A dense aggregation of brittlestars with their arms feeding in
water current.
Photo by NIWA

Census of Marine Life-affiliated scientists, plumbing the secrets of a vast underwater mountain range south of New Zealand, captured the first images of a novel “Brittlestar City” that colonized against daunting odds the peak of a seamount – an underwater summit taller than the world’s tallest building.

Its cramped starfish-like inhabitants, tens of millions living arm tip to arm tip, owe their success to the seamount’s shape and to the swirling circumpolar current flowing over and around it at roughly four kilometers per hour. It allows Brittlestar City’s underwater denizens to capture passing food simply by raising their arms, and it sweeps away fish and other hovering would-be predators.

Discovery of this marine metropolis, announced 19 May 2008 along with important new insights into seamount geology and physics, highlighted a month-long April 2008 expedition to survey the Macquarie Ridge aboard the Research Vessel Tangaroa of New Zealand’s National Institute of Water & Atmospheric Research (NIWA), host of the Census of Marine Life seamount programme, CenSeam. The voyage was largely funded by the New Zealand Foundation for Research, Science and Technology.

Formed at least 12.5 million years ago, Macquarie Ridge stretches 1,400 km south from New Zealand to just above the Antarctic Circle (see map). A multi-disciplinary scientific team from New Zealand and Australia extensively sampled this intriguing ecosystem deep beneath waves familiar to fishing trawlers but rarely reached by scientists.

Macquarie Ridge is one of a few places where the Antarctic Circumpolar Current is detoured in its endless clockwise churn at the globe’s southernmost latitudes – playing a vital part in the global ecosystem, merging and mixing waters of the Atlantic, Indian and Pacific Ocean.

Usually corals and sponges dominate seamount peaks, filtering food that arrives on the current. Biologists aboard the Tangaroa believe Brittlestar City is the first dense aggregation of another filter feeder, the brittlestar, ever found atop a seamount, and they credit the summit’s shape and extraordinary current circumstances there, 750-meters above the ocean floor.

They photographed brown-black brittlestars numbering hundreds per square meter and estimate tens of millions of them populate the 100 square km flat top of the seamount.

Brittlestars are echinoderms, relatives to starfish, sea cucumbers, sea lilies, and sea urchins. The two brittlestar species observed were tentatively identified via photographs sent from the ship to the Museum Victoria in Melbourne, Australia.

There, taxonomist Tim O’Hara determined that the smaller, densely packed brown-black brittlestar species, found living arm tip to arm tip on the sand and cobble substrate of the peak, were likely Ophiacantha otagoensis or Ophiacantha fidelis.

Larger orange-red species discovered down the seamount’s flanks, filmed waving arms in the current to collect passing food, were likely Ophiacantha rosea.

“We were excited to see such a huge assemblage of brittlestars on the Macquarie Ridge seamount. Not only is it amazing to see a vast array of one type of organism but the implications of the find for our understanding of the relative uniqueness of seamount assemblages are potentially far-reaching.” said ecologist Dr. Ashley Rowden of NIWA.