Exploration of deep-sea coral reefs in the Northwestern Hawaiian Islands is just beginning but already has revealed a slew of species previously not known to inhabit the island chain.

National Oceanic and Atmospheric Administration researchers returned last week from a 26-day trip to four of the northwestern islands and the Johnston Atoll National Wildlife Refuge with specimens of new species of deep-water algae and the first recorded specimens of black coral from Johnston Atoll.

Divers on the expedition also spotted more than 20 species of fish never before recorded in the Northwestern Hawaiian Islands and 15 species of fish never before recorded at Johnston Atoll.

Randall Kosaki, deputy superintendent of the Papahanaumokuakea Marine National Monument and NOAA’s chief scientist on the expedition, said the number of discoveries isn’t surprising because advanced scuba technology is allowing researchers to dive far beyond the shallow coral habitats found 20 to 80 feet below the surface to previously unexplored depths of up to 300 feet.

“We are kind of in exploration mode,” Kosaki said Tuesday in an interview. “Every dive we do literally is on a reef no human has laid eyes on.”

The trip marks NOAA’s first full deployment of closed-circuit rebreathers on a research cruise, the administration announced in a news release. The rebreathers recycle the gases divers breathe by removing carbon dioxide and actively maintaining oxygen levels so divers can dive deeper and longer.

Researchers went to Laysan Island, Nihoa, Mokumanamana (Necker Island) and French Frigate Shoals in the northwestern Hawaiian chain.

Research from the expedition will help scientists better understand genealogical differences within species and how they spread throughout the Pacific, Kosaki said.

Johnston Atoll, for example, is believed to be a key jumping-off point for a number of Central and South Pacific marine species to colonize the Northwestern Hawaiian Islands because table coral, which is common throughout the tropical Pacific and at Johnston, is limited in Hawaii to French Frigate Shoals and neighboring atolls.

Kosaki said recent black coral discoveries, such as the population found around Johnston Atoll, have tripled or quadrupled the known area of where black coral formations are found.

“It’s kind of comforting to know that there is a lot of black coral out there — far more than we had known previously,” he said of the coral, a coveted jewelry-making material harvested under strict regulations around the main Hawaiian Islands.

Scientists also conducted archaeological surveys of the Howland, a late-1800s whaling ship that wrecked at Johnston Atoll.

Researchers from the Hawaii Institute of Marine Biology, University of Hawaii and Bishop Museum participated in the expedition along with NOAA.

The Hawaii Institute of Marine Biology reported that the explored coral appeared healthy and exhibited low levels of disease.