Explorers Marvel At 'Brittlestar City' On Seamount In Powerful Current Swirling Around Antarctica
Underwatertimes.com News Service 18 May 08;
Washington, D.C. (2008-05-18 18:36:00 EST) 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” established against daunting odds on 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 today along with important new insights into seamount geology and physics, highlighted a month-long April expedition to survey the Macquarie Ridge aboard the Research Vessel Tangaroa of New Zealand’s National Institute of Water & Atmospheric Research, 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. 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 Oceans.
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.
Piecing together the Brittlestar City ecosystem
The ship towed special sleds to collect seamount organisms. Thousands of specimens of all kinds were gathered from eight seamounts in over 30 sled collections and now fill almost 1,600 vials, jars and bags, to be sent from NIWA to taxonomists in New Zealand, Australia and overseas.
Full identifications may take many years. The eight biologists on board believe some species collected have never before been recorded in the region while some may be new to science.
A Deep Towed Imaging System (DTIS) vehicle, meanwhile, designed and constructed at NIWA by Peter Hill, was “flown” over the seafloor, snapping still photos and high definition video of the animals and their habitat. DTIS captured over 20 hours of video and thousands of photos of a part of Earth never seen before by humankind.
An abundance of deepwater cardinal fishes (Epigonus species) was found sheltering below a rock ledge on the seamount. In the lee of the rock, biologists believe, the fish could both conserve energy and access food.
Several Morid cods (family Moridae) were found in the folds of a large bubblegum coral (nearly two meters high, and likely hundreds of years old). These fish were also believed to be finding shelter from the current and perhaps benefiting in other ways from their close association with the coral.
Oceans worldwide contain an estimated 100,000 seamounts rising at least one km above the seabed; fewer than 200 have been sampled in any detail.
Undersea mountains can be highly productive and biodiverse, sometimes host unique species and serve as feeding grounds for fishes, marine mammals and seabirds. They also may serve as important way stations for marine migrations. The scientists’ work sheds light on factors underpinning seamount biodiversity, suggesting ways to improve their environmental management.
The odd shape and circumstances of Brittlestar City seamount
Tangaroa’s acoustic “multibeam” technology mapped the sizes, shapes and depths of the Macquarie Ridge.
The Brittlestar City seamount displayed several geological faults affecting its shape and geomorphology. The odd rectangular edge of its southern peak was formed by the intersection of two perpendicular faults. Because the upper surface is relatively flat, experts believe it was once at sea level, or slightly submerged. The flat topography suggests wave erosion occurred during the last ice age 18,000 years ago, when sea level was low. Although the base of Brittlestar City seamount is 850 meters below surface, its peak is just 90 meters underwater.
Intrigued by the dynamics of the Antarctic Circumpolar Current as it passed over and through ridge gaps, physicists aboard the Tangaroa calculated its speed over the top of the seamount at a “rattling” 2 knots (about 4 km per hour).
“This current is estimated to be 110 to 150 times larger than all the water flowing in all the rivers of the world,” says Dr. Mike Williams of NIWA. “In terms of the world’s oceans, New Zealand sits right beside the motorway.”
The Macquarie Ridge creates a strategic marine junction. “Understanding this current will shed light on how much water flows into the Pacific as opposed to continuing to circumnavigate Antarctica. This is important for understanding, and ultimately predicting, the impact of potential changes in the current on climate throughout the Southwest Pacific.”
Data was collected from nine strings of metering instruments, anchored a year earlier in two gaps or "choke points" in the Ridge, through which the current squeezes. Experts were astonished to find the current had pushed the top instruments on some strings to a depth of one kilometer below the surface.
Scientists also took this ocean area’s temperature and salinity readings for the first time since the 1960s, looking for climate-related changes, and obtained water samples to measure and compare levels of marine life nutrients.
Underwater mountain a sanctuary for sea life
Paul Eccleston, The Telegraph 19 May 08;
Biologists have found a vast colony of sea creatures living on an underwater mountain close to the Antarctic.
