A rare species of deep-ocean barreleye fish with a transparent head has been filmed by researchers off the coast of California for the first time in 4k high definition, offering a never-before-seen glimpse into the fascinating life of life in the depths.

This is only the eleventh sighting of the Macropinna microstoma to be captured by the team at the Monterey Bay Aquarium Research Institute (MBARI) in its 40-year history of deep-sea exploration. Footage of the species was captured during a recent expedition aboard the research vessel, David Packard.

Researchers from MBARI’s Biodiversity and Biooptics Team were filming and collecting animals around 90 kilometres offshore using the ROV Doc Ricketts when they came across the fish at a depth of around 600 metres.

“Seeing the barreleye fish is a rare treat. Crew from across the ship joined the science team in the control room, mesmerised by this remarkable animal. Deep-sea animals capture the imagination, creating wonder and awe that we hope will inspire stewardship of the largest living space on Earth,” said senior scientist, Steven Haddock.

The encounter came at the close of the eight-day Diving Deep for Delicate Drifters expedition, during which Haddock and his team set out to study gelatinous animals including jellies, siphonophores and ctenophores. By combining blue-water scuba diving near the surface with ROV surveys hundreds of metres down, the expedition supported ongoing work to understand how these animals have adapted to life in the deep water column.

Science operations had wound up for the day, and submersible pilots were “flying” Doc Ricketts back to the surface for recovery when Research Specialist Shannon Johnson spotted the barreleye and called for them to stop. 

The team spent around 20 minutes observing the fish, using the ROV’s high-resolution 4K camera to examine its body and behaviour. They experimented with different lighting conditions to show off its transparent head shield. ROV Pilot/Technician Cameron Law also manoeuvred Doc Ricketts close enough to film the animal with the vehicle’s VR camera, capturing 3D stereo imagery that could be used in future education and outreach work.

This is the first MBARI barreleye sighting to be filmed in 4K, and the first from the institute’s new flagship research vessel. The last sighting was in 2021.

Barreleye fish live in the ocean’s twilight zone, where they scan the waters above for the silhouettes of prey. Large fins provide stability and manoeuvrability as the fish drifts in place, its eyes fixed upwards. When it spots a potential meal, its large tubular eyes rotate beneath a dome of transparent tissue to keep the target in sight. The gelatinous shield also protects the eyes when the fish plucks small crustaceans caught in the stinging tentacles of deep-sea siphonophores.

Earlier MBARI observations by Senior Scientist Bruce Robison and Research Specialist Kim Reisenbichler were instrumental in revealing this unusual feeding behaviour.

The Biodiversity and Biooptics Team studies how marine animals interact with light and has carried out extensive research on the visual systems of deep-sea species. Although the researchers did not collect this barreleye, the footage will be a vital resource for continued study of the elusive fish. A close-up look at its eyes will help the team better understand vision in the dim depths of the twilight zone.

The pigments in the lenses of the fish’s eyes are thought to serve a similar function to the green pigments in the eyes of the strawberry squid (Histioteuthis heteropsis), one of the barreleye’s deep-sea neighbours, which the team has studied previously.

“Macropinna reminds us of the surprising ways that animals have adapted to thrive in the ocean’s midwater,” said Haddock. “So many of them survive by looking for subtle optical cues, like a shadow passing overhead, and this sets off evolutionary competition, pitting bioluminescent camouflage against increasingly sophisticated visual capabilities.”

Earlier this summer, MBARI researchers leading an expedition with Schmidt Ocean Institute in the mid-Atlantic Ocean recorded footage of another species of deep-sea barreleye, the binocular fish (Winteria telescopa).

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