Lophelia II 2012: Deepwater Platform Corals.

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Questions received during the expediton were forwarded to the team at sea, and their replies posted here.

I am familiar with bioerosion in corals, coral rubble, and coral reef carbonate substrate in shallow water. Bioerosion is done by small invertebrates of several taxa boring into carbonate substrate. Small animals (cryptofauna) of many taxa live in holes formed by bioerosion.

Do bioerosion and cryptofauna exist in deepwater corals? If so, it would be useful to study them. For example, what would be the relationship between the taxa from deepwater coral sediments on platforms and any cryptofauna living within bioeroded spaces in the coral skeletons? And what would be the impact of bioerosion as a constraint to hard coral growth?

Question from: Dave Moran, Bureau of Ocean Energy Management (BOEM)

This question has been looked at elsewhere, but not specifically in the Gulf of Mexico. Previous work on Lophelia pertusa in the Northern Atlantic has shown that there are a number of organisms, most prominently boring sponges (eg. Spiroxya heteroclita) and endolithic fungi, that actively erode skeletal material. In addition, there are a number of taxa that encrust the skeleton including hydrozoans, serpulids, bryozoans, bivalves, actinians, and stylasterids. These encrusting invertebrates seem to build up reef structure, either by directly contributing material or increasing sediment trapping, rather than actively eroding skeletal material. Interestingly, the symbiosis between Lophelia pertusa and the polychaete worm Eunice norvegica also strengthens skeletal structures by creating tubes that connect skeletal fragments, resulting in higher rates of reef accretion.

Determining what was causing this was of vital importance to the U.S. Navy (and others) because of the need to understand how to interpret different sonar signals - submarines from whales, undersea mines from shipwrecks - a very difficult task in the early days of sonar development. This phenomenon triggered a variety of research, much of it using mid-water sampling nets to tow through this layer at all times of the day.

The answer soon became clear: this layer, which occurs all over the world in the open oceans, is composed of huge numbers of small fishes, shrimps, plankton, and squids. In fact one member of this community, the diminutive bristlemouth (Cyclothone spp.) may be the most abundant vertebrate on Earth. These animals undergo daily migrations in order to feed. At or near sunset, they move from great depths (about 400-600 meters) to more shallow depths (even all the way to the surface in some cases) where they feed on plankton and other animals. Near sunrise they move back down to their deep-water habitats where they rest and float until the next migration. This world-wide migration is one of the most amazing and extensive animal movements on the planet.

Today, after decades of research, we know a lot about the timing of these migrations, which animals are doing them, and how far they travel. There is still a lot to discover about what impact this abundant community has on oceanic productivity and the movement of nutrients around the oceans.

In general, it appears that bioeroders and encrusters only colonize dead skeletal material and that live tissue prevents colonization. So, I would expect that bioerosion is not a major constraint on growth, but that it almost certainly leads to degradation of reef structure as a whole over time. This is an interesting question however, and probably warrants further investigation, especially in Gulf of Mexico populations, as well as with non-scleractinian cold-water corals.

Answer from: Sam Georgian, PhD student, Temple University