Bivalves are not my forte. In fact, I couldn’t name forty. But despite my obvious failings in the arena of bivalvology, I’ve been captivated by an odd bunch of animals belonging to the family Solemyidae. A local species kept cropping up in samples when a couple of my Honours students were looking for introduced Musculista senhousia (Yes, it’s a bivalve. But I needed someone else with me to identify it.) in sea grass beds in Port Phillip Bay. What makes them interesting (the Solemya not the students) is their mutualistic relationship with chemoautotrophic bacteria.
Solemya live in burrows in sea grass beds. The thin surface layer of the beds is well-oxygenated—a great place to be. But you don’t have to dig very deep to hit anoxic mud. Exposing this black ooze releases the rotten egg smell of hydrogen sulphide. The gas interferes with normal cell processes. Its effect is rapid. For most animals and plants, that mud is lethal.
Solemya excavates a burrow down into the danger zone. It’s not a super-bivalve, immune to hydrogen sulphide, so how does it cope?
Very well as it turns out. Solemya is home to a bacterium that thrives on sulphides. Thiomicrospira lives inside specialized cells in the bivalve’s gills, happily using energy from the gas to transform carbon dioxide into nutrients. (This is a process similar to photosynthesis in plants and algae, except the bacteria employ energy harvested from chemicals rather than from light.) So effective is this relationship that some species of Solemya have not only lost the use of their digestive tracts.
To ensure that the next generation of Solemya gets its share of Thiomicrospira, mothers pass on the bacteria in their eggs. The kids’ inheritance is a permanent food supply.
Lots of deep sea animals cultivate the acquaintance of chemoautotrophic bacteria. They have to—they can’t get any help from photosynthetic organisms because sunlight doesn’t penetrate this far.
But, although it lives in shallow-water, Solemya eschews photosynthesis. It sticks to its sulphur-utilising bacteria. This allows it to exploit a microhabitat that might otherwise be unavailable.