Parasitic snails are sneaking inside sea cucumbers

At first glance, sea cucumbers seem like passive creatures. They sift along the ocean floor, slow-moving and peaceful. Much like sea snails, they tend to keep to themselves.

But unlike sea cucumbers, some species of tiny sea snails aren’t so harmless. These snails aren’t just bottom-feeders – they’re parasites. And for sea cucumbers, that makes them a serious nuisance.

In Japan, where sea cucumbers are prized as seafood, scientists have spent years cataloging the various species of parasites that live on or inside them.

But there’s been a gap in the research: one species, Holothuria leucospilota, has been tough to study. Researchers at Kyoto University decided it was time to change that.

Identifying snails inside a sea cucumber

Holothuria leucospilota defends itself by ejecting sticky threads known as Cuvierian tubules when it’s stressed.

These threads can make dissection nearly impossible. So, to avoid triggering the defense, the team used menthol to gently anesthetize the animals before studying them.

The goal was to identify snails from the genus Melanella that parasitize these sea cucumbers. They did this work in Shirahama, a popular beach area in Wakayama prefecture.

After anesthetizing the sea cucumbers, the scientists examined both the outside and the inside of the animals. They identified the snails by looking at their shell shape and analyzing their mitochondrial DNA.

Four snails, one sea cucumber

The team discovered four species of Melanella parasites. Two lived on the outside of the sea cucumber, and two more were found deep inside the body cavity.

One of the external parasites was identified as Melanella spina – the first time this species has ever been recorded in Japan. The two internal parasites couldn’t be definitively identified yet.

“Our results represent the first record of eulimids exploiting the internal habitat of H. leucospilota,” said study co-author Tomoyuki Nakano.

“To the best of our knowledge, it is also the first record of a single holothurian species, like sea cucumbers, being utilized by both external and internal Melanella parasites.”

How do the snails get in?

The researchers noted that these internal parasites are extremely rare – at least in Shirahama. They found two in 2022, but none in the following two years. That suggests the infection rate is low.

How these tiny snails manage to get inside the sea cucumbers is still unclear, but the team has a few leads.

Both internal snails were found near the host’s mouth. Previous studies suggest that Melanella species often attach around this area. It’s possible that the sea cucumbers accidentally ingest the snails while feeding, along with bits of sediment.

That’s one theory. Another possibility is more direct. One of the snails was found with part of its shell embedded in the sea cucumber’s body wall. That could mean it forced its way in by boring through tissue.

The invasion route is still a mystery – and that’s what the researchers want to investigate next.

Parasitism across the animal kingdom

What the researchers observed in sea cucumbers isn’t unique. Nature is full of strange and sometimes unsettling examples of animals parasitizing others – both in the ocean and on land.

One well-known ocean example is the tongue-eating louse (Cymothoa exigua), a parasite that enters a fish’s mouth, attaches to its tongue, and eventually replaces it. The fish continues to live and eat, even as the louse occupies a central role in its body.

On land, parasitic wasps are notorious for their ruthless tactics. Some species lay their eggs inside caterpillars, and when the larvae hatch, they feed on the host from the inside out. In many cases, the host stays alive until the wasps are fully developed and ready to emerge.

Even in mammals, parasites play a role. Botflies, for instance, lay their eggs on animals like rodents or livestock, and the larvae burrow into the skin to grow.

These examples show that parasitism is a successful – if grisly – survival strategy across the animal kingdom. What makes the Melanella-sea cucumber relationship fascinating is how subtle and understudied it remains.

Scientists are only just beginning to understand how complex these interactions can be, especially when the host makes access so difficult.

The full study was published in the journal ZOOLOGICAL SCIENCE.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–