An expedition by 19 scientists, including five from Australia, studied the geology and biology of eight Macquarie Ridge sea mounts that are part of a string of underwater volcanoes stretching 875 miles from south of New Zealand toward Antarctica.
Expedition leader and marine biologist Ashley Rowden said starfish usually cover only slopes away from the top of the undersea mountains.
"It was unique in that it hasn't been found on the tops of sea mounts before ... (and) it was over a relatively large area" of about 60 square miles, Rowden said.
"I've personally never seen anything like this - all these animals, the sheer volume - all waiting for food from the current," expedition member and marine biologist Dr. Mireille Consalvey said Monday. "It challenged what we as scientists thought we knew."
The starfish are about 0.4 inch across, with arms about 2 inches long.
The expedition began March 26 and returned to port in New Zealand's capital Wellington on April 26.
Melbourne-based marine biologist Tim O'Hara said the vast collection of brittle stars, or ophiuroid ophiacantha, is "like a relic of ancient times."
"Normally fish would prey on them and eat them ... so for whatever reason there's a lack of fish predation there and it's seen this particular animal flourish," he said.
O'Hara, who was not part of the voyage, said the speed of the sea current in the area may partly explain why fish were not feeding on the tiny animals.
The scientists also investigated the world's biggest ocean current - the Antarctic Circumpolar Current - amid expectations they would find evidence of climate change.
The Circumpolar Current merges the waters of the Atlantic, Indian and Pacific oceans and carries up to 150 times the volume of water flowing in all the world's rivers, oceanographer Mike Williams said.
Australian oceanographer Steve Rintoul, who was not involved in the expedition, said there have been few measurements of the Antarctic Circumpolar Current, which "strongly influences regional and global climate" by carrying vast amounts of water and heat across oceans.
Fewer than 200 of the world's estimated 100,000 sea mounts that rise more than a half a mile above the sea floor have been studied in any detail.
