Fish are the oldest vertebrate group. There are more than 21,000 named species, and, in sheer numbers of individuals, they easily outnumber the four terrestrial vertebrate groups combined (mammals, birds, reptiles and amphibians.)
General Groups and Characteristics
Jawless Fishes
- Eel-like body, covered with soft skin containing mucous glands and no scales
- No paired appendages, but has dorsal median fins
- Fibrous, cartilaginous skeleton
- No jaw, but teeth are present in the sucker-like mouth
- Two chambered heart
- Gills
- Dorsal nerve cord and differentiated brain
- Digestive system lacks a stomach
- External fertilization; hagfish have separate sexes, while lampreys are hermaphroditic
Cartilaginous Fishes
- Skin has placoid scales, which are cone shaped with a tooth-like structure. These scales are found only in this group of fish.
- Paired pectoral and pelvic fins; two dorsal median fins
- Completely cartilaginous skeleton
- Ventral mouth with jaws
- Two chambered heart
- Gills
- Brain has multiple lobes
- Digestive system includes a J-shaped stomach
- Internal fertilization; may be oviparous ovoviviparous, or vivaparous
- No swim bladder
Bony Fishes
- Skin with mucous glands; may be scale-less or have one of three types of scales:
Ganoid: Diamond shaped, non-overlapping, pattern like paving stones.Ctenoid: and Cycloid- Thin, flat and overlapping.
- Both paired and median fins
- Skeleton is more or less bony and tail is usually symmetrical
- Mouth with jaws and many teeth (some species are toothless)
- Two-chambered heart
- Gills
- Multi-lobed brain
- Digestive system includes stomach
- Separate sexes (with sex reversal occurring in some species) and fertilization usually external
- Swim bladder
Locomotion
- The trunk and tail muscles provide propulsive power
- Motion is serpentine, with undulations
- Fishes create virtually no turbulence when they swim due to their body motions and surface textures.
Buoyancy
- Many bony fishes have a swim bladder. This organ allows the fish to vary the volume of gas in its body, specifically in the swim bladder. The fish can attain neutral buoyancy and hang motionless at any depth. Special body parts allow the fish to add gas to, and remove gas from, the swim bladder
- All fish are slightly heavier than water and must keep moving forward to prevent sinking
- Large livers of sharks contain saualene, which acts like a buoyant oil
Respiration
- Gills are specialized structures which are richly supplied with blood vessels. The gills remove oxygen from the water and allow the oxygen to enter the blood. Water flows into the mouth, through the gills, and out the gill flap.
- Highly active fish have higher oxygen needs and must constantly swim forward to supply enough water to the gills.
- Some species of fish can spend time outside of the water and have various respiratory adaptations. These include special lungs, use of the skin, use of a lung-like swim bladder, air gulping into a vascular mouth, and specially adapted air chambers for air breathing.
Osmotic Balance
- Fish must maintain the proper balance of salt and water in their bodies and are affected by the salt content of the water.
- Fish fall into one of four categories:
Euryhaline: These fish can pass back and forth from fresh to salt water with ease.Stenolialine: These fish may be fresh or salt water dwellers and can tolerate very little fluctuation of salt content in the water; i.e. they need a certain level of salt ONLYAnadromous: These fish migrate from the sea to fresh water to spawn.
Catadromous: These fish migrate from fresh to sea water to spawn.
Feeding
- Fish devote more time and energy to eating and searching for food than to any other activity.
- Feeding behavior was one of the primary influences in the development of physical adaptations such as jaws, muscles, agility, balance and the improvement of certain senses.
- Foods:
Most fish are carnivores and prey on foods from zooplankton to large vertebrates.A few species are herbivores and eat flowering plants, algae and grasses.
Filter feeders are a third type of fish, and they eat the microorganisms found in the water.
Many fish are filter feeders when in the larval stage. Most filter feeders are pelagic. This
means that they live out in the open seas, rather than in shore areas. These pelagic fish
usually travel in large schools, and they are the most abundant of all fishes. They are an important food sources for larger carnivores. There are also many species of freshwater
fish which are filter feeders.
Another group of fish are the omnivores, which eat both plants and animals.
Scavengers eat organic debris, called detritusParasites are fish which suck the body fluids of other fish- Digestive System. Most fish have a standard vertebrate digestive system in which food is taken in to the stomach and then flows through a tubular intestine.
Coloration and Concealment
- Some of the brightest animals in the world are fish which dwell in coral reefs and other tropical areas. Their colors are not necessarily for camouflage, and these fish depend on alertness, agility or poisonous flesh for protection.
- There are two forms of camouflage patterns present in fish:
Obliterative shading - The upper surfaces of fish with this type of coloration are generally
shades of green, brown or blue, which blend with the stream and ocean floor when
viewed from above. Their bellies are usually pale, grading to silver and yellow-white,
which blends with the water surface and sky when viewed from below.Mimicry - fish may bear a physical resemblance to another item in environment. This
mimicry may be in shape, in color or both.