Why are two groups of tiny sea creatures called bryozoans nearly identical, despite being separated by 1,500 miles of ice? They must have traveled across the continent long ago - on a massive Antarctic seaway.
The bryozoan is a simple marine organism that attaches itself to the sea bed and filters nutrients out of the surrounding water. There are many different bryozoan sub-species, but a new survey of Antarctic organisms turned up some striking similarities between two distant populations. Both live on continental ice shelves, massively thick ice platforms that are created when continental glaciers reach the sea. One population is on the Ross Ice Shelf, and the other is on the Weddell Ice Shelf. As you can see on this map, there's a whole lot of Antarctica between those two populations. So how did such similar species get to such distant locations?
David Barnes, a scientist with the British Antarctic Survey, explains why the animals couldn't have simply worked their way around the Antarctic coastline, and why they must have traveled through the continent:
"When we found groups of strikingly similar bryozoans hundreds of miles apart we knew we were onto something very interesting. Perhaps these species had survived the last ice age whereas in all other regions of Antarctica they were wiped out. We know that after the last ice age groups of bryozoans dispersed freely between many of the regions we studied. But because the larvae of these animals sink and this stage of their life is short – and the adult form anchors itself to the seabed – it's very unlikely that they would have dispersed the long distances carried by ocean currents. For the bryozoans on both the Weddell and Ross sea continental shelves to be more similar to one another than to any of those found in the waters in between is striking indeed. Our conclusion is that the colonization of both these regions is a signal that both seas were connected by a trans-Antarctic seaway in the recent past."
This seaway could have opened up as recently as 125,000 years ago, during an interglacial warm period in which the sea levels were about fifteen feet higher than they are now. We don't know the exact processes that caused this seaway to open up, but they created an opening through what's now over a mile of solid ice. The ancient seaway also calls into question how stable Antarctica's ice is today.
The West Antarctic Ice Sheet, which is the region through which this trans-Antarctic seaway must have traveled, is thought to be unstable and in danger of collapsing, partially as a result of climate change. If the ice sheet does collapse, it could increase global sea levels by 10 to 15 feet, which could displace billions of people from their homes. The bryozoan discovery could, however, help us better understand how a melting Antarctica fits into current climate change. Barnes explains:
"The West Antarctic Ice Sheet can be considered the Achilles heel of Antarctica and because any collapse will have implications for future sea level rise it's important that scientists get a better understanding of big deglaciation events. This biological evidence is one of the novel ways that we look for clues that help us reconstruct Antarctica's ice sheet history. Our new research provides compelling evidence that a seaway stretching across West Antarctica could have opened up only if the ice sheet had collapsed in the past."
[Global Change Biology]