May 3 2008
May 3 2008
So how do corals live in the deep-sea?
One of our target species for this cruise, the Scleractinian hard coral Balanophyllia malouensis.
A beautiful purple Octocoral - Bayergorgia sp.
One of the many species of associated fauna that hang around deep-water.
It’s a question I’ve been asked more than a few times. My name is Dr Rhian Waller, and I’m a biologist at the University of Hawaii, and one of the PIs on this cruise along with Dr Laura Robinson (from the Woods Hole Oceanographic Institution). I completed my PhD in 2003 on the reproductive ecology of cold-water corals and have been working on answering a small piece of the deep-sea ecology question ever since – so how and why do corals live in such extreme environments?
Lets start with a little coral biology. There are basically two main kinds of coral – zooxanthellate (with photosynthetic algae) and azooxanthellate (without photosynthetic algae – also known as cold-water corals). Now zooxanthellate corals are limited to sunlit waters (such as the Caribbean or the Great Barrier Reef) for their algae to be able to photosynthesise and provide food for the corals. But azooxanthellate corals can live anywhere - from shallow waters to the very deepest parts of the oceans, - yet only really thrive in the extremes – waters of just a few degrees Celsius in the higher latitudes; and in the deep, dark depths of the world’s oceans.
These cold-water corals are “suspension feeders”, which really means they eat just about anything that passes within tentacle reach – so dead plankton raining down into the deep-sea, small organisms, larvae and eggs of other creatures that happen to come too close, all become dinner for these corals. These kinds of food source can be surprisingly abundant in the deep-sea, as many smaller organisms (as well as larger ones) are attracted to the habitats some cold-water corals can build. The reefs you see in the sunlit tropical zones are not the only reefs in the ocean – there is actually a cold-water coral reef that is 40km long, 3km wide, 400 meters deep and off the coast of Northern Norway, with over 100 different species of associated fauna using it as their homes and breeding grounds, and that certainly isn’t the only deep-water coral reef around either – try Alaska, Canada, Ireland and Sweden for just a few more.
Now corals in the Antarctic – what a crazy idea! But yes, there are actually many azooxanthellate cold-water corals living here in the Southern Ocean, both on the continental shelves of Argentina, Chile, the Falklands and Antarctica, but also on seamounts and ridges in between. For hard corals alone (scleractinians) there are 37 species living down here, and many more species of soft corals (Octocorals) and black corals (Antipatharians) and stylasterids. In most areas you find cold-water corals they live in waters of around 6-8oC, but here in the Antarctic they can be found down to just 0.5oC – pretty amazing really.
We know very little about how these cold-water corals live in the Antarctic. We do know they can be surprisingly abundant and widespread in places, especially in sediment at around 500m depth on the Western Antarctic Peninsula. From research I’ve recently being doing we now know some of these corals brood large, fat larvae that crawl on the bottom of the ocean – so in theory don’t move very far from the parent – yet the same species are found enormous distances away – how did they get there? I’ve been growing these larvae in aquaria to try and get a sense of how long they take to reach maturity, build a skeleton, and how they begin the transition from larvae to coral, as well as looking at the genetics of different populations spanning almost the whole Western Antarctic Peninsula and across the Drake Passage. It’s all pretty fascinating, especially since so little is known about them, so it’s all new data.
I’ve been on over 20 research cruises in my career so far and one of the things I love most about coming to sea is that cruises often raise more interesting questions than they answer (and often questions you never would have expected either). There is always more to learn, especially when you work in extreme environments. I can’t imagine in my lifetime we will be able to say we really understand how cold-water corals survive in extreme environments, but I intend to solve a good part of the puzzle.
Rhian G Waller
All photos by Dann Blackwood unless otherwise stated