Mysterious Alien-Looking Larva Identified as a Probable Parasite

While many of us are excited about the prospect of finding life on other planets, there's still also so much about life on Earth we have no idea about.

Like, what the heck these floating xenomorph-ish babies become when they grow up.

Scientists have known about microscopic facetotectans since the 1880s, yet still haven't managed to pin the larva on an adult. This has led to speculation that, like their much bigger and fictional Alien look-alikes, facetotectans may have a parasitic phase.

A new study now backs this up.

Related: Adorable Baby Crab Stuns The World With Its Spiky Charm

By collecting over 3,000 of these tiny babies, riding the ocean's surface near Japan, Natural History Museum of Denmark genomicist Niklas Dreyer and colleagues constructed a genetic family tree based on the crustaceans' RNA protein templates.

Facetotecta's position in the crustacean family tree. (Dreyer et al., Curr. Biol., 2025)

"We were finally able to confirm, in the realm of big data science, that they are, in fact, related to barnacles, but they aren't closely related to any of the other parasitic barnacles," says ecologist James Bernot, from the University of Connecticut.

While the facetotectans are not as closely related to parasitic barnacles as expected, their hook-like equipment and response to growth hormone strongly suggest endoparasitism is their lifestyle – they live inside a living host.

Except, instead of doing this at the baby stage, like xenomorphs, facetotectans live freely as babies and infest yet unknown host species as adults.

From baby crustacean (y-larvae), to y-nauplius, y-cyprid, and ypsigon – the last known worm-like stage. Each is about 100 micrometers long. (Niklas Dreyer)

While we usually think of barnacles as harmless, sedentary li'l lumps that like to hug onto rocks and whales, some species prefer to give their hugs from the inside. These inject themselves into their hosts and grow through them like a fungal network.

"The ones that grow like roots inside of crabs castrate their hosts, so their hosts are no longer able to reproduce," describes Berenot.

"They trick their hosts into thinking that the host is pregnant, so it starts taking care of this mass that grows outside of its body, but that mass is part of the barnacle and not actually the eggs of the host, and even if they infect a male crab, the male crab becomes feminized and starts behaving like a pregnant female crab."

Previous research revealed that when facetotectans are exposed to crustacean moulting hormone, they emerge from their exoskeletons as unprotected worm-like creatures (image above), just like those crab-parasitizing barnacles.

The researchers believe this is a case of convergent evolution, where evolutionary selection pressures create the same physical or behavioral features across species that are not direct descendants.

"Our data support the widely recognized scenario that parasitism arose independently multiple times in Thecostraca from free-living ancestors," Dreyer and team conclude.

This research was published in Current Biology.