Wormception: How one parasite lives inside its cousin

Study: Jimi, Naoto, et al. “Alien worm in worm: a new genus of endoparasitic polychaete (Phyllodocidae, Annelida) from scale worms (Aphroditidae and Polynoidae, Annelida).” Systematics and Biodiversity (2020): 1-9.

Where do worms live? In the dirt, on sidewalks after a rain storm, and on the bottom of the sea in muddy sediments. A new study has revealed a new species of ocean worm along with its peculiar living arrangements – inside other ocean worms.

One class of ocean worms is called Polychaetes. This name translates from Latin to “many bristles,” referring to the hard protrusions that cover the worms’ bodies and help them get around. This class contains three families: Phyllodocidae, Oenonidae, and Dorvilleidae. Most of these worms live and hunt freely, squirming around the ocean floor and burrowing in the mud, going wherever their fancy takes them. Only a few species are parasites and make themselves at home inside other creatures, living in their organs or body cavities.

The ocean is cold and full of perils like predators, so it makes sense that some worms have evolved to seek refuge inside other marine critters, feeding off their bodies. But only the Oenonidae and Dorvilleidae families have been known to reside inside their relatives, other ocean worms. Members of the third family, Phyllodocidae, have never been spotted mooching off their cousins – up until now.

A new worm on the map

A Japan-based research team set out on a research cruise to explore the seafloor off Japan’s coast, about 700 feet deep underwater. They used a beam trawl – a fishing device that plows through the ocean floor and catches bottom-dwelling critters into a net attached to a steel beam.

Sea mouse, also known as Aphrodita aculeata, is a free-living worm from the ocean floor. Image source: Wikimedia Commons.

In the net, they found two marine worms: a sea mouse and a scale worm. Those two species of Polychaete worms live in the sediment of the ocean floor and hunt for small crabs. But something about those worms didn’t look right. Something was moving underneath their skins.

The scientists sliced their catch open and pulled out the hidden treasure – bonus worms, one from the sea mouse and one from the scale worm. They had been living in the body cavities of the free-living worms as parasites. But what kind of worms were those parasites? The scientists did not know, so they placed the mysterious parasitic worms into ethanol to preserve them. Back in the lab, they examined their findings, confirming that they had never before been known to science. 

To get a better look at the brand-new species of parasitic worms, the researchers placed the worms under a scanning electron microscope, a very powerful microscope that magnifies the image of a specimen  up to two million times and showcasing all the nooks and crannies. This is like looking at a 2.5 mile-long picture of a grain of rice!

A Russian doll, but with worms

The newly discovered worm parasites were flat and white, less than one-tenth of an inch long. Their bodies consisted of 56 segments called chaetigers. Just like other members of the class Polychaete, they possessed hard protrusions all over their bodies, a hundred per segment.

The newly discovered parasitic worms sported two pairs of paired antennae – a signature Phyllodocidae look. Another giveaway feature was an unusually flexible proboscis, a long thin mouthpart, which could be turned inside out.

To get a better idea of the relationship between the host and parasite, the researchers also examined the bodies of the host worms, the sea mouse and the scale worm. These worms’ insides did not have any scars, suggesting their parasites could not have been swallowed as food and then chewed their way into their hosts’ body cavities. Instead, the scientists speculated, the parasites may have crawled into the free-living hosts by biting through their outside tissues, or possibly wormed their way in as tiny worm babies.

The new parasitic ocean worm was discovered in the North Pacific off the coast of Wakayama prefecture, Japan (shown in red on the map). Image source: Wikimedia Commons

There was also something highly unusual about those new parasitic worms. They were huge – almost the length of their hosts. The free-living sea snail and the scale worm had spent a large chunk of their lives carrying around and feeding parasites of their own size.  Imagine how energetically demanding it would be to have another full-sized human living inside of you?

The researchers named this new worm seisui-yadori-sashiba-gokai in Japanese, after the ship TR/V Seisuimaru that had carried them to the discovery. They also chose its name in Latin, which is the standard international language for species names. This new parasitic ocean worm will now be known to the world scientific community as Endovermis seisuiae, immortalizing the trusty research vessel

This new species of ocean worms makes for an exciting discovery because members of its worm family had never before been known to parasite off other ocean worms. But now scientists know that this family has a black sheep in the ocean floor sediment off the coast of Wakayama prefecture, Japan, living in body cavities of its relatives.

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