An unconventional fish

The three-spined stickleback is difficult for taxonomists to define. Genetically, it is diverse, and variation from fish to fish is rife. Superficially distinct populations may reside within a single stream or pond, refusing to interact. This aquatic tribalism may even exclude inter-population spawning. The sea-dwelling stickleback evolved first. Its ability to flit between fresh and saltwater habitats allowed it to inhabit a vast area. There are sticklebacks in much of the northern hemisphere, including countries as far apart as Japan and Greenland, as well as the northern regions of Africa and America. Current scientific thinking suggests that the freshwater populations across these regions were trapped following an ice age. Sea-dwelling sticklebacks are easily identified by their silvery suits of scaly armour. Freshwater fish have only a few plates, if any. This reflects the variable predation pressures that each population faces. The greater the armour, the higher the number of threats. Marine-goers are preyed upon by seabirds and fish in an environment with few hiding places. Freshwater inhabitants are threatened by kingfishers, aquatic insects and many native freshwater fish. They do at least have the advantage of cover, in the form of plants and fallen wood. For both types, the three dorsal spines that give the fish its name are an impressive anti-swallowing deterrent when erected and held firm. Marine sticklebacks may reach 41/2in (11cm) fully grown. Those in freshwater will peak somewhere closer to 2in (5cm). The marine types are nearly exclusively silver. Across the belly the freshwater fish are platinum, but green, brown, turquoise and grey decorate their upper, dorsal sides. This colouring doubles as a form of camouflage known as countershading. It makes them as hard to spot by predators stalking from beneath as by those above.