Every time we think we've figured out the limits of where life can survive, something new turns up and shows that life is hardier than we thought. During a recent expedition to the Gulf of California, a team of marine biologists discovered huge schools of fishes living in an environment that's almost completely devoid of oxygen – levels well below the amount previously thought necessary for these animals.

The discovery was made during a series of dives in the Gulf in 2015 by researchers at the Scripps Institution of Oceanography and the Monterey Bay Aquarium Research Institute (MBARI), using a remotely operated vehicle (ROV). Over 1,000 m (3,280 ft) down, this part of the ocean has some of the lowest oxygen concentrations in the world, which should make it deadly to most animals. And yet, the researchers found thriving communities of fishes down there, defying expectations.

In these extreme environments the team identified two species in particular – the lollipop catshark, Cephalurus cephalus, and the cusk eel, Cherublemma emmelas . In fact, they were living in waters with up to 40 times less oxygen than other fish species normally considered low-oxygen.

"I distinctly remember the moment when we went over a ledge with the ROV and saw hundreds of these cusk eels, along with several other species of fish, swimming around," says Natalya Gallo, lead author of a study describing the discovery. "I could hardly believe my eyes. We were in a suboxic habitat, which should exclude fish, but instead there were hundreds of fish. I immediately knew this was something special that challenged our existing understanding of the limits of hypoxia tolerance."

These fish are not only tolerating the low-oxygen environments in the Gulf, but they may actively prefer them. The team found that their numbers were high in the oxygen-poor areas but they weren't found in areas with more oxygen. The researchers suggest that may be a deliberate survival strategy – after all, there's far less competition in waters that would suffocate most other fish.

Exactly how these animals are able to live in these extreme environments is still unknown, but the team says their body types could hold some clues. Both species are fairly small, coming in under 30 cm (1 ft) and have soft bodies, which should help them conserve energy. Their heads and gills, meanwhile, are relatively large, possibly to help them absorb as much of the limited oxygen as they can.

The goal of the study was to examine how marine life might adapt to changing ocean conditions brought on by climate change, which is expected to include warming waters and reduced oxygen levels.

The research was published in the journal Ecology and the fish communities can be seen in the video below.

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