Scientists Solve Mystery of Golden Orb Shed by Relicanthus daphneae Sea Anemone

In 2023, scientists piloting a remote vehicle through waters so deep that sunlight cannot penetrate made a puzzling discovery on the ocean floor.

Firmly attached to a rock was a spherical clump of golden material. Under the powerful lamps of the ROV Deep Discoverer, it glinted as though it might be something nobody had encountered before.

Early theories suggested the enigmatic object could be the discarded egg case of a creature that lives in the deep. Three years later, scientists have finally identified it – but not as they first expected.

It remains decidedly strange: the lustrous lump of tissue was a piece of ‘skin’ shed by an impressive sea anemone, perhaps left behind when the animal relocated or attempted to reproduce.

Detailed analysis also identified the species as Relicanthus daphneae, a deep-sea cnidarian whose tentacles can exceed 2 metres in length.

Golden orb found on the Gulf of Alaska seafloor

The original discovery both perplexed and excited the scientists. The blob was roughly 10 centimetres across, with an opening on one side, and was fixed to a rock on the floor of the Gulf of Alaska, about 3,250 metres beneath the ocean surface.

At that depth, the sea is intensely cold and completely dark, while the surrounding pressure is immense – all major obstacles to human exploration.

Researchers aboard NOAA’s Okeanos Explorer spotted the mass while watching a live video stream from the remotely operated vehicle Deep Discoverer.

"I don't know what to make of that," said one researcher during a livestream of the 2023 expedition.

"It's definitely got a big old hole in it, so something either tried to get in or tried to get out," another speculated.

"I just hope when we poke it, something doesn't decide to come out," one researcher said. "It's like the beginning of a horror movie."

Using the ROV’s robotic arm, the team cautiously retrieved the sample and sent it to a laboratory for analysis. They expected it to prove to be either an egg case, a dead sponge or coral. Instead, the puzzle became more complicated.

"We work on hundreds of different samples and I suspected that our routine processes would clarify the mystery," explains zoologist Allen Collins of NOAA Fisheries' National Systematics Laboratory.

"But this turned into a special case that required focused efforts and expertise of several different individuals. This was a complex mystery that required morphological, genetic, deep-sea, and bioinformatics expertise to solve."

Relicanthus daphneae cuticle identified through deep sequencing

The team established that the sample lacked the anatomy normally expected of an animal.

Rather, it was fibrous and densely filled with stinging cells known as cnidocytes, which are commonly found in corals and anemones. The cnidocytes identified within the blob were spirocysts, found exclusively in cnidarians of the Hexacorallia class.

At that stage, however, the researchers hit a problem.

Initial DNA testing produced no clear result because the blob contained numerous other microscopic organisms. Only after sequencing its complete deep genome did the scientists find a close match: R. daphneae, a species first described in 2006.

The researchers concluded that the blob was a cuticle shed by one of these anemones. A cuticle is a thin, multi-layered covering secreted by the outer tissues of certain anemones. It forms pliable, sheet-like structures that may detach and remain on the seabed.

It seems to consist chiefly of chitin, the durable fibrous substance that forms the hard structures of organisms including beetle cases and fungal cell walls.

"Observations of animals in situ suggest that cuticle is left behind as the animal moves, suggesting that the animal can detach from it," the researchers write.

Samples of R. daphneae collected by scientists seldom retain a cuticle, and its capacity to move away while leaving this layer behind could account for that. The discarded cuticle may also offer insight into the animal’s reproduction, which is hard to study in species inhabiting such an inaccessible environment.

"Although genetic and morphological data confirm the identification of the taxon in question, explanation of the golden orb morphology remains a vexing issue," the researchers write.

"One possible interpretation is that the orb is a remnant of incomplete asexual reproduction. Some sea anemones are capable of pedal laceration, whereby the base of the polyp is abandoned, and the upper portion of the animal moves away, leaving a stump of the body that then regrows a new polyp."

It is not yet known whether R. daphneae reproduces this way. Yet even if the object represents incomplete reproduction, the process would still be suited to survival in the hostile deep sea.

Shed anemone skin may support microbial activity

The vast number of microorganisms detected on the cuticle indicates that it could function as a tiny hotspot of microbial activity. There, microbes consume and decompose the decaying tissue, contributing to a key stage of the nitrogen cycle.

In short, an anemone shed its ‘skin’ and provided microbes with a free meal.

"This is why we keep exploring – to unlock the secrets of the deep and better understand how the ocean and its resources can drive economic growth, strengthen our national security, and sustain our planet," acting director of NOAA Ocean Exploration Captain William Mowitt says.

A preprint describing the team’s findings is available on bioRxiv.