"as a stickleback swam past"

Stickleback
GNU Free Documentation LicenseStickleback - Credit: Ron Offermans
Sticklebacks are fish of an unusual sort, in that they have no scales. They are distinguished by a strong and clearly isolated spine in the dorsal fin.  Some species have bony armour plates. Their maximum length is about 4 inches.  They’re most commonly found in the ocean, and are related to seahorses.  However, they do also occur in some freshwater lakes in Europe, Asia and North America, where they were trapped after the ice age. 

Their mating habits are also unusual.  The male constructs a nest, the female lays her eggs inside, the male fertilizes the eggs, and then guards them until they hatch.