The Southern Ocean: hospitable for some, bleak for others

May 19th

I study chrysogorgiid corals (http://deepseacoral.lifedesks.org/pages/63), a group of predominantly deep-sea corals found down to 4200 m depth. Chrysogorgiids are diverse (about 100 species described) and found in all major oceans, within a broad latitudinal range (see figure). Despite this broad distribution, chrysogorgiids are rare in the Southern hemisphere below 45 degrees latitude, a pattern that contrasts with our observations from the Northern hemisphere. Only one specimen of the genus Chrysogorgia was ever identified in these southern waters, on the shelf of Antarctica. In particular, these corals were apparently never collected in the Drake Passage, despite numerous attempts to explore the deep benthic ecosystems of the area. While the Southern Ocean, and the Drake Passage in particular, may be a hospitable place of other coral groups, it seems bleak for the chrysogorgiids, who may not be able to survive there.

Sampling in the deep sea is not a trivial task, and it may well be that chrysogorgiids are present in these southern waters, but, out of random chance or maladapted sampling strategies, were never collected. Chrysogorgiids, for instance, are often found on the hard substrates of slopes, canyons, and vertical wall, areas that are typically difficult to sample with traditional gear. During the Drake Passage Expedition, many different types of sampling gears are utilized, within a wide depth range, multiplying our chances to collect or observe chrysogorgiid corals.

Unfortunately, after a week of work, and sampling at 20 stations, still no signs of chrysogorgiid corals. So what does it mean, if indeed these corals are so rare in these waters? Why would corals that are apparently able to grow and reproduce everywhere else in the world, would not be successful at colonizing the Drake? The absence of chrysogorgiids in this area has strong implications for the biogeography (past, current and future) of these animals. First, if waters South of the tip of South America are inhospitable, the Atlantic and Pacific coral fauna are expected to be strongly differentiated. Depending on how long ago these southern waters became inhospitable to chrysogorgiids, this dispersal barrier might have had a strong effect on the overall diversification history of these corals. The Drake shelters the typical habitats in which chrysogorgiids are usually found, so water chemistry characteristics might be the reason why these corals do not survive there. Understanding the link between coral geographic distributions and ocean chemistry may help us predict where corals lived in the past, and were they will be able to survive in the future.

By Eric

Weather: temperature 30°F; windchill -14°F; windspeed 5-15 knots; sunny but hazy

The geographic distribution of chrysogorgiid corals as we know it. Each location at which chrysogorgiids were sampled in marked as a blue dot. Gray dots are locations were over 16,000 octocorals (soft corals, sea fans and gorgonians) were collected, based on museum records, the taxonomic literature, and our own explorations of the deep sea (Pante, unpublished data, please do not use without permission). While chrysogorgiid corals are pan-distributed overall, they seem to be absent from the Drake Passage. Why is that?

A Chrysogorgia specimen freshly collected from the deep waters off New Caledonia (Southwestern Pacific Ocean). This specimen is about 30 cm tall, and shelters a little galatheid crab. Chrysogorgiids are among "some of the most beautiful and interesting of all the known Gorgonians" according to Verrill, a prominent 19th century coral taxonomist. To see pictures of chrysogorgiid corals live in their natural environment, and learn more about their biology, please visit the Deep-Sea Corals Portal (http://www.ucs.louisiana.edu/~scf4101/Bambooweb/).