The deepest species off mainland Australia

06/06/2022 | 2 mins

Researchers at the Minderoo-UWA Deep-Sea Research Centre have uncovered the deepest fish off the Australian mainland, more than six kilometres underwater off the southwest coast. 

 The two new species of snail fish were uncovered during an expedition to the Diamantina Fracture Zone, a valley that was formed when Australia separated from Antarctica more than 50 million years ago.

Deep-Sea Centre Director, Professor Alan Jamieson, led the expedition as part of a five-year program to map the deepest parts of the East Indian Ocean. In March, the Centre sailed to the far south coast to map the bottom of the ocean along the most eastern point of the Diamantina Fracture Zone, where they uncovered the new species. 

“Most of the very deepest fish in the world are snailfish but they can also be found in shallower water,” Professor Jamieson said. “In adapting to survive a low food environment, they don’t have scales and skin and they don’t have a lot of muscle because there’s no need to be fast in the dark depths of the deep sea. Instead, they are made of a gel and that makes them hydrodynamic, so they use less energy to swim through the water. The gel gives them the buoyancy because the pressure is so high at those depths, they don’t have a swim bladder like normal fish.”

The researchers caught one specimens of each species. The first measuring approximately 25 centimetres long and the second 10 centimetres long juvenile, both were living in temperatures below two degrees Celsius.  

The Diamantina Fracture Zone was not the only location on the deep-water itinerary, they also visited the Naturaliste Plateau which is a large feature that rises from the fracture zone to 2000 metres deep.

“The vast majority of Australia is very, very deep water,” Professor Jamieson said. “I don’t’ think people appreciate how deep it drops off at the edge of the Continental Shelf.  Coral reefs have been very well researched, but in comparison, very little has been looked at below 500 metres. Much of it is within marine parks that are providing conservation on a large scale, yet no one has ever seen the bottom.”  

The Diamantina Fracture Zone expedition deployed three full ocean depth lander vehicles equipped with baited imaging systems, collection devices and environmental monitoring sensors that can operate anywhere in the world and are rated to 11 kilometres underwater.

The landers freefall off the research vessel for 2.5 hours to reach 6000 metres. Once on the bottom, the bait attracts fish and other species in, which is captured on video for up to eight hours. The landers also measure oxygen, current speeds, direction, salinity, tides, and carry small traps to enable specimens to be brought to the surface.

In addition to the deep snailfish, the team also film several other rare species from the fracture zone and plateau. For example, Enypniastes, a sea cucumber often called ‘the headless chicken monster’.  This species is a deep red transparent bottom feeder that rises into the water to relocate.  It was filmed at 3000 metres deep on the Naturalise Plateau in very high abundance.  Also, at 4000 metres, they filmed an unusual siphonophore, which are colonies of animals that appear as one.

Among these rarities, the team captured a significant volume of footage of the deep-water fishes off Australia, namely large cusk eels, Blue Hake and Cut-throat eels.

These data will be combined with previous visits to the East Indian Ocean and many more site further north that are scheduled for the next few years.

Media references

Annelies Gartner (UWA PR and Media Manager) 08 6488 6876

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