Monday, January 16, 2012

Deep Sea Nudibranchs and Ocean Acidification: A Meditation

My favorite nudi pick ever, Nembrotha kubaryana Credit: NatGeo
I would like to apologize in advance for this long post, this is really just a stream of though exercise for me. Please comment!
Nudibranchs have always been some of my favorite marine animals. Their defensive mechanisms and bright coloration (called Aposematism) have always elicited fascination from me. My research project (please see Research Project ) on one particular species, Onchidoris bilamellata, revealed to me the amazing toughness of these shell-less creatures, despite their delicate appearance. They are found all over the world, from beneath the ice in the frigid Antarctic Ocean to temperate tidal pools to warm tropical reefs. They may be tiny, but they are found in every single habitat on earth. Everywhere, it seems, but the deep ocean, specifically hydrothermal vents, cold seeps, and methane seeps.
Dendronotus comteti Credit: SSF
    Why is this? Sure, the deep sea is an enormously harsh environment. But hundreds, possibly thousands, of species of animals live at these sites, from almost every phylogenetic clade of life (except Plantae). The species here are of such great diversity and have such fantastic adaptation that one would think that one would think that such  fantastic adapter like the nudibranch would be able to adapt to these demanding habitats. Yet, this is not the case. Only one species has been named to date (Dendronotus comteti)in the Atlantic and a few unnamed specimens seen in the Bering Sea (Source 1).Why are there so few specimens in such a small area of this hardy organism in the deep sea?
Vent Snails Credit: DSN
Vent Mussels Credit: MBARI
        Although this question is unanswerable at the moment, I’d like to pose another interesting quandary. Nudibranchs may be rare in the deep sea, but mussels and snails are hugely abundant at these depths. Why would other shelled mollusks be much more present in the this environment than an animal as adaptive as the nudibranch? The mystery deepens when we realize that hydrothermal vents often are extremely acidic – around 2.8 pH. Since high acidity keeps dissolved calcium carbonate from settling out, how are there shelled mollusks that live at these vents? Calcium carbonate is necessary for building a shell, and if high acidity prevents snails and mussels from taking calcium carbonate out of the water, how can they grow? Acidic waters can even begin to dissolve shells, so how do they survive? One would think that snails and mussels would not be able to live in these acidic environments, leaving a niche open for the shell-less nudibranchs.

      What does this mean for the ocean of the future? Increased carbon dioxide in the atmosphere and in the water is quickly acidifying our oceans, and it will be more and more difficult for shelled mollusks (as well as a host of other hard-shelled creatures) to grow, much less exist. One would think that being shell-less like the nudibranch would be a great benefit in these future acidic seas. Yet, we still face the perplexing dilemma of the acidic vents. Perhaps these snails and mussels have a special mechanism for absorbing CaCO3, and perhaps nudibranchs have one physiological trait that prevents them for living successfully at depth or near vents. Either way, I am perplexed by the question I’ve raised, if anyone has a hypothesis, please comment below!

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