Antarctic icefish rewired their skulls to win an evolutionary arms race

(Press-News.org) Antarctica’s Southern Ocean is one of the most demanding places on Earth when it comes to survival. Its waters plunge below freezing, long periods of darkness restrict growth and feeding, and food webs shift with relentless climate swings. Yet one group of fish — the notothenioids, or Antarctic icefish — not only survived here but flourished.

From a single ancestor tens of millions of years ago, they evolved into dozens of species. Some cruise near the surface, others prowl the seafloor, and still others dart through the open water. A new study led by Rice University, published in the Proceedings of the National Academy of Sciences, reveals the secret behind this success: Icefish reorganized their skulls in ways that unlocked new feeding strategies and ecological opportunities.

“Modularity sounds abstract, but the idea is simple,” said Kory Evans, assistant professor of biosciences at Rice and a lead author of the study. “When a body is broken into semi-independent blocks, or modules, those parts can evolve on their own. That gives you more evolutionary degrees of freedom. And in the case of icefishes, it meant they could retune their feeding strategies as Antarctica changed around them.”

Organisms everywhere show modularity: Bird beaks evolve independently from wings, and human limbs can vary without altering other traits. But the icefish story stands out because they didn’t just reshuffle existing modules — they added a new one.

Using micro-CT scans of more than 170 fish species, Evans and his team built 3D maps of eight skull bones across the notothenioid family tree. Their analysis revealed that icefish split their oral jaws into upper and lower modules, effectively giving the skull a new “tool” to work with.

“That’s unusual,” said Mayara P. Neves, a former postdoctoral researcher in Evans’ lab and co-lead author. “Most animals keep their number of modules consistent. Icefishes actually added one.”

The consequences were dramatic. Freed from moving in lockstep, the upper and lower jaws could adapt independently. Some species evolved crushing jaws for bottom-dwelling prey, while others fine-tuned suction feeding to capture fast-moving targets in open water.

“By decoupling the jaws, notothenioids could tweak suction and biting mechanics without redesigning the entire head,” Evans explained.

The evolutionary shifts coincided with some of the Southern Ocean’s biggest environmental upheavals: the onset of the Antarctic Circumpolar Current, pulses of glaciation and swings between frozen and thawed conditions.

“Environmental shocks don’t just test organisms; they can rewire which traits evolve together,” Evans said. “In icefishes, that rewiring seems to have happened inside the skull.”

The team found that during times of climate instability, correlations among bones broke down. This decoupling freed key elements — like the maxilla, essential for suction feeding — to evolve more rapidly.

“The maxilla’s tempo stood out,” Evans said. “Small shape tweaks there can remake how a fish grabs prey.”

The story began more than 30 million years ago with a single ancestor that drifted south from South America. It carried a rare advantage: antifreeze proteins in its blood.

“Imagine dropping all the tropical fishes of Florida into Alaska in December,” Evans said. “Most would die. But one fish had antifreeze in its blood, so it stayed. With no competition, it radiated into all these new forms.”

For Evans and his colleagues, the story of icefish is about more than Antarctic biology — it’s about how life adapts to change. And as climates continue to shift and reshape the poles, this discovery carries a broader lesson: Modularity may be nature’s way of preparing for the unexpected.

“Modularity didn’t just accompany diversification,” Evans said. “It likely enabled it in one of Earth’s toughest environments.”

END