These teensy sea creatures, known as phoronids or horseshoe worms, might look relatively straightforward, but they’re keeping marine biologists up at night. That’s because they are simply the larval stage of an organism. Although we know a bunch of stuff about these babies, we’re pretty clueless about what form they take as adults.

Their name comes from “Phoronis”, another name for the Egyptian goddess Isis. The larvae look like dancing aliens, with vibrant yellow or pink patches of pigment on a translucent body, but they will eventually sink to the sea floor and develop into adults, completing their life cycle. But which larva turns into which adult is a bit of a mystery. 

Adult phoronids are filter-feeding worm-shaped animals that hook onto rocks typically between the shoreline and 400 meters (1,312 feet) below the water's surface. The band of animals these creatures belong to has been described as one of the "smallest and least familiar phyla" known to science.

Writing in the journal Invertebrate Biology, researchers from the Smithsonian Tropical Research Institute (STRI) detail the trials and tribulations of pinning down the nature of these beasts off the coast of Panama. 

Scientists have been tracking phoronids since the 19th century and there’s now up to 15 known species worldwide. However, it’s also been a challenge to pair up the larvae to the adults. As such, we know very little about their evolutionary history, their behavior, or even their role in the ecosystem.

"The global diversity of small, rare marine animals like phoronids is grossly underestimated," Rachel Collin, study co-author and STRI staff scientist, said in a statement. "We don't know what animals are out there, and we know even less about what their role might be in the world's oceans."

This recent project collected over 50 phoronid larvae from the waters around Panama, 23 from the Pacific and 29 from the Atlantic, and carried out some genetic analysis. The team managed to distinguish seven phoronid species, none of which have previously been part of GenBank, a global collection of DNA from more than 300,000 organisms. However, they have no idea what these new species actually turn into. Their adult forms are like ghost organisms.

"Because of the cryptic lifestyles of phoronids, the matching adult worms may never be found, yet the presence of their larval forms in plankton confirm that they are here, established and reproducing," added study co-author Michael J. Boyle.