Simon Wightman, of Balclutha Primary School, asks :-
Do crabs only live in the salty sea?
Keith Probert, a marine zoologist at Otago University, responded.
The short answer, perhaps surprisingly, is no. There are several thousand different types, or species, of crabs, and the great majority of them do indeed live in the sea. But there are also hundreds of species - most in tropical countries - that live in freshwater lakes and rivers, and some even on land. In fact we have a type of freshwater crab in New Zealand: a small spider crab (Amarinus lacustris) that grows to about 1 cm shell width and occurs in some North Island lakes and rivers, though it prefers water that's brackish (only slightly salty).
As you move from the open sea into a river mouth and up a river, then the water gets less and less salty until it's completely fresh, with no saltwater mixed with it. Many groups of animals that evolved in the sea have species that have made this journey, moving from salt to freshwater. This presents problems though, as the animal has to keep a balance between the saltiness of its own body tissues and that of the surrounding water.
Some crabs spend only part of their life in saltwater, like the Chinese mitten crab (Eriocheir sinensis), so called because of its furry front claws. In this species the young crabs live in freshwater, but then migrate down to the sea to live as adults. There is a real concern that this crab could be introduced into New Zealand in ships' water ballast tanks, and that its burrows would damage river banks.