With more than 2200 described species, brittle stars (Ophiuroidea) form a particularly abundant and diverse group of echinoderms. They have a relatively simple body shape, with usually five elongated arms and a central round disc. They do not have eyes nor a digestive tract, but a mouth connected to a sac-like stomach, out of which food waste is simply vomited after digestion. And yet, they are very successful animals as they colonised virtually every marine habitat, from tropical to polar latitudes, and from shallow to the deepest seas. They are really common on seamounts and muddy deep-sea plains, making this group one of the dominant components of deep-sea ecosystems.
Seamounts are extinct underwater volcanoes. Ocean currents flow over the seamounts bringing a rich food supply to the animals that live around the peaks. Slow growing animals such as sponges and corals feast on tiny marine animals and bits of organic matter that float by. On Tasmanian seamounts, one particularly abundant species of coral (Solenosmilia variabilis) forms dense thickets or reefs that offer shelter to many benthic invertebrates such as sea stars, brittle stars, crabs and shells. About 40 brittle star species have previously been recorded on Tasmanian seamounts. Many have not been scientifically described and we know little about how they live and reproduce.
From a beam trawl on Investigator, we just collected a beautiful specimen of the brittle star Ophiocreas sibogae. It is almost always found clinging to a deep-sea coral, in this case Paramuricea sp. By climbing on the coral, the brittle star gets a better access to drifting particles on which it feeds. It also feeds on debris on the coral, a process that helps to keep the coral clean of silt. While this may at first seems like a one-sided interaction, deep-sea corals may also benefit from brittle stars, as was recently shown in the Gulf of Mexico. There, scientists could observe that brittle stars were most likely helping the corals to brush off the smothering “floc” of oil and dispersants after the catastrophic Deepwater Horizon oil spill of 2010. Much more remains to be discovered about the ecology and evolution of these fascinating creatures, and how they interact with other deep-sea animals.
So far, my experience on the ship has been very exciting and enriching! I just needed a couple of days to adjust to the ship movement and to my new “time zone” (I am on the 2 am–2 pm shift) at the beginning of the voyage, but now I am fully enjoying it. It is the first time that I have the opportunity to examine freshly sampled deep-sea brittle stars; until now I only worked with shallow water species or deep-sea specimens from museum collections. Many species we just collected were very colorful, a feature that is lost when the specimens are preserved in ethanol. I am really looking forward to the rest of the voyage!