Deep-Sea Footage Reveals *Scalicus engyceros* Moonwalking Along the Seafloor

The vast, abyss-like darkness of the deep ocean evokes the mystery and magnificence of outer space.

Eerily bioluminescent animals glow like nebulae; fish-excrement-filled ‘marine snow’ sparkles like cosmic dust; and strange, luminous sharks patrol the depths.

Much as spacecraft, telescopes and rovers allow us to explore space, underwater submersibles can carry us into the ocean’s darkest, coldest regions and expose the animals that inhabit them.

Conventional taxonomic research frequently relied on specimens that had been caught, killed or washed ashore. Modern submersibles, however, enable scientists to watch marine creatures in their natural underwater homes without harming them.

That approach can produce startling discoveries.

Scientists in China have now recorded an unusual shrimp-like fish moving sideways across the seabed like a crab, as well as ‘moonwalking’ in reverse – behaviour never previously documented in any other fish.

A remarkable collection of anatomical features makes this lively movement possible, and the researchers were able to examine them more closely while the fish was active.

Scalicus engyceros and its unusual seabed walk

Scalicus engyceros, an armoured searobin species from the western Pacific Ocean, was first formally described in 1872 and had already been recognised for its distinctive ‘walk’.

New video from the South China Sea shows that the fish uses thick, reinforced fin rays to scuttle like a shrimp. When threatened, it can also make an abrupt, jerky leap.

"These walking fish harbor far more novel behavioral and evolutionary adaptations than previously assumed," says Han Tian, a marine scientist at Sun Yat-sen University and the study’s first author.

Writing in the journal Ocean-Land-Atmosphere Research, the team also describes observations of two further armoured searobin species from the family Peristediidae. They were seen at depths of between 350 and 500 metres using the human-occupied vehicle Shenhaiyongshi, meaning deep-sea warrior, and the remotely operated vehicle Haiqin.

"By observing deep-sea animals alive in their surrounding environment, we can gain insight into the adaptation and evolution of the species," Tian says.

Current taxonomy indicates that the three peristediid fishes descend from a shared ancestor that lived roughly 60 to 70 million years ago. Over time, they developed striking adaptations for coping with their harsh environment.

At the depths where the searobins were found, several hundred metres beneath the surface, the equivalent of about 40 atmospheres of water pressure bears down on their shoulders – and fish do, in a sense, have shoulders.

Their bony body plates are the reason for the ‘armoured’ description.

They also possess branched barbels on their snouts, resembling a garden rake. Working rather like whiskers, these structures let the fish detect food and obstructions physically and chemically as they move across the seafloor.

The researchers watched S. engyceros drag these rake-like structures over the seabed during its walk, "occasionally touching sediment bumps or rocks".

While this may assist the fish in ‘sniffing’ out prey and negotiating obstacles, the researchers say it "seems awkward because it appears inconvenient for walking".

To support its movement, the armoured searobin has two pectoral fins that extend sideways, seemingly helping it maintain balance.

Deep-sea senses and armoured searobin adaptations

Like many other animals of the deep, S. engyceros has notably large eyes, although they may offer limited benefit. After extended evolutionary exposure to darkness, its vision could be declining.

When the scientists drew near to S. engyceros in their submersibles, the fish appeared neither to hear nor to see their arrival.

One of its eyes did, however, turn towards the vehicle’s beam of light before the fish swam off, perhaps in an effort to avoid small talk.

"This raises the question of whether this species has not resided in this dark habitat long enough for complete degradation of light sensing," the researchers ponder in their paper, "or whether it may still move upward at times, perhaps into the twilight zone."

The team also spotted six individuals of Peristedion liorhynchus in an underwater canyon, rather than on the seamount where S. engyceros was observed.

What the Scalicus engyceros footage reveals

The energetic walking skills of S. engyceros clearly distinguish it from the roughly dozen other fishes that have developed skeletal modifications for walking on land. Those species include the charming amphibious mudskipper and the alien-looking, eyeless cave angel fish.

The backwards-shuffling, side-stepping S. engyceros may occupy a category of its own. Yet it is impossible to know what other creatures may be crossing the seabed in places that humans have never remotely managed to observe.

Fortunately, ever more flexible submersible technologies should enable many further spectacular discoveries in the deep sea.

Whether it involves the spine-tingling unease of underwater ‘backrooms’ or golden spheres deposited by tentacled, virgin-birthing ‘spaghetti monsters’, the oceanographic Bingo card appears to have no limits.

"Ultimately, this research will reshape scientific perceptions of deep-sea biodiversity and reveal how extreme marine environments drive the emergence of one-of-a-kind biological structures and behaviors unseen in any other aquatic organism on Earth," Tian concludes.

This research was published in Ocean-Land-Atmosphere Research.

This article was fact-checked and edited by Clare Watson. Although we take pride in our process, we are only human. If you spot an error, please let us know.