Tens of millions of starfish-like brittlestars cling together at the 2,500ft summit, which is higher than the world's tallest building.
Scientists were so astonished at the vast numbers of the brown-black creatures covering the 60 sq mile flat top of the mountain - known as a seamount - that they christened it Brittlestar City.
The ecosystem was found on an expedition to survey sea life on the 12 million-year-old Macquarie Ridge, which stretches almost 900 miles south from New Zealand to just above the Antarctic Circle.
Thousands of specimens were gathered and will be sent for classification. The biologists believe some species collected have never before been recorded in the region while others may be new to science.
Dr Ashley Rowden, an ecologist who took part in the survey, said: "Not only is it amazing to see a vast array of one type of organism but the implications for our understanding of the relative uniqueness of seamount assemblages are potentially far-reaching."
Millions of tiny starfish inhabit undersea volcano
Ray Lilley, Associated Press Yahoo News 20 May 08
Marine scientists surveying a large undersea mountain chain were amazed to find millions of tiny starfish swirling their arms to capture food in the undersea current.
An expedition by 19 scientists, including five from Australia, studied the geology and biology of eight Macquarie Ridge sea mounts. They are part of a string of underwater volcanoes — dormant for millions of years — that stretches 875 miles from south of New Zealand toward Antarctica.
The scientists also investigated the world's biggest ocean current — the Antarctic Circumpolar Current — amid expectations they would find evidence of climate change in the Southern Ocean.
While the expedition's cameras found a wide range of corals, a high density of cardinal fish and the huge coral, the vast collection of brittle stars was the highlight of the voyage.
"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."
Expedition leader and marine biologist Ashley Rowden said starfish usually cover only slopes away from the top of the undersea mountains.
"It got us excited as soon as we saw it," Rowden said of the site, dubbed "Brittle Star City."
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, a brittle star specialist, 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 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.
'Brittlestar City' discovered on underwater mountain
Yahoo News 19 May 08;
A unique colony of tens of millions of starfish-like creatures has been discovered on a mountain peak lying below the ocean in sub-Antarctic waters south of New Zealand, scientists said Monday.
Tens of millions of brittlestars are estimated to inhabit the vast, flat-topped marine mountain colony, dubbed Brittlestar City by scientists, New Zealand's National Institute of Water and Atmospheric Research (NIWA) said.
Scientists from New Zealand and Australia also collected some species never before recorded from the underwater peak -- known as a seamount -- on the Macquarie Ridge, which stretches 1,400 kilometres (870 miles) underwater from south of New Zealand to near the Antarctic Circle.
They photographed hundreds of brown-black brittlestars on every square metre (yard), and estimate tens of millions of them populate the 100-square-kilometre (39-square-mile) flat top of the seamount.
The brittlestars live arm-tip to arm-tip on the summit, 750 metres above the sea floor and 90 metres below the sea surface.
Their arms wave in the current, catching food which floats past on the circumpolar current, flowing at a rapid four kilometres an hour as it forces its way between peaks in the Macquarie Ridge.
"We were excited to see such a huge assemblage of brittlestars on the Macquarie Ridge seamount," said ecologist Dr Ashley Rowden of the NIWA.
He described the vast colony of brittlestars as amazing and said it would make a significant contribution to understanding life found on seamounts.
Corals and sponges are usually the dominant life forms on seamount peaks, filtering food that arrives on the current.
Biologists on the believed "Brittlestar City" was the first ever find of its kind on a seamount, attributing the unique discovery to the unusually flat shape of the peak and the rapid current.
"This current is estimated to be 110 to 150 times larger than all the water flowing in all the rivers of the world," said Mike Williams of NIWA.
"In terms of the world's oceans, New Zealand sits right beside the motorway."
The discovery was made during a month-long expedition on NIWA's research ship in April.
It is part of a study of seamounts being done as part of the Census of Marine Life, an ambitious global survey to document the life of the world's oceans.
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