| 1a |
Dorsal spines usually 9 (7-12), short and inclined alternately to left and right; gill membranes united, but entirely free from isthmus; no distinct bony plates on sides |
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| 1b |
Dorsal spines 3-6; gill membranes united to isthmus (membranes united but free from isthmus in Culaea); lateral bony plates present or absent |
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| 2a |
Dorsal spines 4, 5, or 6, short; pelvic spines also short; length of all spines usually less than eye diameter; pelvic skeleton without large, wide, median posterior processe |
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| 2b |
Dorsal spines 3 (rarely 2 or 4), usually with distinct vertical bony plates along sides; pelvic skeleton with large, wide, median posterior processes |
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| 3a |
Dorsal spines 4, 5, or 6, short; pelvic spines also short; length of all spines usually less than eye diameter; pelvic skeleton, if present, with narrow median posterior processes |
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| 3b |
Dorsal spines 4 (rarely 50, of varying lengths and long, inclined alternately to left and right; length of first and second dorsal spines and pelvic spines distinctly greater than eye diameter; pelvic skeleton with lateral posterior processes |
Fourspine Stickleback, Apeltes quadracus |
| 4a |
Pelvic fin of 1 spine and 1 soft ray, spine with 1 pointed cusp at base; caudal peduncle usually with a keel; body without round black spots; colour in life green, blue or silvery |
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| 4b |
Pelvic fin of 1 spine and 2 soft rays, spine with 2 well-developed pointed cusps at base; caudal peduncle keel-less; many round black spots along sides; colour in life lemon-yellow |
Blackspotted Stickleback, Gasterosteus wheatlandi |