Why these bizarre, slow-moving fish traded speed for a suit of armor

Nature · Published 3 October 2026 Swipe more like this →

In short: Tetraodontiformes are the oddballs of the ocean, abandoning the typical streamlined fish shape for globes, boxes, and squares. By sacrificing speed, these creatures evolved extreme defensive strategies, from inflatable bodies and toxic skin to fused, beak-like jaws. They are a masterclass in evolutionary adaptation for a life of survival.

The order Tetraodontiformes, or Plectognathi, represents a radical departure from the standard fish anatomy. Comprising roughly 430 species across 10 extant families, these fish are defined by highly modified skeletons that lack common features like lower rib bones or specific skull structures. Instead, their jaws are fused into a beak, with visible sutures that allow scientists to categorize them by the number of teeth-like bones—such as the two-toothed Diodontidae or the four-toothed Tetraodontidae. This specialized mouth, paired with powerful muscles and pharyngeal teeth, is perfectly suited for crushing the hard-shelled crustaceans and shellfish that make up their diet.

Because most members of this order are rigid, ostraciiform swimmers, they cannot rely on body undulation for movement. Instead, they use their fins for precise, slow-motion maneuvering. To compensate for this lack of speed, they have developed formidable defenses. Many species are encased in strong plates, spines, or leathery skin. Pufferfishes and porcupinefishes can inflate their bodies by swallowing water to deter predators, while others, like the Tetraodontidae, carry the potent neurotoxin tetrodotoxin in their internal organs. The ocean sunfish, the largest bony fish at up to 3 meters and 2 tonnes, stands out as a pelagic outlier that feeds on jellyfish rather than hard-shelled prey.

The evolutionary history of the group remains a subject of active research. While traditional classifications once pointed to the Plectocretacicoidei from the Late Cretaceous as the oldest members, recent studies have questioned this placement, noting that these fossils appear too ancient for the group's position as a highly derived percomorph lineage. If these early fossils are excluded, the definitive record begins in the earliest Eocene. Today, the order is recognized as a monophyletic group within the Percomorpha, though some authorities argue it should be classified as a clade within the Acanthuriformes, closely related to anglerfishes.

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