Josh Alien, of Ilam Primary School, asks :-
Why do fish have scales?
Jo Knight, a zoologist and the curator of community programmes at the Auckland Zoo, responded.
The short answer Josh is that fish have scales to protect themselves.
All fish need protection from abrasion, being eaten, and from bacterial and parasite attack from organisms living in the water. Some primitive fish, such as the hag fish, use a layer of slime for protection and defence. Most use scales.
Scales are really overlapping bony plates in the skin of the fish arranged like tiles on the roof of a house. Many primitive fish had thick heavy plates which were often quite large and armour-like. A few of these relics of the past exist today, for example the gar and the coelacanth, but in the main they have been overtaken by more efficient bony fish. These modern fish have small thin scales which, while still protective, allow much more freedom of movement so that the fish's body can flex as it moves through the water.
The sharks do not have true scales but have `dermal denticles' (skin teeth) which are actually tiny teeth that serve the same purpose as scales and give the skin its sandpaper feel.
If a fish gets damaged it will quickly replace the lost scales, but anyone who keeps an aquarium will tell you that this is a time when a fish often gets an infection.
An alternative answer to the question Josh is "so they can sing in the school choir!"