichthyes) because they have skeletons made of bone.

5. Skin The skin of most bony fish is covered with bony scales that look like shingles on a roof. Bony fish scales are waterproof and help protect the fish. Glands in the skin in which the scales are embedded secrete a layer of mucus that covers the entire body

6. Mucus helps protect fish from infection. Handling fish removes this mucus coating and can be harmful to the fish.

8. Gills Oxygen enters the bloodstream and carbon dioxide diffuses out at the gills, which are feathery structures found along the sides of the head. The gills of a healthy fish are bright red due to the high level of oxygen in the blood that is very close to the surface of the gills. (Without oxygen, the gills would be brown.) In bony fish, the gills are covered by a bony place called an operculum.

11. Shape The shape of a fish's body tells a lot about its lifestyle. Fish with fusiform, or streamlined, bodies are usually fast swimming predators that may swim at high speeds much of the time or are capable of great bursts of speed.

12. Many tropical fish are laterally compressed (flattened from side to side). Fish with this shape are not very streamlined but they do not rely on speed for catching food or escaping from predators. Their body shape is perfectly adapted for hiding in the cracks and crevices of rocks and reef.

13. They can move into these areas to hide, to escape predators or to get at food that cannot be reached by other fish. Fish with this body shape, like angelfish, are very maneuverable and capable of short bursts of speed. They are often camouflaged with disruptive coloration.

14. Other fish are flattened from top to bottom. Fish with this body shape spend most of their time at the bottom. They are usually camouflaged or can change color to match the bottom.

15. Fusiform (Streamlined) Laterally Compressed (Flattened from side to side)

16. Depressed (Flattened from top to bottom) Eel-Like (Body shaped like a snake)

18. Fins

19. Fins are used for swimming and sometimes for protection. Some fins are paired and others unpaired. The paired fins are the pectoral and pelvic fins. The unpaired fins are the dorsal, caudal (tail) and anal fins.

20. The way the fins are used varies among different groups of fish. Most fish use their tails to move through the water and their other fins to steer with. Fins are most bony fish are flexible and supported by visible spines and rays. The shape, location and size of a fish's fins are closely linked with its way of life.

21. Pectoral Fins The paired pectoral fins are usually responsible for turning, although they can be used for other functions such as tasting, touching, support and as a source of power for swimming.

22. Pelvic Fins Paired pelvic fins add stability and are used for slowing some bony fishes.

23. Dorsal Fin This may be a single fin or be separated into several fins. In most bony fishes, the dorsal fin is used for sudden direction changes and acts as a "keel" to keep the fish stable in the water.

24. Caudal (or tail) fin This is responsible for propulsion in most bony fishes

25. Anal Fin The anal fin adds stability.

26. Tails The shape of the tail can be an indicator of how fast a fish usually swims. Crescent-shaped: Fish with crescent-shaped tails are fast swimmers and constantly on the move. Forked: Fish with forked tails are also fast swimmers, though they may not swim fast all of the time. The deeper the fork, the faster the fish can swim. Rounded: Fish with a rounded or flattened tail are generally slow moving, but are capable of short, accurate bursts of speed.

27. Fish with continuous caudal fins (dorsal, caudal, and anal fins attached) are able to swim in and around cracks and crevices. Fish with lunate caudal fins tend to be the fastest fishes and maintain a rapid speed for long durations.

28. Many continuously swimming fish have forked caudal fins. Fish with truncate caudal fins are usually strong, but slow, swimmers. Fish with rounded caudal fins are usually strong, but slow, swimmers.

29. Eyes Fish are visual predators. Many nocturnal fish have large eyes to help them feed at night. Fish such as sharks have pupils that dilate and constrict, and some sharks also have an eyelid that closes from the bottom upward. Bony fish eyes lack both of these characteristics.

30. Mouths The position of a fish's mouth gives a general indication of where it feeds in the water column. Fish with up-pointing mouths like hatchetfish primarily feed on the food flakes that float or hang near the water surface. Some fish with mouths on the underside of their head, like the catfish, feed on the bottom.

31. Large For eating whole fish or chunks of fish Small For nibbling on plants and small animals Dorsal For eating near the surface

32. Anterior For eating in the water column Ventral For eating on the bottom

33. Breathing Underwater Fish, like most organisms, need oxygen to survive. The oxygen that fish "breathe" is dissolved in the water. The oxygen enters the water surface by diffusion or in the water from plants as a byproduct of photosynthesis.

34. Water enters the fish's mouth, moves across the gills and passes out the gill slits or operculum. The gills are made up of a bony or cartilaginous arch supporting a large number of paired gill filaments. Numerous small projections with very thin membranes on each filament are the sites of gas exchange (oxygen to carbon dioxide).

35. Beneath the thin membrane is a network of blood vessels. Oxygen diffuses from the water through the membrane into the blood and carbon dioxide diffuses outward.

37. Senses

38. Sight: Most fish have well-developed eyes on the sides of their heads. This placement allows them to see in all directions. The structure of the eye is very similar to that of other vertebrates.

39. In fish, focusing on near and distant objects is achieved by moving the lens backward and forward by muscular action. Fish that are colorful can probably see in color. Bony fish that are active at night have large eyes that help them see in the dark.

40. Smell: Fish can smell odors in the water with receptors inside blind sacs, called nares, located on the head. Nares are similar to our nostrils, but fish do not use them to breathe. The sense of smell is well developed and is useful in detecting distant odors.

41. Taste: For fish, taste is more of a close range sense. Taste buds, which can be found on different parts of the fish, are used in the identification of food. Taste buds are located in and around the mouth, and, on some fish, on the skin and even on the fins. Barbels, whisker-like structures near the mouth of fish such as catfish, are covered with taste buds and are used in locating food.

