Vent Crabs: Shallow Opportunists
Biodiversity abounds near deep-sea hydrothermal vents,
which spew energy-rich sulfides and minerals that sustain
life. But although most of the action—and the
variety—of life at thermal vents is found in the
deep, some organisms flourish in shallower extremes.
Take, for example, Xenograpsus testudinatus. These crabs live off the coast of Kueishan Island in northeastern Taiwan and inhabit crevices on the sea floor near shallow-water vents. Unlike deep-sea vents, which emit nourishing plumes of sulfides, the Kueishan vents discharge elemental sulfur and toxic volcanic gases into the sea. This creates a highly acidic and nutrient-poor environment that is not conducive to the chemosynthesis-based ecosystems found at deep-sea vents. The energy-rich sulfides are missing.
Scientists from Taiwan and Singapore wondered what X. testudinatus were eating in such a nutrient-poor environment. As reported in the December 30, 2004, issue of Nature, they discovered that the crabs are opportunistic feeders that take full advantage of their unusual environment.
At slack tide around Kueishan, when there is no current moving in or out, sulfurous plumes billow from as many as nine vents at once, killing everything in their path. Zooplankton caught in the deadly billows die and fall to the ocean floor like "marine snow." Suddenly the crabs swarm out of their crevices and, bombarded by a steady rain of dead zooplankton, begin a frantic feeding frenzy. By peeking into the guts of several X. testudinatus specimens the scientists confirmed that the crabs were indeed feasting heartily on the zooplankton (mostly ocean-dwelling copepods).
As soon as the currents around the vents pick up again,
the sulfurous plumes shift, the rain of food ceases,
and X. testudinatus retreat to their crevices.
Scientists believe the crabs probably feed twice a day,
following the pattern of Taiwan’s semidiurnal
tides. A second crab species, X. novaeinsularis,
has also been observed feeding on the ocean floor, but
thus far its diet remains unknown.
—Emily Huhn
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ZooGoer 34(2) 2005.
Copyright 2005 Friends of the National Zoo.
All rights reserved.
