The Gasterosteiformes

Alwyne Wheeler (1969) in "The Fishes of the British Isles and North West Europe":

STICKLEBACKS

Gasterosteidae.

STICKLEBACKS are fishes of very wide distribution in the northern cold-temperate zones, two of the European species being found on the American continent, as well as in northern Europe and Asia. They are also extremely adaptable to their surroundings and can survive and even thrive in areas where other fishes would be discomforted. In addition they are tolerant of sea water, and while one European species is entirely marine, another is equally at home in both sea and fresh water and the third species will tolerate a low salinity.
All three species have complicated nest building habits, the males constructing the nest and guarding it and the eggs during incubation. The development of this habit leads to territorial behaviour, defence of a bounded site and intricate displays before spawning on the part of the male. The males also show striking colour changes before and during the spawning season.
Sticklebacks have certain features in common; a rather elongate body form, particularly of the head and snout, two dorsal fins, the first of which is composed of a series of isolated, alternate spines, the second dorsal fin and anal fin being opposite and similar in shape. The pelvic fins are much reduced, but have one long, sharp, stout spine. Some authorities regard the sticklebacks as closely related to the pipefishes and unite them in a common order - the Thoracostei.

A. Eight or more small isolated spines on the back, all approximately of equal length; spines all shorter than the second dorsal fin rays. B B. Eight, nine or ten short isolated spines on the back; body moderately, but not very elongate, its depth about equal to head length. Fresh water; locally distributed.
Ten-spined Stickleback Pungitius pungitius
AA. Three isolated spines (sometimes two or four) on back; the first two as long as, or longer than the dorsal fin rays, the third spine short. Abundant in fresh water, also occurs in estuaries and, in the north in the sea.
Stickleback Gasterosteus aculeatus BB. Fifteen short isolated spines on the back; body extremely long and thin (may be mistaken for a pipefish); body depth less than half the head length. Wholly marine, usually close inshore.
Grows to 10 in (25 cm).
Sea Stickleback Spinachia spinachia

A. Eight or more small isolated spines on the back, all approximately of equal length; spines all shorter than the second dorsal fin rays. B	B. Eight, nine or ten short isolated spines on the back; body moderately, but not very elongate, its depth about equal to head length. Fresh water; locally distributed. Ten-spined Stickleback Pungitius pungitius
AA. Three isolated spines (sometimes two or four) on back; the first two as long as, or longer than the dorsal fin rays, the third spine short. Abundant in fresh water, also occurs in estuaries and, in the north in the sea. Stickleback Gasterosteus aculeatus	BB. Fifteen short isolated spines on the back; body extremely long and thin (may be mistaken for a pipefish); body depth less than half the head length. Wholly marine, usually close inshore. Grows to 10 in (25 cm). Sea Stickleback Spinachia spinachia