THE Ventana glides slowly along the bottom of Monterey Bay, just a metre
above the silty seafloor. The bottom here is mostly barren mud with a few
cobbles and the occasional fish or gorgonian coral. Abruptly, a dense knot of
whitish clams comes into view, standing out like a village on the featureless
plains. This is a cold seep, a mild-mannered, unheated equivalent of the more
famous and violent hydrothermal vents, and it is one of James Barry鈥檚 study
sites. But Barry, a benthic ecologist at the Monterey Bay Aquarium Research
Institute in Moss Landing, California, has never been here. He sits almost a
kilometre above, in the forecastle of a ship, at the far end of a fibre-optic
tether.
Barry and his colleagues are in the vanguard of a revolution in deep-sea
research. Until the 1990s almost everything scientists knew about the ocean
floor came from trawls or dredges that scraped or grabbed blindly at the seabed,
bringing back samples to be picked over on the surface. It was a strategy much
like that of 19th-century naturalists who sat at home and examined the exotic
specimens that collectors brought from overseas. But today, Ventana and a small
fleet of other submarine robots鈥攌nown as remotely operated vehicles, or
ROVs鈥攁re allowing marine researchers to see, touch and study the seafloor
more cheaply, more safely and more often than ever before. These ROVs, a few
manned minisubs and an arsenal of other high-tech equipment are at last bringing
20th-century biology to the ocean floor, allowing ocean researchers to come to
grips with the ecosystem they study.
Without submersibles, oceanographers would never have discovered cold seeps
in the late 1970s. Seeps are small patches, rarely covering more than a few
square metres, where sulphide-rich water emerges from the ocean floor to mingle
with seawater. Like freshwater in the Sahara, this sulphide creates an oasis of
life in the perpetual darkness of the deep sea. Microorganisms that live at the
seeps have evolved a way to convert sulphide to elemental sulphur. This
liberates chemical energy, which they use instead of sunlight to convert carbon
dioxide to sugars. Some of these microorganisms form symbioses with clams and
other seep animals. The clams provide a steady supply of sulphide to the
bacteria living in their gill tissues, and their house guests pay rent in the
form of energy to nourish their hosts.
Advertisement
An intertidal ecologist by training, Barry is one of the first biologists to
study the ecology of these underwater oases. 鈥淭hese seeps are sort of like the
intertidal, with sharp environmental gradients,鈥� he says. Water at the centre of
a seep contains more sulphide than water near its periphery, and this affects
what lives where.
Two species of vesicomyid clams dominate Barry鈥檚 seep communities. One,
Calyptogena kilmeri, tends to live near the centre. The second,
Calyptogena pacifica, is more likely to be found in the outer zone of a
seep. Manoeuvring Ventana鈥檚 robotic arm, Barry and his colleagues swapped the
clams between low and high-sulphide sites to see if each could live in the
other鈥檚 favoured conditions. They found that C. pacifica was as happy
as a clam in sulphide-rich areas, but C. kilmeri could not survive in
the poorer waters with less sulphide.
Simple experiments like this are commonplace for ecologists working on land
or along the shore. On the ocean floor they are almost unheard of, because most
deep-sea ecologists cannot visit a study site regularly enough to keep an
experiment going. 鈥淚f you鈥檙e lucky, you get to go to sea with an ROV or
submersible once or twice a year. If you get out twice in a year, it鈥檚 really
remarkable,鈥� says Craig Smith, a benthic ecologist at the University of Hawaii.
Barry is luckier: he has the Ventana at his disposal for 30 days each year,
which gives him time to tinker with the communities he studies. 鈥淗is situation
is the envy of many a marine biologist,鈥� says Smith.
In his laboratory ashore, Barry has been keeping the two clam species in
water with different sulphide concentrations. He finds that both species capture
sulphide from seawater and store it in their blood for their bacterial partners
to use. However, C. pacifica is much better at concentrating sulphide
than C. kilmeri. This means that C. pacifica can provide its
guests with enough sulphide to keep them satisfied even in the sulphide-poor
water near the edges of seeps, where C. kilmeri would fail.
Back on the ocean floor, Barry has been running another experiment to measure
the growth rates of the two clams. He collected some of each species and marked
and measured their shells. Then he returned them to the seeps and fenced them
off from their fellows in plastic corrals. Almost a year later, he checked how
much the clams had grown. The central zone clam, C. kilmeri, turned out
to grow quickly, as much as 20 millimetres per year when young, and reached full
size within 5 to 10 years. Its neighbour, C. pacifica, lagged far
behind. 鈥淣o animal grew as much as a millimetre a year,鈥� says Barry. 鈥淚t only
grows to 60 millimetres, but it鈥檚 taking 60 to 100 years to get there.鈥� Barry
believes that C. kilmeri鈥檚 rapid growth could be the crucial factor
that enables it to defend its claim to the sulphide-rich centre of a seep.
Simple experiments like these are routine in most ecological circles. But in
the deep sea, the fact that Barry can do them at all is a breakthrough.
Experiments allow ecologists to go beyond merely describing patterns: now they
can learn something about the ecological forces that produce them. This approach
may hold the key to understanding how species interact at seeps, just as it
unlocked the secrets of the intertidal zone in the 1960s and 1970s.
But the deep-sea experimenters still face huge practical challenges. Barry
cannot visit his study sites in person. He has to view them second-hand on a
video screen. And he can only 鈥渉andle鈥� his clams with the help of a professional
submarine operator who controls the craft鈥檚 robot arm. 鈥淛ust picking up an
animal can take as much as an hour. You really have to scale back your
expectations of what you can get done in a day.鈥� ROV researchers need strong
stomachs, too, to sit in a dim control room on a heaving ship watching a second,
independent motion on video.
Despite this, ROVs are easily the most popular choice for undersea work,
because they are cheaper to build and operate than minisubmarines designed to
carry people. MBARI鈥檚 latest ROV, Tiburon, which can dive to 4000 metres, will
carry researchers鈥� eyes and arms still further into the ocean. 鈥淗aving
submarines down there brings our personal perspective down into that habitat,鈥�
says Barry. 鈥淲e鈥檙e able to see organisms in their natural habitat behaving the
way they ordinarily behave.鈥� That鈥檚 something landlubbers take for granted, but
for deep-sea biologists, it marks the beginning of a renaissance.