Gasterosteoidei

Gasterosteoidei is a suborder of ray-finned fishes. This group includes the sticklebacks and several close relatives. These fish are important for studying how species change and evolve. They provide a clear look at how different fish families relate to one another. Scientists study them to understand the history of life in the oceans and rivers. Most of these fish live in the northern hemisphere. They are mostly found in temperate and Arctic regions.

These fishes have very specific physical traits. Their mouths are quite small. They have a protractile upper jaw, which means it can move forward. The premaxilla, a bone in the upper jaw, has a strong upward-pointing process. Many species have bony armor on their bodies. This armor is made of dermal plates. Some plates grow in rows along the sides of the body. Other plates grow from the pelvic girdle to create a protective shell. These unpaired plates grow from the middle radials of the dorsal and anal fins.

There are three main families within Gasterosteoidei. The first is Hypoptychidae, which contains sand-eels such as the genus Aulichthys. The second is Gasterosteidae, the family of sticklebacks. This family includes genera like Gasterosteus, Pungitius, and Spinachia. The third is Aulorhynchidae, which consists of tubesnouts like Aulorhynchus. The placement of the genus Aulichthys can change depending on the study. Some scientists place it in Hypoptychidae, while others put it in Aulorhynchidae. Currently, Eschmeyer's Catalog of Fishes keeps it in the Hypoptychidae family.

Scientists have long debated the exact place of these fish in the tree of life. This is called systematics, the study of how organisms are related. In some classifications, Gasterosteoidei is a suborder within Scorpaeniformes. Other studies treat them as an infraorder called Gasterosteales. They might also be seen as a sister clade to the Zoarcales. Some researchers believe the Indostomidae family belongs in Gasterosteoidei. However, others argue this makes the group paraphyletic, meaning it does not include all descendants of a common ancestor. They suggest Indostomidae belongs in a different group called Indostomoidei.

We can learn about the history of these fish through fossils. Fossils of the genera Gasterosteus and Aulichthys date back to the Middle Miocene. This time period suggests these fish were already diverse by then. Their different families likely branched out before the Middle Miocene began. Some relatives, like the Zoarcoidei, may have lost the bony armor that Gasterosteoidei still possess. This loss is called an apomorphy, a special trait that evolves in a specific group. Studying these fossils helps us map the timeline of fish evolution.

Many of these fishes are quite small in size. The largest species is the sea stickleback, known as Spinachia spinachia. This fish has a maximum published standard length of 80 millimeters. Some members of the group have very unique biological processes. For example, the kidneys of some gasterosteoids produce an adhesive chemical. Male fish use this sticky substance to build nests from plant material. It is not yet known if every single species in the group uses this method. This behavior shows how they use their own biology to create habitats.

Understanding Gasterosteoidei helps us see the bigger picture of marine biology. Their ability to live in different environments is very impressive. The Gasterosteidae family can live in fresh, brackish, and salt water. In contrast, the other two families are strictly marine, meaning they live in the ocean. This shows how different branches of the same group can adapt to different needs. By studying their skeletons and habitats, we learn more about the complex systems of life in the water.