Joshua Sellwood of Balclutha Primary School asks :-

Why do fish need gills?

Jean McKinnon, a marine zoologist at Otago University, responded.

Fish need gills to breathe. Fish gills extract oxygen from seawater; lungs don’t work in water only in air. This means that a fish generally has to stay wet so that its gills can work. There are a few odd fish though which don't seem to follow the rules!

Mudskippers are funny little fish that can be found throughout the Pacific in swampy areas. What makes them strange is that they spend three quarters of their time out of water. They have gills just like any other fish but they also have a special space behind their gills where they store sea water, when their gills start to dry out they rotate their eyes, which squashes the cavity and squirts water onto the gills. Every now and then they return to the sea to collect more water for their storage space!

Lungfish have gills but also have a simple lung which can extract oxygen from the air, sometimes they can be seen gulping air at the waters surface. Lungfish live in areas where droughts are common (e.g. South America, Africa and Australia), if the river dries up the lungfish buries itself in the mud and produces a tough leathery cocoon and the animal lies dormant until the rivers water returns. They can stay like this for up to two years!

Mexican walking fish (Axolotl) are not actually fish, they are amphibians like frogs! They spend most of their lives in a larval (baby) form where there are feathery gills on either side of its head. It has primitive lungs but they are not used much.