Description
The largest of all sticklebacks, it is usually 5 cm (2 in) long (but may reach, exceptionally, twice that length). The body is laterally compressed. The base of the tail is slender. The caudal fin has 12 rays. The dorsal fin has 10-14 rays; in front of it are the three spines that give the fish its name (though some individuals may have only 2, or 4). The third spine (the one closest to the dorsal fin) is much shorter than the other two. The back of each spine is joined to the body by a thin membrane. The anal fin has 8-11 rays and is preceded by a short spine. The pelvic fins consist of just a spine and one ray. All spines can be locked in an erect position, making the fish extremely hard to swallow by a predator. The pectoral fins are large, with 10 rays. The body bears no scales but is protected by bony plates on the back, flanks, and belly. There is only one ventral plate, but the number of flank plates varies greatly across the distribution range and across habitat types (see below); it is normally higher in marine populations (some freshwater populations may in fact lack lateral plates altogether).
Dorsal coloration varies but tends towards a drab olive or a silvery green, sometimes with brown mottling. The flanks and belly are silvery. In males during the breeding season, the eyes become blue and the lower head, throat, and anterior belly turn bright red. The throat and belly of breeding females can turn slightly pink. There are a few populations, however, where breeding males are all black or all white.
Read more about this topic: Three-spined Stickleback
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