Emily Stangel, Collections Assistant and Dan Distel, Director

May 29, 2026

Antarctic icefish are known to be the only vertebrates on Earth to have lost functional red blood cells during their evolution … until now.  

H. William Detrich, long-term Ocean Genome Legacy (OGL) Center collaborator and Marine and Environmental Sciences professor at Northeastern University, recently discovered another fish species that lacks red blood cells: the Asian noodlefish.  

With this fantastic discovery, though, one caveat emerged: Antarctic icefish and Asian noodlefish are completely unrelated, so how did both species independently develop this unusual trait?  

How fish lose their red blood cells  

Most fish rely on red blood cells to efficiently transport oxygen from their gills to other tissues via a protein called hemoglobin—the same protein that makes human blood red. Because water contains much less oxygen than air, fish must work harder and smarter to get the oxygen they need to swim and survive. Some develop enlarged hearts and greater blood volume to pump more oxygen-carrying blood through their bodies, while others absorb oxygen directly through their skin. Either way, their circulatory systems need to be effective and efficient at extracting oxygen from the water.   

So why don’t Antarctic icefish have red blood cells, and how do they survive without them?  

The cold-water advantage 

Years ago, Detrich and his lab conducted a genomic analysis of Antarctic icefish and found that these fish had lost the ability to produce red blood cells because their hemoglobin genes were deleted from their genomes millions of years ago. However, a unique feature of their environment—cold water—allowed them to survive. 

Cold water holds more oxygen! The perpetually cold waters of the Southern Ocean contain enough dissolved oxygen for Antarctic icefish to survive, even without red blood cells or hemoglobin. Detrich calls this the cold-water advantage. 

Detrich’s cold-water advantage theory explains both why Antarctic icefish’s hemoglobin genes have disappeared and how they remain equipped to survive without red blood cells.

But what about the Asian noodlefish?  

Two completely different fish, one remarkable convergence  

Antarctic icefish and Asian noodlefish inhabit vastly different worlds. Asian noodlefish inhabit warmer coastal and freshwater systems across East Asia—far from the frigid Southern Ocean—making it all the more surprising that both independently lost their red blood cells.  

To investigate, Detrich and collaborators at the Qingdao Marine Science and Technology Center analyzed the genomes of all 12 Asian noodlefish species. The results revealed something striking: These fish didn’t just repeat what Antarctic icefish did, but rather, they took an entirely different molecular road to the same destination.  

Unlike Antarctic icefish, Asian noodlefish have lost another gene entirely, the myoglobin gene, and carry multiple mutations in their hemoglobin genes. Myoglobin plays a key role in storing oxygen in muscle cells, and Antarctic icefish still retain it. The Asian noodlefish’s loss of myoglobin points to a distinct evolutionary strategy that remains an open-ended question for researchers to explore.  

What this means for evolution  

These findings offer new insight into how extreme oxygen transport adaptations can independently arise in unrelated species, revealing evolution’s remarkable ability to solve the same biological challenge in different ways. 

Discoveries like this also underscore a broader theme: The ocean’s biodiversity holds answers to biological questions we haven’t thought to ask yet. Preserving and studying biodiversity at the genetic level, which is the work organizations like OGL exist to do, is how those questions get answered. 

To learn more about the Asian noodlefish, click here: Asian noodlefish. Want to help OGL study and protect the ocean’s biodiversity? Support OGL here.