42. Hearing: Fish have ears but they do not open to the outside and cannot be seen. Hearing in fish is well developed. Water carries sound much better than air and the sound waves are picked up through the fish's body.

43. Lateral Line: A line of holes runs along both sides of a fish from the head to the tail. The holes have nerve connections that are sensitive to vibrations. The lateral line is used form schooling, avoiding obstacles and detecting prey and predators.

45. Electroreception: Some bony fish can detect weak electrical fields through pit organs on their heads and faces. This ability is used for locating prey (all living things give off weak electrical fields), avoiding obstacles in murky water and in migrating.

47. Buoyancy Neutral buoyancy, the ability to remain motionless in the water without rising or sinking, is of great importance to a fish. It allows a fish to spend energy feeding, escaping predators or mating that would otherwise be needed to maintain its position in the water.

48. Several strategies help achieve neutral buoyancy. Most bony fish have a structure called a swim bladder, which is a balloon-like organ in the body cavity. Gases from the blood are added or removed to control buoyancy at different depths.

49. Fish Behavior

50. Behavior is the action of a fish in response to its environment including other animals. The most interesting and sometimes the most obvious behaviors involve interactions with other individuals.

51. Migration: Many species of fish migrate during their life cycle. It is well known that salmon make long migrations to spawn. Other fish migrate in response to changing environmental conditions, such as temperature or abundance of food.

52. Schooling: Many fish congregate in groups called schools. A school is a group of similarly-sized fish of the same species aligned and swimming in the same direction. Sight seems to be the primary means of maintaining the school. The lateral line may also be used.

54. Aggression: Aggressive behavior is an interaction between two fish of the same species (intraspecific) or different species (interspecific). It is usually associated with reproduction or the defense of feeding territory. It includes posturing, direct attacks and displays such as fin flaring and changes of color.

56. Resting: Some fish spend a good portion of their day resting. Resting reduces competition, provides protection from predators and may help in energy conservation.

57. Communication: Senses play an important part in fish communication. Visual communication is important to most fish. Body movements, postures, colors and color patterns are the primary means of visual communication.

58. Sound is also used for communication. Sounds are produced by grinding teeth, flexing or contracting muscles and vibrating the swim bladder.

59. Fish also communicate by releasing chemicals called pheromones. These are chemical signals produced by an animal that, when released, influence the behavior of others of the same species. The sense of smell is important for this form of communication.

61. Cleaning: Some tropical fish are cleaners. They pick parasites from other fish. Cleaning fish, such as wrasses, are brightly colored. They establish cleaning stations that other fish approach in order to be cleaned. Body posturing, a form of visual communication, is used to initiate cleaning.

63. Color

64. Red Red is a common color in fish. You might think that red fish would be very easy for a predator to find. However, most fish that have this coloration live in dark or deep water, or are nocturnal (active at night). In deep water, red light is filtered out quickly so red is a good camouflage. At night, red-colored objects appear gray.

66. Countershading Many fish are dark on top and light on the bottom. When seen from above they "disappear" by blending in with the dark color of the depths of the bottom. Seen from below, the light belly blends into the sky above.

67. Disruptive Coloration This is a form of camouflage. The patterns and lines break up the outline of the fish and help it to blend into the background. This confuses predators since the fish shape is not easily identifiable.

68. Eye Spots Eye spots are a form of mimicry. They eye spot, usually found near the tail, may be used to draw attention away from the real eye which is a target that a predator might strike.

69. Warning Coloration Many fish use bright colors to "advertise" the presence of venomous spines or some other defensive mechanism

70. Camouflage Many fish have colors or patterns that match their backgrounds. Some fish can even change color to match different backgrounds either to hide from prey (if an ambush is planned!) or to hide from a predator.

72. Food

73. As a group, bony fishes have a diverse range of food preferences. They may be herbivores (plant-eaters), carnivores (meat-eaters), omnivores (plant- and meat- eaters) or detritivores (animals that eat decomposing plants and animals). Some of the animals common in the diets of bony fishes include worms, marine snails, mussels, clams, squids, crustaceans, insects, birds, amphibians, small mammals and other fishes.

74. The amount of food a bony fish eats is directly related to its size, its metabolic rate and the temperature of its environment. Warm water fishes, such as tropical fish, generally have higher metabolic rates and require more food than coldwater species of the same size.

75. The esophagus in bony fishes is short, expandable and layered with muscles so that large objects can be swallowed. Most species of bony fishes have a stomach with gastric glands that release substances that break down the food to prepare it for digestion. The intestine is where the majority of food absorption takes place and the digestive system terminates at the anus.

76. Reproduction in Fish

77. In most species of bony fishes, sperm and eggs develop in separate male and female individuals. Fertilization is predominately external and, in some instances, internal. Males and females may look similar, or they may look very different. Male/female characteristics may include size, coloration, external reproductive organs, head characteristics and body shape.

78. While reproduction in bony fishes is generally cyclic, various factors may influence bony fish breeding such as changes in the duration of sunlight, temperature changes, moon stages and presence of spawning areas.

79. Bony fishes show at least three types of embryonic development: egg layers (oviparous), egg retainers (ovoviviparous) and live bearing (viviparous). Depending on the species, fish parents (male and/or female) may scatter, hide, guard or brood eggs.

80. There is great variation in the development stage at which offspring are released from the parent fish. The number of offspring is inversely related to the chance a single egg has to reach maturity and reproduce; in general, species whose eggs have little chance to reach maturity lay the most eggs. For the most part, many species give no care to their eggs or young.