Tetraodontiformes

The Tetraodontiformes are a diverse order of ray-finned fishes. This group includes well-known animals like pufferfishes and boxfishes. They are famous for having body shapes that look nothing like typical, streamlined fish.

While most species live in the salty ocean, some live in freshwater. There are about 430 known species spread across 10 different families. They are found in waters all around the world. This group is a highly specialized lineage of fish.

Most Tetraodontiformes use a strategy of defense instead of speed. Many species have scales that have evolved into hard plates or sharp spines.

Others, like filefishes and the ocean sunfish, have tough, leathery skin. Pufferfishes have a very unique way to protect themselves. They can inflate their bodies to increase their diameter. They do this by sucking water into a diverticulum, which is a pouch in their stomach.

Some species, including many in the Tetraodontidae family, also use a neurotoxin called tetrodotoxin. This powerful chemical is concentrated in their internal organs to stop predators.

These fishes have highly modified skeletons that differ from most other fish. They lack several bones, such as the nasal, parietal, and infraorbital bones. They also usually lack lower rib bones. Their jaws are modified and fused into a structure that acts like a beak.

This beak is divided by visible lines called sutures that look like teeth. Scientists use these "teeth" to distinguish between different families. For example, the Tetraodontidae are "four-toothed" and the Diodontidae are "two-toothed." Because they eat hard-shelled prey like crustaceans, they use powerful muscles to operate these jaws. Many also have pharyngeal teeth to help process their food.

Body shape and movement vary greatly within this order. Some fish, like boxfishes, are nearly square or triangular. Others are globose, which means they are round like a ball. Filefishes and triggerfishes are laterally compressed, meaning they are flattened from side to side.

Most members are ostraciiform swimmers. This means their bodies are rigid and cannot bend sideways. Because they cannot undulate their bodies, they are relatively slow movers. They rely on their pectoral, dorsal, anal, and caudal fins for propulsion. This allows them to be very precise when maneuvering.

The size range in this order is truly massive. The smallest member is a filefish called Rudarius excelsus, which is only a few centimeters long. In contrast, the ocean sunfish is the largest bony fish in the world.

An ocean sunfish can reach lengths of up to 3 meters. They can also weigh more than 2 tonnes. The ocean sunfish is very different from its relatives. It lacks a swim bladder and spines. It also lacks a caudal peduncle, which is the part of the body before the tail. Instead, its tail is a stiff, rudder-like structure used for steering.

Taxonomy, or the science of naming and grouping, has changed for this order over time. In 1817, the group was proposed as the Plectognathi. Later, in 1940, Lev Berg introduced the name Tetraodontiformes. This name is based on the type species, Tetraodon lineatus, which was named by Linnaeus in 1758. The name comes from Ancient Greek words for "four" and "tooth" and the Latin word for "form."

Recent studies have changed how scientists classify them. While once considered a suborder of Perciformes, they may actually be a sister taxon to the anglerfish order, Lophiiformes.

Fossil evidence helps us understand the history of these fish. The oldest recognized ancestors are the extinct Plectocretacicoidei from the Late Cretaceous period. These were found in the former Tethyan region, including modern-day Italy and Slovenia.

However, some scientists question if these fossils truly belong to this order. They note that these ancient fossils have traits not seen in modern Tetraodontiformes. If these are excluded, the earliest definitive fossils appear in the Eocene epoch. This history shows how much these fish have changed and specialized over millions of years.