Three-spined Stickleback - Gasterosteus aculeatus
This tiny fish measures usually no more than 10cm and gets its name because of three spines on its back: two large ones and one smaller at the base of the tail. These backwards pointing spines make it impossible for the fish to be swallowed by predators attacking from behind. However, they can't avoid airborne beaked predators, such as Dippers and Kingfishers because the spines won't pierce the beaks and the birds are careful to manouevre them round and swallow them head first.
In spring the male Three-spined Stickleback turns bright red along the flanks and silvery-blue along its back. Males fight for territories in coastal waters, in large rockpools, in estuaries and along stream and riverbeds and among reeds. Here they each build a nest and vie to attract females.
This often involves forcefully nudging passing females towards the nest. If the female is impressed by the construction she will lay and egg and the male will promptly fertilise it and hide it in the nest.
They become so aggressive in the breeding season that they will attack much larger fish, even pike.