The Three Spined Stickleback, Gasterosteus aculeatus, is usually thought of as a freshwater fish, shoaling in both running and still waters. It is characterised outwardly by its very silvery appearance when viewed laterally but with a greenish brown appearance from above. The Stickleback has plates in place of scales to protect its body which enhances this shiny image. It is one of three Stickleback species found in and around the British Isles.
This species - far less known and widespread than the Three-spined - is not found in Northern Scotland. It is small - seldom reaching three inches in length. It has from 8 to 11 short spines on the back. Its colour is greenish-olive - with many dark dots; in the Spring the male is blue-brown - much less conspicuous than its red-bellied larger brother. The habits of the two species are very similar - but the nest of the Ten-spined is hung on to weeds above the river bottom - unlike that of the Three-spined.