Three-spined Stickleback

Fascinating facts about the Three-spined Stickleback

• Over 10 million years ago a fossil stickleback from California’s Monterey Formation looked just like today’s Gasterosteus aculeatus, showing that the species’ basic shape has barely shifted since the Miocene. This longevity makes it a living window into ancient marine ecosystems.
• An adult anadromous stickleback typically sports 30 to 40 bony plates along its sides, while many freshwater descendants carry none to a dozen. The stark reduction in armor happened repeatedly as populations colonized lakes, illustrating evolution’s ability to strip away defenses when they’re no longer needed.
• When a male builds his nest, he glues together algae, sand and debris with spiggin – a protein secreted from his kidneys. The same male’s eyes turn bright blue and his throat and belly flash red.
• All three dorsal spines can lock upright, turning the stickleback into a tiny, spiky brick that predators struggle to swallow. This mechanical defense is a simple yet highly effective survival trick across its wide range.
• Some stickleback populations can change dramatically in just over a decade: after a 1982 chemical cleanup in Alaska, oceanic sticklebacks recolonized a lake, but within 12 years the frequency of the fully armored form fell from 100% to 11%, while low-plate forms rose to 75%. The rapid shift highlights how quickly genetic variation can reshape a species when environments flip.

At a glance

About the Three-spined Stickleback

The three-spined stickleback (Gasterosteus aculeatus) is a diminutive, laterally-compressed fish that rarely exceeds 8 cm, with most adults maturing at 3–4 cm. Its naked skin is armored by bony plates along the back, flanks and belly, and a trio of dorsal spines, sometimes two or four, can lock upright, rendering it almost impossible for predators to swallow. Males in breeding season flash bright red throats and bellies, their eyes turning a startling blue, while some isolated populations sport entirely black or white males. Inhabiting the slow-flowing waters north of 30° N, this species thrives in ditches, ponds, quiet rivers, marshes and sheltered bays, tolerating fresh, brackish or salty conditions. Anadromous individuals spend most of their adult lives at sea, feeding on plankton and small fish before returning to freshwater to build nests, whereas freshwater forms exhibit a dazzling range of armor and body shapes, from the sleek, limnetic types with large eyes that hunt plankton near the surface to the stout, benthic types that scour lakebeds for chironomid larvae and amphipods. Males secrete spiggin, a kidney-derived protein, to glue together algae, sand and debris into a nest that shelters the eggs.

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