An expedition to the Bounty Trough off the Otago Coast uncovers a treasure trove of deep-sea creatures - including some species new to science. Veronika Meduna meets slimy fish, snails, and tiny shrimp-like critters from the ocean depths.

The Bounty Trough is one of the world's least explored deep-ocean ecosystems.

Earlier this year, an expedition set off to explore this deep underwater rift valley off the Otago coast as part of the international Ocean Census project, under the umbrella of the United Nations Ocean Decade.

In partnership with Te Papa and NIWA, the voyage aimed to fill gaps in our knowledge of what lives in the deep.

Hundreds of new species discovered

Onboard the research vessel RV Tangaroa were different sets of traps and samplers as well as a deep-tow camera system to allow the scientists to watch what goes on at depths of up to 5,000 metres. The expeditioners returned with hours of video footage and 1,800 specimens. Hundreds of them are new to science - including a slender, slimy bottom-dwelling fish known as an eelpout.

The number of new fish species may be countable on one hand, but Dr Rachael Peart expects to identify several new small crustaceans known as amphipods and isopods. They look a bit like tiny shrimp and dominate life in the deep ocean.

Te Papa mollusc curator Kerry Walton has already identified 78 new species of snails and mussels from the Bounty Trough, including a parasitic snail stuck to a gummy squirrel - a weird sea cucumber with a large sail-like extension.

Te Papa fish curator Andrew Stewart says the museum's collection is limited to shallower depths of about 1,200 metres and mostly commercial species. Being able to explore the Bounty Trough opened up a treasure trove of sightings and catches from the deep.

It also brought home the importance of ocean life.

"This is the world's largest habitat, by a vast margin. It plays a massively important role," he says. "We really need to know what's down there because shifts down there are the canary in the coal mine. We lose those, we're in trouble. Maybe not today, maybe not tomorrow, but we're in trouble."

Why explore the deep sea?

Professor Alex Rogers, the science director of the Ocean Census programme, says there are many reasons why we need to know what lives in the deep.

About half of the oxygen we breathe is produced by tiny plants that live in the ocean. They also kick off the marine food web - from microorganisms to fish - which ultimately feeds millions of people. …

Go to this episode on rnz.co.nz for more details