What is it about?
A surprising evolutionary workaround allowed codfish eggs to remain buoyant - and helped shape marine ecosystems and human history. Around 60–84 million years ago, the ancestors of modern codfishes (Gadiformes) lost two aquaporin genes, aqp1ab1 and aqp1ab2, that other marine fishes use to hydrate their eggs. This posed a fundamental reproductive challenge: without sufficient water uptake, eggs cannot remain buoyant in the water column, where ocean currents disperse them. The study, published in Proceedings of the National Academy of Sciences (PNAS), reveals how codfishes overcame this apparent evolutionary dead end. They repurposed a different water channel, Aqp4, from an ancestral role in ovulation to facilitate the extensive hydration of the oocytes prior to the release of their marine eggs.
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Why is it important?
Codfish eggs can contain approximately 93–97% water, allowing them to float and survive even in relatively low-salinity waters. This extraordinary hydration depends on a solute gradient generated mainly as yolk proteins break down into free amino acids, together with rapid water movement across the oocyte membrane. The new findings identify Aqp4 as a crucial component of this process in Atlantic cod. In ancestral freshwater fishes, Aqp4 helps mature oocytes swell slightly and be released from their follicles during ovulation. In Atlantic cod, the channel is also present in the oocyte membrane, but its role has been expanded: inhibiting it prevents the massive water uptake during oocyte maturation which is needed later for egg buoyancy. This provides functional evidence of evolutionary gene co-option—an existing biological mechanism acquiring a new, essential role.
Perspectives
The success of gadiform fishes has had consequences far beyond reproduction. These species are important predators and links in marine food webs, and Atlantic cod has supported fisheries, coastal societies and North Atlantic trade for more than a millennium. Dried cod, or stockfish, was particularly important to Norse voyages across the North Atlantic. Without codfishes, the ocean would have lacked its most exploitable, shelf-stable protein. Human exploration of the North Atlantic would have slowed down, the economic foundation of early America would have been hollowed out, global trade routes would have evolved around entirely different resources, and the cod wars that changed maritime law with the establishment of 200 nautical mile Exclusive Economic Zones (EEZ) – would not have occurred. The discovery illustrates how evolutionary innovation can preserve a lineage despite the loss of genes once considered essential. It also provides new insight into the reproductive biology of ecologically and economically important marine fishes as ocean conditions change.
Dr Roderick Nigel Finn
Universitetet i Bergen
Read the Original
This page is a summary of: Codfishes survived essential gene loss by functionally co-opting freshwater ovulatory pathways to marine egg hydration, Proceedings of the National Academy of Sciences, November 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2626713123.
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