Fish anatomy is primarily governed by the physical characteristics of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, which is much denser

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 than air, holds a relatively small amount of dissolved oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and absorbs more light than air does.
Fish have a variety of different body plan

Body plan

A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

s. Their body is divided into head, trunk, and tail, although the divisions are not always externally visible. The body is often fusiform

Fusiform

Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends.* Aneurysms can be classified as saccular or fusiform...

, a streamlined body plan often found in fast-moving fish.