Fish
Three-spined Stickleback
Gasterosteus aculeatus · also called tiddler, sticklebat
The three-spined stickleback is the “tiddler” that generations of children have caught in a jam jar from a ditch or a stream. It is also, quietly, one of the most important fish in science: a tiny, spiny, armoured animal that has taught biologists how instinct works, how evolution repeats itself, and how parasites hijack their hosts.
Overview
Gasterosteus aculeatus is a small fish, usually 3 to 8 cm long, with no scales but a row of bony side plates, two or three locking spines on the back, and a stout spine in place of each pelvic fin. It lives in fresh, brackish, and coastal waters right around the Northern Hemisphere, in ponds, ditches, streams, estuaries, and rock pools, and it eats small invertebrates and fish eggs.
Armour that evolves away
Sea-run sticklebacks are heavily armoured. When populations become landlocked in lakes and streams, they repeatedly evolve reduced plating and shorter spines within decades to a few centuries, a change driven largely by one gene. This fast, repeatable evolution has made the stickleback a favourite study system.
The red male and the zigzag dance
In spring the male turns red-throated and blue-eyed, builds a tunnel nest glued with a kidney secretion, and courts females with a zigzag dance. He alone guards and fans the eggs. Niko Tinbergen’s work on that red belly and that dance helped found the science of animal behaviour.
Status
The three-spined stickleback is Least Concern and hugely abundant. It matters to science, not to conservation, though a few isolated freshwater forms are locally distinctive.
Three-spined Stickleback: common questions
Why is the three-spined stickleback so important in science?
It combines several features that are rare in one animal. It is small, hardy, quick to mature, and easy to keep and breed in a tank, so it suits laboratory work. Its courtship is a fixed, elaborate sequence of movements, which made it the ideal subject for Niko Tinbergen's Nobel-winning studies of instinct in the mid-twentieth century, including the finding that a crude model with a red underside triggers male aggression better than an accurate model without one. And its recent evolutionary history, with sea-run fish repeatedly invading fresh water and losing their armour, has made it one of the best systems for studying how the same genetic changes produce the same adaptations over and over.
Do sticklebacks really lose their armour in fresh water?
Yes, and it happens fast enough to watch. Marine three-spined sticklebacks are heavily armoured, with a continuous row of bony plates along each side and long pelvic spines, defences against the many predators of the sea. When populations become landlocked in lakes and streams, the calcium and phosphorus needed to build that armour are scarcer, predators are often different, and the plates and spines can slow the fish and give dragonfly larvae something to grab. Over decades to a few centuries, freshwater populations repeatedly evolve reduced plating and shorter or absent pelvic spines, and the main plate change is driven by different versions of a single gene called Eda.
How does the male stickleback look after the eggs?
The male does everything. He first stakes out a small territory on the bottom and builds a nest, gathering bits of plant material and cementing them into a short tunnel with a sticky protein, spiggin, secreted from his kidneys. He then courts passing females with a zigzag dance, leads a ripe female through the tunnel so she lays her eggs, and fertilises them. From that point he guards the nest against all comers, repairs it, and constantly fans water through it with his fins to keep the eggs oxygenated. After they hatch he shepherds the fry for a few days, retrieving strays in his mouth, before they disperse.
Are sticklebacks a threat to other fish?
On their own, not seriously, but they can be a nuisance where fish are being bred. Sticklebacks eat the eggs and newly hatched fry of other species, and in a hatchery pond, a trout stream, or a garden pond stocked with other fish, a dense stickleback population can take a real toll on young fish and compete for invertebrate food. They are hard to remove once established because they breed prolifically and tolerate poor conditions. In the wild, sticklebacks are part of the natural community and are themselves heavily eaten by larger fish, birds such as kingfishers and herons, and diving beetles.