May 19th 2026|3 min read

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The Japanese eel is big business. Japan alone consumes over 130,000 tonnes of the fish every year. Unlike such species as trout and salmon, which are hatched and reared in fish farms, eel fry are captured in the wild and raised in ponds before going to market. This is done because artificial insemination is required to breed these eels in captivity and the associated fertility rates are poor. New work led by Feng Zhao at the East China Sea Fisheries Research Institute, published in MDPI Biology, has revealed one possible reason artificial insemination may not be working well: Japanese eels have two types of sperm.

There is much that is unusual about Japanese eels. They spend spring and summer in Asian rivers, but begin migrating into the Pacific Ocean during the autumn. They then swim into the Mariana Trench, the deepest point on the planet, in order to breed and lay their eggs. The eggs hatch in the bitter cold of the Stygian trench and the fry then swim back to the rivers that their parents came from. It is along this portion of the journey that they are caught and transferred to ponds.

The capture and farming of fry is decimating the wild population. When the Japanese eel was classified as endangered in 2014 its numbers were thought to have collapsed by 90% over the preceding three decades. The decline has since continued. It was the threat of population collapse, along with the hope of breeding eels more cheaply in captivity, that led Dr Zhao to study eel sperm.

Working with colleagues, Dr Zhao manually injected hormones into 20 male eels that had been captured in China’s Pearl River estuary. The hormones drove the eels to sexual maturity, making it possible for the team to extract sperm by applying pressure to their abdomens. When they put these samples under the microscope, they were astonished to discover two types of sperm cell. One type, which has been seen before, was shaped like an eyebrow. The other was round.

Few animals have two types of sperm. Most that do are insects that engage in sperm competition, in which sperm from several males compete for access to the eggs inside one female. Under these circumstances one sperm type is usually the “fertiliser” that actively swims towards the eggs while the other either impedes or draws the attention of opposing sperm.

Whether the round sperm that Dr Zhao has discovered is a decoy or something more mysterious remains to be determined with fertility experiments. If it plays no active role, ditching it in favour of the eyebrow-shaped fertilisers during insemination ought to make breeding easier.

What would make it easier still would be getting the eels to sexually mature on their own. Such work is under way. One of the challenges of replicating this process artificially is that eels encounter major environmental changes during migration that are thought to drive them to mature. Dr Zhao is developing indoor systems that use salinity, temperature and lighting to replicate trench conditions. He describes his early results as encouraging, with the eels in his lab having engaged in courtship and breeding behaviours. Dr Zhao cautions, however, that proper experimental verification has yet to be done. ■