Stunned Japanese Scientists Find That Eels Have Two Types of Sperm, and They’re Not Sure Why

Growing eels might not be as simple as we thought.

Japanese eels are born far out at sea. Their larvae drift toward the coasts of East Asia, where the young eels enter rivers, estuaries, and coastal waters. Years later, as autumn and winter arrive, the adults begin the journey back to the ocean to reproduce.

That last part is still full of mystery. Somewhere offshore, their gonads mature and they spawn. But for scientists and eel farmers, the details matter enormously.

In Japan, eel, or unagi, has long been a prized food. Yet Japanese eels remain notoriously difficult to breed in captivity. Farmers can raise young eels, but they still depend heavily on wild-caught “glass” eels. That puts pressure on wild populations, and the Japanese eel is listed as Endangered by the IUCN.

Now, researchers in China have found a strange new piece of the puzzle: male Japanese eels can produce two structurally different types of sperm.

One type looks more familiar. It has a rounded head and a tail, much like sperm seen in many other fish. The other type has a longer, curved, eyebrow-shaped head and a tail built differently on the inside.

So, the obvious question is: why?

Operation Eel Sperm

The researchers studied 20 wild male Japanese eels captured in the Pearl River Estuary in China. First, they acclimated the fish to saltwater. Then, once a week, they injected the males with carp pituitary extract and human chorionic gonadotropin to stimulate sperm production.

Once the eels matured, the researchers collected milt (the sperm-containing fluid) by applying pressure to the abdomen. All 20 males released sperm, and milt from 10 fish was collected for detailed analysis. They then prepared sperm smears for light microscopy and preserved samples for scanning and transmission electron microscopy.

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Under the microscope, two forms stood out. The first had a round or nearly round nucleus. These sperm were smaller, with nuclei roughly 2.57 micrometers along their long diameter and 2.11 micrometers along the short one. Their flagella, the whip-like tails that move sperm through fluid, were about 37 micrometers long.

The second form had an eyebrow-shaped nucleus. These sperm were much longer in the head, measuring about 7.66 micrometers along the long axis, though their short axis and flagellar length were similar to the round sperm.

But these were only the apparent differences. Electron microscopy let the researchers see the internal architecture of the sperm, not just their outer shapes.

That difference became more striking under electron microscopy.

Electron microscopy showed cross-sections through the sperm, revealing internal structure. It showed that the tail isn’t wired the same way for both types of sperm. The eyebrow-shaped sperm’s tail is missing part of the central support structure. So, the big point is the two sperm types are not just shaped differently on the outside. They also have different inner machinery, which could affect how they move or mature.

So… What Does This Mean?

Believe it or not, eels aren’t the only species that have two types of sperm. This is called sperm heteromorphism, and it pops up in some groups. Almost all butterflies and moths have it, as do fruit flies and some gastropods.

But this is extremely rare in vertebrates. It’s speculated that some fish do it, but this study puts Japanese eels in a very select club.

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In sperm heteromorphism, one type is usually the “fertile” sperm that can fertilize eggs. The other type may be infertile or have a helper role. In some snails, for example, males make ordinary fertilizing sperm and also larger nonfertile “parasperm” that may help transport or protect the fertile sperm. This could also be used to prevent females from mating again.

This could be the case in eels, though researchers note that the artificial maturation may also play a role.

For now, however, the honest answer is that we don’t know yet. The study discusses possibilities, but stops short of claiming a definitive answer.

This Matters for Aquaculture and Conservation

Japan’s eel industry is a massive, multi-billion dollar sector. The country consumes over 130,000 tonnes of freshwater eel (unagi) annually, which accounts for over 70% of the entire global eel supply. Farmers can grow eels, but the industry still relies heavily on wild-caught juveniles because closing the full life cycle in captivity remains difficult at commercial scale.

That is a serious bottleneck. If researchers can better understand sperm development, sperm quality, and fertilization success, hatcheries may eventually become less dependent on wild glass eels.

The new study does not solve that problem. But it adds an important clue. If one sperm type swims better, fertilizes eggs more reliably, or indicates healthier maturation, it could become useful for captive breeding programs.

It may also help explain why artificially induced reproduction in eels remains so tricky. A sperm sample may not be a simple matter of “good” or “bad.” It may contain different cell types with different roles.

Understanding eel reproduction also matters because wild eel populations have declined sharply in many regions. Japanese eel is listed as Endangered, while other freshwater eels, including European eel, are also in serious trouble. The pressures include overharvesting, habitat loss, dams and other river barriers, pollution, climate-related changes, and illegal trade.

Eels already have one of the strangest life cycles of any fish. They cross huge distances, transform their bodies several times, and spawn in remote oceanic waters that scientists are still trying to understand.

Now, it seems they may keep secrets even at the scale of micrometers.

The study was published in the journal Biology.