This new species of crab, found near methane seeps, “farms” the fluid released to derive energy for the bacteria on its appendanges, which it uses for food. Photo: Andrew Thurber, Oregon State University |
A species of crab found a thousand feet below the surface of
the Pacific Ocean near Costa
Rica lives off the bacteria on its claws—bacteria
that it fertilizes by waving them in methane and sulfide released from the
seafloor.
This “farming” behavior was described for the first time in
detail by the scientists in PLoS One.
This new species of the Yeti crab, called Kiwa puravida, was first discovered in
2006, according to Andrew Thurber, a post-doctoral researcher at Oregon State
University and lead
author of the study. It is only the second member of the Yeti family of crabs—first
discovered in 2005—and illustrates how little scientists know about the deep
ocean environment, the researchers say.
“We watched the crabs wave their claws back and forth in
fluid from a methane seep, and rather than trying to capture bacteria, it
appeared that they were providing food to the bacteria already growing on their
claws,” Thurber says. “There isn’t sufficient food that deep that is derived
from the sun’s energy, so vent and seep animals harness chemical energy
released from the seafloor.”
“These bacteria are specialists and can be found on a
variety of crustaceans—crabs, shrimp and barnacles—near seeps and vents,”
adds Thurber, who is in OSU’s College
of Earth, Ocean, and
Atmospheric Sciences. “But we hadn’t before seen that kind of ‘farming’
behavior in which the host waves its symbionts in seep fluid.”
Thurber, who did much of the research as a doctoral student
at the Scripps Institution of Oceanography, wasn’t part of the 2005 that found
the first Yeti crab, but participated in the 2006 expedition that discovered Kiwa puravida, and follow-up cruises in
2009 and 2010 that collected the crabs using the submersible Alvin.
Having the specimens allowed the scientists to more closely
examine the bacteria on their claws and run their genetic code through GenBank—an
international database that includes thousands of species of bacteria. They
discovered that it is most similar to bacteria found on crabs and shrimp living
near hydrothermal vents.
“We don’t know for certain whether hydrogen sulfide alone
fuels this species’ symbionts,” Thurber says, “but we suspect it may use both
hydrogen sulfide and methane released from the seafloor to exist so far from
the sun.”
Thurber says symbiotic behavior in nature is common, but few
animals are known to behave in quite the same way as Kiwa puravida. Some organisms, including mussels and tubeworms,
have symbionts inside of them that allow them to harness chemical energy, while
others that do not have symbionts—including barnacles—wave their appendages to
grab food as it goes by. This new species is the only one that combines the
two, by using symbionts on its appendages and waving those bacteria-laden
appendages in seep fluid to capture chemical energy as a food for themselves.
Lipid and isotope analyses showed that these epibiotic
bacteria are the crabs’ main food source, though Thurber says they may be
getting a small amount of sun-derived energy from dead plankton that have
filtered down through the water column.
Thurber says the crabs harvest the bacteria growing on their
claws by using a specially adapted appendage to scrape the bacteria off their
bodies and bring it to their mouths, and then continually waving their claws
near methane seeps to boost the bacteria’s productivity.
Only one specimen of the original Yeti crab, K. hirsuta, has been collected and that
was near a hydrothermal vent. About 30 to 40 specimens of Kiwa puravida have been examined and the scientists believe they
may exist at similar methane seeps.
“Since this entire family of crabs wasn’t even discovered
until 2005, there is a strong possibility other species are out there,” Thurber
says.
