Why fish can’t evolve both powerful jaws and big teeth

In the watery world of fish evolution, feeding is everything. Success depends on catching food efficiently, whether it’s darting prey or algae stuck to rocks.

A recent study published in the journal PLOS Biology shows that fish face a basic anatomical choice: grow big teeth or evolve mobile jaws – they can’t do both.

Nick Peoples and colleagues at UC Davis used high-speed videography and evolutionary models to investigate this trade-off across 161 ray-finned fish species.

Ray-finned fishes dominate the aquatic world, forming about half of all vertebrate species. Their feeding success hinges on two traits: long teeth that help damage or grip prey, and extendable jaws that produce suction for capturing elusive prey. Yet both features rarely appear in the same species.

Bigger teeth, less mobile jaws

The researchers analyzed videos to track how fish approached prey using three techniques: swimming forward (body ram), extending the jaws (jaw ram), or sucking prey in (suction). Most species (76%) relied on body ram, while suction rarely played a primary role.

The team found that tooth size closely matched feeding strategy. Fish with big teeth nearly always relied on body ram. Those with small teeth used jaw protrusion more effectively.

There was no strong link between tooth size and suction use. The results support a simple pattern: the bigger the teeth, the less mobile the jaws.

Tooth size limits feeding flexibility

To explore the evolutionary basis of this mismatch, the team ran statistical models comparing species with different feeding behaviors.

The results confirmed a tight trade-off: species that use jaw ram evolved to have teeth four times smaller than species that rely on swimming strikes.

The study also showed that as teeth get larger, the diversity of feeding strategies drops sharply. Species with smaller teeth had more flexibility, combining jaw movement and suction in various ways. This flexibility was lost in fish with bigger teeth.

Jaw structure shapes fish evolution

Most ray-finned fish grow replacement teeth inside the jawbone. This requires extra space, especially if the teeth are big. But extendable jaws are slim and fast-moving, with little room for large internal replacements.

The result? Fish with mobile jaws can’t support very large teeth. “This presents a dilemma for species with high jaw protrusion,” noted the researchers.

Slim jaws that allow fast strikes limit space for growing new, large teeth. To make things harder, large teeth could slow a suction strike by creating drag or interfering with water flow. Fish that use suction likely keep their teeth small on purpose.

Fish adapt to keep big teeth

Some species sidestep this problem using special modifications. These include fused teeth in parrotfish, which form a beak, and stacked tooth structures in pufferfish that become dense cutting tools.

Triggerfish attach teeth using soft tissue for more support, while butterflyfish evolved flexible tooth shafts.

These adaptations allow large-toothed species to exploit unique diets – like coral scraping, detritus collection, or even wood eating.

“When we looked into which species had exceptionally large teeth, we were surprised to find that they also have interesting modifications to the dental system, like the beak of parrotfish,” said Peoples.

Feeding success has limits

Fish evolution illustrates a simple but powerful rule: you can’t always have it all. Both big teeth and mobile jaws help fish survive, but trying to evolve both seems to backfire.

“Many feeding innovations have evolved in fishes, and our results show that it is important to consider the interactions between these traits as some may actually be incompatible with each other,” noted Peoples.

“High-speed videography really provides a unique view into fish feeding. IIt often reveals subtle movements and behaviors that you would not notice if only observing the fish in the aquarium.”

Bite hard or strike fast

Fish species that evolved big teeth gave up jaw mobility but gained new dietary options. Species that evolved flexible jaws gained versatility but lost the power to grip or damage prey.

This study highlights how physical constraints can shape evolution. Even in a group as diverse as ray-finned fishes, nature imposes limits.

Those limits may explain why fish face a fork in their evolutionary path: bite hard or strike fast – but never both.

The study is published in the journal PLOS Biology.

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