Fish and amphibians have necks after all, new research suggests

Fish and amphibians do have a form of neck, new research suggests, challenging one of the longest-standing divisions in animal anatomy.

After filming trout and salamanders feeding, scientists found that the animals bend parts of their backbone to move their heads independently.

The discovery calls into question a principle taught in biology for generations: that only four-limbed animals, including humans, possess a true neck.

It also bears on a much older question about how the first creatures emerged from water and began living on land.

What counts as a neck?

In textbooks, the neck is the series of bones between the skull and shoulders that lets the head turn and tilt without moving the rest of the body.

That definition works well for mammals, birds and reptiles. It has never suited fish, which have long been classified as having no neck whatsoever.

Roxana Taszus, a postdoctoral research associate at the University of Liverpool, suspects that this clear-cut distinction has obscured something.

As she and her colleagues reviewed decades of anatomical research, they repeatedly encountered the same problem: studies did not define the neck consistently across different animal groups.

The inconsistency matters. With no common definition, comparing necks throughout the animal kingdom is almost impossible, making their evolutionary history much harder to reconstruct.

Rethinking the vertebrate neck

Taszus and her team set out to create a definition of the vertebrate neck that applies to any animal with a backbone, whether it swims or walks. Their approach considers both form and movement.

Dr Ariel Camp, a University of Liverpool biologist who researches animal movement, described the idea in an email to Earth.com. “We think ‘neck’ should mean the bones of the spine that allow the head to move independently of the body.”

By focusing on function rather than a fixed group of bones, the researchers can compare necks that look entirely different.

Reading the backbone

To observe how fish use their spines, the team recorded X-ray video of the animals lunging at food. They then reconstructed the moving skeletons as three-dimensional computer models.

Examining the footage frame by frame revealed where the bones bent, how much they bent and where movement began and ended along the vertebral column.

The technique had already produced a surprise. In a 2021 study led by Camp, footage of feeding fish showed that they raised their heads in an unexpected manner.

Rather than pivoting solely at the joints immediately behind its skull, a rainbow trout flexes up to two-thirds of its spine to lift its head towards food. Frogfish do something similar, moving large sections of their backbones to direct their heads.

Fish necks move in unexpected ways

“I was really surprised by how complex these neck motions can be in fish,” Camp told Earth.com when asked what had surprised her most.

As a frogfish raises its head, its spine coils before uncurling. A trout bends its spine upwards to lift its head, while also flexing from side to side to swim. That distinction between form and movement ran throughout the project.

The researchers distinguished the morphological neck, meaning the vertebrae close to the head, from the functional neck: the bones that actually move the head.

Almost all mammals have seven neck bones, but fish follow no single pattern. Some species have no vertebrae behind the head at all; in others, those vertebrae fuse into one rigid block or develop unusual specialised forms.

The idea that fish might have necks is not new. In the 1940s, anatomists observed that the vertebrae immediately behind a fish’s head can make up a distinct region, separate from the rest of the spine.

What has changed is the ability to demonstrate this in a living animal. “Only recently, with tools like X-ray video, have we been able to show the functional neck of fish and demonstrate their ability to move the head independently from the body,” Taszus explained.

The neck’s ancient history

The wider significance reaches back hundreds of millions of years. A movable head was among the features available to the first animals that pulled themselves onto land.

The oldest neck identified in the fossil record belonged to Tiktaalik roseae, a flat-headed predatory fish that lived about 375 million years ago.

In most fish, bony plates connect the skull to the shoulders, holding the head in place. Tiktaalik no longer had those plates.

A separate study of its skull suggested that their loss allowed the head to swing freely, a change linked to hunting in shallow water and, later, on land.

Those shallow-water pioneers stood at the origin of the tetrapods, the four-limbed lineage from which every land vertebrate, including humans, descends.

Their loosened, mobile heads were an early form of the same ability now being revealed in living fish.

Salamanders set the scene

That evolutionary connection is why the team has now turned its attention to salamanders. These amphibians are the closest living stand-ins for the earliest four-limbed animals, which first divided their lives between water and land.

The researchers have only just started filming salamanders feeding underwater and on dry land, mapping how each animal coordinates its head and spine in both environments. Biologists already agree that salamanders have some kind of neck.

What remains unclear is precisely which vertebrae perform that role and how those bones behave when a salamander lunges at prey. Finding out would do much more than resolve a dispute over terminology.

Onto dry land

A neck able to lift and aim the head allowed early land animals to feed and look around while keeping their bodies braced against the ground. Identifying where that ability began helps fill in a chapter of our own journey out of water.

The broader goal is one consistent measure of the neck that works across every branch of the vertebrate family tree.

“We think this new, broader definition of the neck will be a helpful tool that lets the community better compare and combine these studies,” Camp said in response to questions from Earth.com.

Long classified as neckless, fish are proving both stranger and more familiar than textbooks suggested. A disagreement over a definition is developing into a clearer account of how heads first came to move independently.

The research was presented at the Annual Conference of the Society for Experimental Biology, held in Florence.