How does the nine-spined stickleback differ from the three-spined stickleback?

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Lined seahorse (Hippocampus erectus) — gasterosteiform Lined seahorse (*Hippocampus erectus*).

The three-spined stickleback (Gasterosteus aculeatus) and the nine-spined stickleback (Pungitius pungitius) are both widely distributed species within the family Gasterosteidae, typically reaching similar lengths of 5 to 10 cm (2 to 4 inches).1

The primary difference between the two is indicated by their common names: the three-spined stickleback possesses three dorsal spines, while the nine-spined stickleback features a higher number of dorsal spines. Both species share the characteristic Gasterosteiformes anatomy, including a slender caudal peduncle, a squared tail, and a row of spines positioned in front of a soft-rayed dorsal fin.1