The sun provides
energy for nearly everything on Earth, although some alien exceptions
exist. The sulfur and hydrogen
loving bacteria of Yellowstone or the methane worms of deep-sea vent
communities are just two examples. There might be more familiar species that
also don’t use solar energy. In
the September Marine Ecology Progress
Series, Dr. Tina Treude from Leibniz Institute of Marine Sciences in
Germany and colleagues discovered shark and skate (a flat fish similar to a
ray) egg capsules at two deep sea cold seep sites and discovered fossil
evidence of similar behavior in Washington. This indicates that these fish use
cold seeps as nurseries.
Like hydrothermal
vents, cold seeps are locations in the deep sea that provide energy to a
community that cannot rely on photosynthesis. They look very much like the
vents; however, temperatures at cold seeps are usually less than 10 degrees
Celsius. Instead of geothermal heat, energy is released in the form of
hydrocarbons like oil and methane that oozes, or “seeps” out of fissures in the
sea floor. Initially extreme species of bacteria colonize the area, then
mussels and eventually tubeworms can move in to form a community. More complex
organisms may eventually follow.
Dr. Treude and her
team used remotely controlled devices to examine two locations: a mud volcano
in the Mediterranean Sea and a cold seep off the coast of Chile. At the Mediterranean Sea site, hundreds
of shark egg cases were strewn across a dense field of tubeworms. Based on the
size and shape, the team guessed they belong to deep-water catsharks. In
addition to the egg cases, anemones, shrimps, a squid, and other fish were
found living in the tubeworm field. At the Chilean site, hundreds to possibly
thousands of large skate egg cases were discovered on a large sheet of rock
exposed to open ocean. The sheer number of embryos and a possibly pregnant
skate seen at the site indicate this may be a nursery for generations of the
bottom-dwelling fish.
In the second part
of the study, the research team looked at the fossil record, taking samples
from a 35 million year old seep deposit in Washington state. Fossils of mussels
dominated the deposit while tubeworms and snails were also present. The
researchers collected 30 fossilized egg cases and fragments. Based on the shape
of the fossilized capsules they determined that the egg capsules belonged to
some species of deep-water catshark.
Fossil tooth evidence proves that catsharks lived in that part of the
ocean at that time.
Putting together
the prehistoric evidence with modern observations, Dr. Treude concluded that
cold seeps serve as nurseries for elasmobranches, which are cartilaginous
fishes like sharks, skates, and rays. These locations provide protected and
ventilated holdfasts for the egg cases to cling to. While more research needs
to be done, Dr. Treude suspects that the presence of small fishes, crustaceans,
and mollusks could provide food for newborn elasmobranches. While not fully
understood the presence of a large predator in a nonsolar-based community is
intriguing because higher level predators are normally found in a solar-based
food chain. Humans have considered using methane hydrate deposits from the
ocean floor as an alternative fuel source. If a baby shark can gain enough
energy to live eating chemosynthetic organisms, perhaps humans can use and farm
extreme bacteria for fuel.
Treude, et al. “Elasmobranch egg
capsules associated with modern and ancient cold seeps: a nursery for marine
deep-water predators” Marine Ecology
Progress Series. September 15h, 2011.
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