The tiny, armored fish that build nests with their own biological glue

Nature · Published 3 October 2026 Swipe more like this →

In short: Gasterosteoidei are a group of small, northern-hemisphere fish known for their distinctive skeletal armor and unique reproductive habits. While they are often associated with sticklebacks, their evolutionary history remains a subject of intense scientific debate, with researchers constantly reclassifying where these peculiar creatures fit on the tree of life.

The suborder Gasterosteoidei, which includes sticklebacks, tubesnouts, and sand-eels, is defined by a suite of specialized physical traits. These fish possess a protractile upper jaw, very small mouths, and a body often protected by dermal plates. A particularly striking feature is the development of paired pelvic plates that emerge from a membranous outgrowth of the pelvic girdle. Furthermore, their skeletal structure is unique; they lack a postcleithrum, and their pectoral radial plate is fused into a single unit. Perhaps most fascinating is their reproductive behavior: males of at least some species synthesize an adhesive chemical in their kidneys, which they use to bind plant material into nests.

Despite these clear physical characteristics, the classification of Gasterosteoidei is notoriously fluid. In the 5th edition of Fishes of the World, they are placed within the order Scorpaeniformes, while other phylogenetic models suggest they belong to the infraorder Gasterosteales or act as a sister clade to the Zoarcales. The inclusion of certain families, such as Indostomidae, is also contested; some researchers argue that including them makes the entire group paraphyletic. Even the placement of specific genera like Aulichthys remains unsettled, with some catalogs assigning them to Hypoptychidae and others to Aulorhynchidae.

The evolutionary origins of these fish are equally complex. Fossils of Gasterosteus and Aulichthys dating to the Middle Miocene indicate that the group had already radiated into its component families by that time. Some scientists propose that the Gasterosteoidei are closely related to the Zoarcoidei, suggesting that the latter may have evolved from a common ancestor but lost their characteristic bone armor over time. These fish are primarily found in temperate and Arctic regions of the northern hemisphere. While Gasterosteidae thrive in fresh, brackish, and salt water, the other families are strictly marine, with the sea stickleback representing the largest species at a maximum length of 22 cm.

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