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On July
12th, 1997, our colleagues Charles Fisher from Pennsylvania
State University and Pilot Phil Santos from Harbor Branch
Oceanographic Institution were diving in submersible Johnson
Sea-Link II to examine a hydrocarbon seep and the community
of organisms associated with it, 550 meters deep in the northern
Gulf of Mexico. As they maneuvered to investigate a large
mound on the seafloor, Santos noticed movement on the exposed
underside of the deposit. They had discovered a dense colony
of a new worm species, living in a network of burrows that
covered the entire exposed surface of the mound.
Hydrocarbon
seeps are places where gas and oil flow naturally out of the
seafloor. Seeps are quite common on the continental slope
of the northern Gulf of Mexico. Rather than harming the marine
fauna, they support dense biological communities.
The primary
producers in these communities are bacteria that can subsist
by using the chemical energy contained in compounds like methane
and hydrogen sulfide, which are produced by seepage. Certain
animals, such as tube worms and mussels, form a symbiotic
partnership with bacteria. By helping the bacteria obtain
the chemicals and oxygen they need, the hosts are able to
attain huge densities compared with that normally observed
in the deep sea.
A variety
of other animals, including fishes, crustaceans, and mollusks,
are attracted to the resulting food supply. The result is
diverse assemblages comprised of chemosynthetic fauna, specialized
for the seep environment, and predators and browsers that
commonly occur across continental slope, but attain unusually
high abundances at seeps.
The mound
where Fisher and Santos discovered the worms was made of gas
hydrate, an ice-like substance that forms under pressure when
methane or other hydrocarbons are caged in a lattice of water
molecules. (For more information about gas hydrates see "Gas
hydrate gardens of the Gulf of Mexico") Hydrate is
common on the Gulf of Mexico continental slope and the discovery
of the worms, informally known as "ice worms," demonstrates
existence of a previously unknown ecological niche.
Chilly
burrows have all the comforts of home
Ice worms
are a type of polychaete worm, meaning they are characterized
by pairs of segmented appendages, belonging to the family
Hesionidae. Species of this small and poorly known family
are most common in shallow water associated with hard substrates
but have been found in the deep sea. Eyeless hesionids are
associated with hydrothermal vent systems in the Pacific,
but are not the same species found on the Gulf of Mexico gas
hydrates.
The hydrate
dwellers can be as much as four centimeters in length when
actively crawling and swimming, quite large for worms of this
family. The formal description of the worm is being carried
out by two French experts, D. Desbruyères and A. Toulmond,
who believe the new species is in the genus Hesiocoeca.
We do
not yet know how the worms survive in the mounds of gas hydrate,
or what role the worms play in their ecosystem. One of the
most striking features of the polychaete was its occupancy
of burrows that penetrate the hydrate surface. These burrows
are generally shallow depressions on the exposed surface,
but evidently extend into the hydrate mass where the surface
remains covered with sediment. Individual worms rarely leave
their burrows and, as video footage showed, will actively
defend their burrows against would-be interlopers.
We observed
a number of predatory species in close association with the
ice worms and the gas hydrate deposits-including isopods,
eels, and fish. The worms would appear to be quite vulnerable
to predators once their burrows are exposed, but direct evidence
for predation is still lacking.
Hydrate
habitat
The hydrate
mound where the worms were found was about two meters wide
and had a maximum height of roughly 1.3 meters. Gas hydrate
beneath the sea floor had evidently increased its volume sufficiently
to break through the sediment. Such mounds are common at gulf
hydrocarbon seeps, but this one was unusual because it exposed
so much hydrate.
The exposed
hydrate had two distinct layers, one yellow and one white,
capped with a drape of sediment. Hydrate of different colors
seems to have different characteristics. While yellow hydrate
is granular, the white variety is more dense and contained
fewer inclusions of sediment. Sediment surrounding the hydrate
mound was coated with mats of bacteria. Oil was also plentiful
in the sediments surrounding the hydrates.
Since
the worms were discovered, we have been collaborating with
scientists at Louisiana State University, Harbor Branch Oceanographic
Institution, Pennsylvania State University, San Francisco
State University, the University of California at Davis, and
the University of Virginia.
A number
of very basic questions remain to be answered concerning the
polychaete. Together with our collaborators, we have material
on hand to address its food supply, its early life history,
and its geochemical environment. Early indications are that
it is a very ordinary polychaete in every respect except its
habitat, but future expeditions to the lair of the ice worm
will undoubtedly produce new surprises.
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The iceworm is one of many organisms in the seafloor ecosystem
supported by naturally seeping hydrocarbons in the Gulf of
Mexico. (Photo by Ian MacDonald)
(Photo by Ian MacDonald)
(Photo by Jonathan Blair)
(Photo by Jonathan Blair)
Bacteria
are the microscopic workhorses of the chemosynthetic community,
converting sulfide and hydrocarbon molecules into substances
edible to others. Tube worms (top, bottom) and mussels (middle)
live in symbiosis with internal bacteria, then become prey
to starfish and Rochinia crabs. The scallop-like Acesta
bullisi (top) attaches itself to tube worms to obtain
food, but does not harm or benefit the worms.
Ice worms usually hide safely in their burrows, but disturbances
expose them to possible predators such as this isopod, a deep-sea
crustacean distantly related to pillbugs.
(Photo by Michael Peccini)
Gas hydrate in the lab briefly resembles snow before it
decomposes into water and free gas. Holes in this piece are
ice-worm burrows.
(Photo by Michael Peccini)
Oceanography graduate student Susan Escorcia captures methane
gas escaping from various sections of a hydrate core cample
that was brought to the surface.
(Photo by Ian MacDonald)
Oceanographers use a manned sumersible, the bubble-shaped
R/V Johnson Sea Link, to visit hydrate mounds
and study their ecology. Back on the ship's deck ice-worm
discoverer Chuck Fisher examines tube worms collected on the
seafloor with its pilot-controlled robot arm. (Photo by Jonathan
Blair)
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