Amazing Dive - Part 2: Crab Convention

June 3rd @ 12:45pm – As we ‘flew’ across the seafloor, gliding over the forest of sea pens (and here I must note that it seemed every sea pen has a brittle star wrapped around it, making me wonder if there is some kind of mutualistic symbiotic relationship in the abyss), we came across a high concentrated group of crabs. Also of note was the fact that the seafloor was letting loose hundreds of bubbles from the depths of the mud. This didn’t bother the crabs, rather they seemed to be almost seeking out the bubble vents and mining the sediment around them.

It turns out that the bubbles are methane gas percolating up through a fairly significant fault below the seabed. The methane is consumed by mats of chemosynthetic bacteria, which is what the crabs feed upon by sifting through the sediments. Interestingly enough, as the crabs feed, some of the bubbles get trapped on the underside of their bodies and the temperature and pressure down here are in just the right combination for the methane gas to sublimate as it reacts with the water. One question that came up was whether the white crust we observed on the underside of the crabs was, in fact, gas hydrates. At least that is one possible theory.

Another possibility is that the crabs are cultivating methane and sulphur-eating bacterial colonies on their undersides and forarms, and they purposefully wade into the bubble streams to ‘feed’ their crop of bacteria. The crabs then eat a small amount of the bacteria for their own sustenance.

Apparently, this has never been witnessed by anyone before, so the discussion around the ROPOS control room was quite animated. I asked if we had anything on ROPOS that could obtain a gas sample for isotopic analysis, allowing us to possibly source the methane, but we didn't have a gas tube sampler on board on this dive.

I fortuitously managed to capture a video on my iPad of one crab who got too many bubbles trapped under him and it lifted him off the seafloor and flipped him end over end. The gas hydrates are a very interesting phenomenon, and potentially a huge source of energy... but also of vast quantities of a greenhouse gas (methane) that makes CO2 look like nothing.

Stay tuned for Part 3…

Blog post and photos by Scott Doehler 
Marine Educator