*Scalicus engyceros* Is the Deep-Sea Fish That Moonwalks Backwards

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

Eerie bioluminescent animals glow like nebulae, fish-faeces-rich ‘marine snow’ sparkles like cosmic dust, and strange luminous sharks patrol the depths.

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

Conventional taxonomic research frequently relied on animals that had been caught, killed or washed onto shore. Modern submersibles, however, enable scientists to observe marine life in its natural habitat without harming it.

That capability can produce striking discoveries.

Scalicus engyceros and its unusual seafloor walk

Scientists in China have now recorded an odd, shrimp-like fish moving sideways across the seabed like a crab and even ‘moonwalking’ backwards – behaviour never previously seen in any other fish.

A remarkable combination of traits makes this lively movement possible, and the researchers were able to examine those features more closely while the animal was active.

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

New recordings from the South China Sea show that the fish uses its thick, reinforced fin rays to scuttle like a shrimp. When threatened, it can also make a sudden, 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.

In Ocean-Land-Atmosphere Research, the team also describes observations of two other armoured searobin species from the Peristediidae family. These were found 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.

Armoured searobins in an extreme environment

Existing taxonomy indicates that the three peristediid fish share an ancestor that lived roughly 60 to 70 million years ago. Over time, they developed unusual adaptations for surviving in their harsh surroundings.

At the several-hundred-metre depths where the searobins were seen, around 40 atmospheres of water pressure bear down on their bodies – and yes, fish do have something resembling shoulders.

Armoured searobins are sheathed in bony plates, which accounts for their “armoured” name.

Their snouts also carry branching barbels resembling a garden rake. Acting rather like whiskers, these structures let the fish detect food and obstacles both physically and chemically while moving across the ocean floor.

The scientists watched S. engyceros drag these rake-like structures across the seabed as it walked, “occasionally touching sediment bumps or rocks”.

While the appendages may assist with locating food and avoiding obstacles, the researchers say this “seems awkward because it appears inconvenient for walking”.

For support, armoured searobins possess two pectoral fins that extend out to the sides and seemingly help them balance.

Eyes, movement and future deep-sea discoveries

Like many deep-sea animals, S. engyceros has notably large eyes, although they may be of limited use. After such a long period in darkness, its vision could be deteriorating.

When the submersibles approached S. engyceros, the fish appeared not to hear or see them arriving.

One of its eyes did turn towards the vehicle’s beam of light before the fish swam off, perhaps 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 researchers also encountered six individuals of Peristedion liorhynchus in an underwater canyon, rather than the seamount habitat where S. engyceros was found.

The agile walking ability of S. engyceros clearly distinguishes it from the dozen or so other fish that have developed skeletal adaptations for walking on land, including the endearing amphibious mudskipper and the alien-looking, eyeless cave angel fish.

The backwards-shuffling, side-stepping S. engyceros may be unique, but there is no telling what else might be crossing the seafloor in locations humans have never come close to observing.

Thankfully, ever more versatile submersible technologies promise to deliver many more spectacular discoveries from the deep sea.

From the skin-crawling strangeness of underwater ‘backrooms’ to golden spheres deposited by tentacled, virgin-birthing ‘spaghetti monsters’, the oceanographic Bingo card appears limitless.

"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 by Clare Watson and edited by Clare Watson. Although we take pride in our process, we are only human. If you notice an error, please let us know.