Mystery Over Pristine Shipwrecks, Dead Whales Solved
Both shipwrecks and dead whales tend to sink to the seafloor and gradually disappear due to erosion and scavengers. In the Antarctic, however, shipwrecks tend to stay pristine, whereas finding a dead whale carcass is an extraordinarily rare event, found for the first time off Antarctica only this year.
The difference is due to the type of scavengers that thrive in the Southern Ocean, and those that don’t.
Adrian Glover of The Natural History Museum and his team have discovered two new species of bone-eating Antarctic worms: Osedax antarcticus and Osedax deceptionensis. And unlike Osedax in warmer regions, O. antarcticus at least scavenges in droves.
Glover refers to shipwrecks, other wood waste, and dead whales as “organic falls,” since all tend to be large and very slow to break down. If you put a giant dead whale in your compost bin, you’d probably see and smell it for quite some time. In other parts of the ocean, Osedax worms and flesh-eating fish spend generations munching on a single whale fall. But little is known about what happens in the ocean depths of Antarctica.
Glover explained, “The deep sea that surrounds the Antarctic continent is one of the least explored ecosystems on Earth. The organisms that live there are dependent on a supply of food from the surface, in extreme examples this can be the remains of a whale or piece of wood. These large ‘organic-falls’ are unstudied in Antarctica.”
For this latest study, the researchers left planks of pine and oak as well as whalebone sitting for more than a year on the seafloor in three different locations along the West Antarctic Peninsula.
After the team recovered their samples, they found through microscopic analysis that the wood was nearly as good as new – albeit water laden. The whalebones from two of the sites, on the other hand, were infested with the newly identified O. antarcticus worms to the point where the bones looked like they still had skin attached, only it was the skin of the worms. The newly identified O. deceptionensis emerged from a hole in the bone from the third site after several days in an aquarium. The findings bring the number of known bone-munching Osedax worms to seven.
The findings are published in the latest Proceedings of the Royal Society B.
Usually wood is attacked by wood-boring bivalves that can penetrate solid materials. Scavengers such as teredinid shipworms and deep-sea xylophagainid clams consume the wood directly, but gain nutrients from their meal only after the fibers are broken down by their own personal bacterial endosymbionts. Other wood-boring bivalves don’t eat the wood directly, but dig into it with attachment bristles. As sometimes the only topographic feature for miles around, shipwrecks can provide an impressive amount of shelter. Until that is the bivalves dig, or with their bacteria eat, the wooden wrecks to the ground.
(Certain species of bacteria can even feast on rusting metal on their own, which is one reason why famous wrecks like the Titanic are in danger of eroding away. The hull of the Titanic, for example, is laden with bacteria-formed “rusticles.”)
Shipwreck hunters that may have avoided the Southern Ocean in the past because of the danger and difficulty of exploring the region may find their efforts well worth the trouble. Glover and his colleagues suspect that wood-boring species are either absent or in low numbers in this region because the cold water and its current form a barrier that may act to keep the species away.
“It is possible that our experiments were not left long enough, or that the size of the lots of wood, or the presence of whalebone, has inhibited larval development” of wood-boring species, the researchers acknowledge. But similar experiments at other latitudes have shown that wood-boring species are capable of infesting wood at the end of three months, and in some cases, destroying a sample completely within a year, they reported.
But for more than 30 million years, the Antarctic continent was tree free.
“Since humans first started exploring the Antarctic, wood has been deposited on the seafloor in the form of shipwrecks and waste; our data suggest that this anthropogenic wood may be exceptionally well preserved,” the authors wrote.
Stay tuned for possible future discoveries!
IMAGE: Parborlasia corrugatus, or the proboscis worm, is a scavenger that will comb the ocean floor eating sponges, jellyfish, anemones, and fish. This photo was taken in the Ross Sea, Antarctica, under 5 meters of sea ice. An underwater field guide to McMurdo Sound is available at: http://scilib.ucsd.edu/sio/nsf/fguide/index.html. (Henry Kaiser, National Science Foundation)