Shallow coral reefs are often known as the rainforests of the oceans as they thrum with life and biodiversity. Most assume that these abundant ecosystems are limited to the sunlight-rich upper waters, but dive a little deeper into the twilight zone and things are not as they seem. Researchers have documented that the deeper waters off the coast of Hawaii are surprisingly abundant, and even contain more endemic species than anywhere else in the ocean.
After two decades of documenting the waters around the archipelago, which stretch for 2,590 kilometers (1,609 miles), the scientists found there was a surprising amount of diversity between the depths of 30 and 150 meters (98 and 492 feet). They discovered vast algae meadows made of more than 70 different microalgae species, as well as extensive areas of 100 percent coral cover. These were supporting massive amounts of fishes and invertebrates in unique communities.
“These are some of the most extensive and densely populated coral reefs in Hawaii,” explains US Fish and Wildlife biologist Anthony Montgomery, who co-authored the study published in PeerJ, in a statement. “It’s amazing to find such rich coral communities down so deep.”
The researchers used a multitude of techniques to survey the twilight zone – known officially as the mesophotic coral zone – including remotely operated cameras, submersibles, towing camera systems behind boats, and even getting specially equipped divers down to the bottom.
“Submersibles can go much deeper and stay much longer, but divers can perform more complex tasks to conduct experiments and collect specimens,” says lead author Richard Pyle of the Bishop Museum. “Combining both together on the same dives allowed us to achieve tasks that could not have been performed by either technology alone.”
Collaborating with geologists, biologists, and botanists, the team were able to fully analyze the entire island chain. While in the shallower reefs around 17 percent of species are thought to be unique – or endemic – the researchers found that below 70 meters (230 feet) it was more like 50 percent of species. This means they were able to document the highest rates of endemism ever found in a marine ecosystem.
The coral fields at this depth were also vast, completely covering areas extending for tens of square kilometers. These were mainly populated by the reef-building corals of the Leptoseris genus.
The discovery of such incredible life living so far beneath the waves has important implications for how we manage these hidden environments. Cable laying, dredging dump sites, and deep sewer outfalls, for example, may be irreparably damaging such ecosystems. With the Obama administration recently giving protection to the Papahānaumokuākea Marine National Monument surrounding the islands, it seems these reefs might be protected for now.