Three-spined Stickleback
The Three-spined Stickleback, Gasterosteus aculeatus, is a fish native to much of northern Europe, northern Asia and North America. It has been introduced into parts of southern and central Europe.
Gasterosteus aculeatus aculeatus is found in most of the species range, and is the subspecies most strictly termed the Three-spined Stickleback; its common name in Britain is the Tiddler, although "Tittlebat" is also sometimes used.
The anadramous form spends most of its adult life eating plankton in the sea, and returns to freshwater to breed.
The adult fish are typically between 6 and 10 cm long, and have 30 to 40 lateral armour plates along their sides. They also have long dorsal and pelvic spines. The anadromous form is morphologically similar all around the Northern Hemisphere, such that anadromous fish from the Baltic, the Atlantic and the Pacific all resemble each other quite closely.
Source: en.wikipedia.org/wiki/Three-spined_stickleback - 29.09.2011
Three-spined Stickleback
During the spawning season only the male is so brightly coloured.