Two entirely separate lineages of fish have independently lost functional red blood cells, providing evidence that white blood cells evolved on more than one occasion across vertebrate evolution. These research findings were detailed in the journal Current Biology.
The discovered data forces a re-evaluation of how extreme oxygen transport capabilities might arise, showing that identical outcomes do not necessarily stem from a shared genetic heritage.
This understanding was reinforced upon recognizing that the slender species of labyrinth fish exhibited the identical blood condition—lacking red cells—previously attributed solely to Antarctic icefish.
The basis for this parallel was established when H. William Detrich of Northeastern University documented that the bloodlessness in icefish resulted from the deletion of oxygen-carrying genes. He then collaborated with Chinese colleagues to determine if a parallel situation existed elsewhere.
Genome-wide comparisons demonstrated that the labyrinth fish achieved bloodlessness through a distinct mechanism of genetic disruption, rather than by retracing the icefish’s steps.
Taken together, the results establish a common physiological end-point shaped by divergent evolutionary trajectories. This opens the door for deeper comparisons of how each lineage managed survival without red blood cells.
The majority of fish rely on red blood cells because these cells accumulate oxygen in small spaces and rapidly distribute it throughout the body. Within each cell, hemoglobin, the protein that binds and releases oxygen, captures it at the gills and transports it to body tissues.
Muscles also utilize myoglobin—a protein that holds oxygen within cells, enabling swimmers to sustain activity when oxygen demand spikes.
When genes for these pigments are inactivated, the fish must depend on dissolved oxygen and restructure their circulation, which narrows the environments where the animals can thrive normally.
In icefish, entire blocks of hemoglobin genes were eliminated, causing the animals to cease red blood cell production over millions of years. The frigid waters of the Southern Ocean contain more dissolved oxygen, reducing the pressure required to pack oxygen inside cells.
However, this cold-water advantage could not account for how the labyrinth fish survived without red blood cells in habitats far removed from polar seas.
Researchers sequenced the genomes of 11 labyrinth fish species, tracing the impairments affecting oxygen genes within their populations, and identified a pattern that did not align with the icefish findings.
Across all 12 species examined, the team found the absence of the myoglobin gene, suggesting a single, early loss of this gene in their common ancestor.
Instead of shedding hemoglobin genes, each line of labyrinth fish carried smaller mutations that hampered the production of functional proteins within their red blood cells.
These mismatched disruptions led to the same result—white blood cells—but left distinct genetic footprints in the genomes of labyrinth fish versus icefish.
Asian labyrinth fish have extremely short lifespans, typically only one year, a brevity that causes them to exhibit a form of neoteny—a retention of juvenile characteristics into adulthood.
Adults reproduce near the end of their brief year but remain slender and translucent, their blood never turning red.
Studies on fish larvae revealed that the skin can supply much of the organism’s oxygen in early development, before the gills take on the primary load.
By maintaining juvenile biological functions into maturity, the labyrinth fish rendered red blood cells unnecessary, allowing damaged oxygen genes to persist without causing mortality.
As blood oxygen content dropped, both fish groups managed this by increasing the volume of fluid pumped and expanding their network of small vessels.
A new analysis pointed to angiogenesis—the growth of new blood vessels—and heart development genes as structures that changed under intense selection.
Earlier studies on Antarctic icefish had already detailed enlarged hearts and greater blood volumes, adaptations that helped deliver sufficient oxygen to tissues.
These enhancements demand energy and space, which partly explains why hemoglobin-lacking fish remain rare and their populations are strictly localized.
Biologists designate this as convergent evolution—the independent emergence of similar traits, as the two lines did not inherit bloodlessness concurrently.
The paper also employed a historical contingency approach—based on how random events influence subsequent development—to rationalize why the genetic disruptions look so disparate.
One potential trigger involved a transposon, a genetic sequence capable of moving and disrupting gene function in a single step.
Once these mishaps occurred, natural selection could only act upon what remained, favoring organisms that still met their fundamental oxygen needs.
The Asian labyrinth fish’s range spans from Eastern Russia to Vietnam, traversing China, Korea, and Japan in warmer coastal and riverine waters.
“They don’t have the advantage of a cold, oxygen-rich environment like the Southern Ocean,” Detrich noted.
The labyrinth fish remained slender, often only five to 25 centimeters long, a body plan that keeps tissues close to blood vessels. While this structure reduces the distance oxygen must travel, warmer environments can inherently have less oxygen, adding extra stress.
The discovery regarding the labyrinth fish suggests that researchers may have overlooked other vertebrates quietly jettisoning oxygen-carrying proteins when conditions allow for survival.
Teams can now scan fish genomes for disrupted oxygen transport genes and subsequently test how heart function, blood volume, and behavior are affected.
“It turns out there are a lot more species out there than we ever thought that don’t use red blood cells for oxygen transport,” stated Detrich.
Each new instance would refine what biologists can predict based purely on genetics versus what remains tied to organismal history.
These two fish groups reached the same endpoint—white blood—yet their genetics and life histories revealed divergent pathways to attain it.
Field observations coupled with laboratory analysis will be needed by researchers to determine which compensatory mechanisms proved most effective, and which fell short.
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