Three-spined Stickleback

Overview

The Three-spined Stickleback (Gasterosteus aculeatus) is one of the most famous, scientifically celebrated, and biologically adaptable teleosts on planet Earth. Belonging to the stickleback family (Gasterosteidae), this tiny, spiny fish has served as a cornerstone model organism in evolutionary biology, animal behavior, and genetics for over a century, immortalized by Nobel laureate Niko Tinbergen in his pioneering foundational studies of animal ethology.

Ecologically, the three-spined stickleback is a vital mesoconsumer and keystone forage fish across coastal and freshwater ecosystems throughout the Holarctic. Thriving in marine, estuarine, and inland river habitats, it transfers massive amounts of invertebrate energy to wading birds, kingfishers, and predatory fish like pike and trout. Its astonishing evolutionary plasticity has allowed isolated post-glacial populations to repeatedly and independently evolve specialized benthic and limnetic morphs across thousands of isolated northern lakes.

Biology and Physical Characteristics

Gasterosteus aculeatus possesses an armored, slender body characterized by three sharp, erectile, isolated dorsal spines preceding its soft dorsal fin, and two pelvic spines that can be locked rigidly outward like switchblades to choke predatory birds and larger fish that attempt to swallow it. Instead of true scales, its body is shielded by vertical bony lateral plates (scutes); marine populations are fully armored with 30 or more plates from head to tail, whereas freshwater populations rapidly evolve reduced armor, bearing only a few anterior plates to save metabolic calcium. In spring, breeding males undergo a dazzling transformation: their backs turn bright shimmering blue-green, their eyes flare iridescent turquoise, and their throat and belly become flushed with intense, fiery crimson-red. Internally, the male’s kidneys hypertrophy to produce ‘spiggin’—a specialized, elastic glycoprotein mucus that functions as organic underwater glue.

Behavior and Ecology

Sticklebacks are famous for their complex reproductive and parental behaviors executed entirely by the male. In spring, a male establishes a small territory on a sandy or weedy riverbed. Collecting algae, moss, and decaying plant fragments, he weaves them into a neat, hollow, barrel-shaped nest, gluing the fibers together with secreted spiggin glue from his kidneys. When a gravid female with a swollen silver belly approaches, the male performs his legendary ‘zig-zag dance’: darting rapidly between the female and the nest entrance, he guides her to swim inside the tunnel, poking her tail to induce spawning. After the female deposits up to 300 eggs and departs, the male swims through the nest to fertilize the clutch. For the next two weeks, the male remains dedicated to the nest, tirelessly fanning his pectoral fins day and night to oxygenate the eggs, cleaning away fungus, and aggressively herding stray fry back into the nest inside his mouth.

Conservation and Human Impact

The Three-spined Stickleback is evaluated as Least Concern on the IUCN Red List, boasting vast, resilient populations across three continents. Its extreme physiological hardiness allows it to tolerate brackish water, low oxygen levels, and urban drainage ditches. However, distinct post-glacial species pairs in isolated coastal lakes in British Columbia (such as Enos Lake) have suffered tragic genetic collapses and extinction due to the introduction of invasive crayfish that disrupt courtship and muddy water clarity.

Interesting Factoid

During the spring breeding season, the male Three-spined Stickleback’s kidneys turn into a living hot-glue gun, producing a super-strong biological glue called ‘spiggin’ that he uses to weave and paste together a hollow underwater home out of water plants.