Functional morphology of fish

Author: Susanne Driessen, EZNC
Functional morphology is a branch within biology that looks not only at the anatomy (structure, appearance) of an organism (plant or animal) but also at the function that correlates with that structure (behaviour, feeding habits, suitable habitat).
Fish are, in terms of evolution, at the basis of the development of vertebrates. There has thus been much research on fish morphology, as the findings can be extrapolated to higher vertebrates.

A fish’s appearance is one determinant of both how and where that fish lives. In turn, the environment is a determining factor for the fish’s appearance, behaviour and feeding habits. That behaviour then also influences the fish’s appearance and habits. Appearance, environment and behaviour form a triangle of interrelationship (see illustration: -logy means ‘study of’).

In determining the shape of a fish, measurements are taken of several internal and external body parts; the sizes of these parts define the species of fish. Everything the fish does is aimed at finding and processing food in order to have enough energy to reproduce.

The eyes are the instruments in visually searching for food, which mainly occurs in clear water. A ‘sight hunter’ has fairly large eyes. If the water is more murky, the eyes are larger. In truly muddy waters, ‘sight hunting’ is useless, and eye size is of no import.

Mouth size determines the size of the morsels that can be taken in. The mouth is measured at ‘gape’ (open to maximum), so mouth size is limiting. The water taken in is eliminated through the gills, which filter out food particles while also filtering oxygen (as land animals breathe in air and their lungs take up oxygen). More gill area means greater oxygen uptake, which is especially necessary for fast hunters, who have high oxygen needs.

Body shape and proportions say something about the streamlining and swimming method. A long body with approximately equal height and width has the best streamlining, which makes swimming more efficient in terms of speed and distance. A high, short body is right for maneuvering (between rocks, seaweed, etc.) but is not very fast. Sometimes the body provides extra protection in the form of spines, armouring or ways to disappear from view (in hiding places or with camouflage).