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Deep-Sea "Nursery" Found Off California

John Pickrell in England
for National Geographic News
September 23, 2003
 
Researchers are still in the dark about most aspects of deep-ocean life, and perhaps least known are the breeding strategies of deep-sea fish. Now, researchers carrying out geological work off the coast of California have uncovered a completely new type of brooding site.

At the Gorda Escarpment, 100 miles (160 kilometers) off the coast and one mile (1,600 meters) below the surface, octopus and fish aggregate densely in the darkness of the ocean floor to lay eggs and care for young. The brooding site may be a totally new type of deep-sea biological hot spot, teeming with life in the otherwise sparsely populated depths.


Using a remote-controlled submersible robot, researchers have captured images of thousands of lilac-colored fish eggs, plastered across boulders by the little-known blob sculpin, Psychrolutes phrictus. Plump and pale, two-foot-long (60-centimeter-long) parent fish are to be seen resting nearby. These fish nests are interspersed with Graneledone and Benthoctopus octopuses tightly wrapped around rock surfaces and ledges harboring their own precious eggs.

The colorful site is also home to an array of crabs, anemones, sea stars, crinoids, and sea fans.

Biological Hot Spot

The find represents the first example yet recorded of deep-sea fish caring for their young. "This is [also] the first time we've ever seen a multiple species congregation for reproduction in the deep sea," said Jeffrey Drazen of the Monterey Bay Aquarium Research Institute (MBARI) in California, who revealed the find at the Deep Sea Biology Symposium held in Coos Bay, Oregon, last month.

Other types of deep-sea hot spots include hydrothermal vent communities, where specially adapted crabs, clams, and odd-looking tube worms gather around mineral-rich hot springs. Cold seep communities thrive on cooler, mineralized water leaking from the muddy sea floor. In both these environments, "the minerals fuel an entire ecosystem, the way the sun would in a terrestrial environment," said Drazen. Tube worms and other animals use bacteria to convert sulphurous minerals and alike into useful substances.

Similarly densely populated seamount communities thrive on the peaks of submerged mountains where ocean currents gather pace as they negotiate the obstacles. Fast moving currents concentrate plankton and other goodies for filter-feeding animals that fuel these entire ecosystems.

The new find at the Gorda Escarpment reveals some of the greatest densities of fish and octopus ever seen in the deep sea and represents a new kind of biological hot spot, said Drazen. The animals are present at hundreds of times their typical numbers at these depths, he said.

MBARI geologist Debra Stakes first spotted the unusual site in the summer of 2000, whilst using the remote-controlled submersible to map the structure of the sea floor. MBARI biologist Shana Geoffredi went along the following year, and Drazen organized a third dive, in July 2002, to carry out the first accurate census of the animals and their eggs. The discovery is also detailed in an article authored by Drazen, Stakes and Geoffredi in the current edition of the science journal Biological Bulletin.

"Exploitation Potential"

Though the scientists behind the find have no firm idea of what might encourage these seasonal aggregations, Drazen has a hunch that it could be related to environmental conditions. Water currents increase in speed over the nesting sites, which are situated at the leading edge of submarine plateau. These currents could be useful for removing wastes and increasing oxygen flow to the eggs. The sites also have lots of boulders and rocky outcrops for egg attachment, said Drazen.

Perhaps the two species aggregate because they use similar strategies when it comes to caring for their offspring, added Drazen, "the [Gorda Escarpment] sites could be particularly advantageous for brooding eggs."

Even though we've only just discovered these communities, we may already have to look towards conserving them, commented deep-sea biologist John Gage at the Scottish Association for Marine Science in Oban, who commended the find. "The exploitation potential of such aggregations can make the organisms highly vulnerable," he said, pointing to the example of the orange roughy, a deep-sea fish found near Australia and elsewhere.

That deep-ocean species is known by fishermen to aggregate on seamounts and submarine ridges. The fish were "famous for having been subject to serial depletion," said Gage. Fisherman in trawlers swept them up to order, trashing corals and entire seamount communities in the process, he said.
 

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