Three-spined Stickleback Gasterosteus aculeatus

Three-spined Stickleback male
Three-spined Stickleback
Three-spined Stickleback marine
The Three-spined Stickleback is a small fish with a distinctive shape - an elongated, thin "wrist" before a tiny tail. Also, as its name indicates, it has 3 small, erectible spines on its back. It has different populations, many living entirely in freshwater (streams, lakes, ponds, even ditches) and some living in coastal salt or brackish water and returning to rivers to breed. There are populations native to much of the northern hemisphere.
Three-spined Stickleback marine
Three-spined Stickleback marine
Three-spined Sticklebacks marine
It grows to 3-5cm long, but can reach 8cm. In the breeding season, males in particular become more colourful including red undersides and blue eyerings. The fish immediately above were in coastal, marine waters. Stickleback are closely related to Seahorses and Pipefish.
Three-spined Sticklebacks
Three-spined Stickleback
Three-spined Sticklebacks marine
Males build nests for females and guard the eggs and young. Outside of the breeding season, the fish often form shoals.