What is a fish?

This is a difficult question to answer because fishes have a huge variety of body forms.

The popular concept of a fish as an animal that has fins and scales, and lives in water, is not strictly correct. Many species of fishes such as the clingfishes lack scales, and others such as some species of eels have no fins. Some fishes such as the lungfishes can spend considerable time out of water.

All fishes have a backbone or a notochord, and all breathe using gills. Some animals that are not fish, such as the axolotls also breathe using gills. These animals however have fully formed limbs that are lacking in fishes.

Berra (2001) states that 'If we allow room for these and other exceptions, we can define a fish as a poikilothermic, aquatic chordate with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales'.

Nelson (1994) states that fish are 'aquatic vertebrates that have gills throughout life and limbs, if any, in the shape of fins'.

References

1. Berra, T.M. 2001. Freshwater Fish Distribution. Academic Press. Pp. 604.
2. Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
3. Nelson, J.S. 1994. Fishes of the World. 3rd Edition. John Wiley and Sons, Inc. Pp. 600.