Once the tide comes in, all a mussel has to do is open its shell and beat the minute hairs that line some of its body surfaces to waft tiny particles of food across the gills for nourishment and to deliver oxygen to the animal. However, Jörn Thomsen, Martin Tresguerres and colleagues from the Scripps Institution of Oceanography, USA, and two German institutions – Christian Albrechts University and the Alfred Wegener Institute – explain that it wasn't clear whether the mollusc also takes advantage of these micro-currents to dispose of the bivalve's ammonia waste at negligible cost. Many aquatic invertebrates excrete their toxic ammonia waste through expensive ion pumps that consume energy in the form of ATP, while some waste is carried out of cells by passive diffusion through a group of gas-permeable channels known as Rhesus-like proteins, ready to be washed away by the water. Little was known about how mussels excrete nitrogenous waste and through which organs; were the animals using the kidney, the gills or even the plicate organ, which is unique to this family of mussels and was believed to contribute to gas exchange for respiration? Thomsen and his colleagues decided to investigate.
Searching for the presence of Rhesus-like proteins in the mollusc's tissues (only one had previously been found in bivalves or crustaceans), Thomsen and Tresguerres located the channels in the plicate organ and to a lesser extent in the gill. However, there was no evidence of the channels in the kidney, which was surprising, as the team had suspected that the kidney would contribute to ammonia excretion. And when Thomsen and Franz-Josef Sartoris cautiously inserted a fine capillary into the mollusc's kidney to extract urine to find out whether the ammonia concentration in the urine was higher than that of the haemolymph, they discovered it was not.
Having ruled out the kidney as a site of ammonia excretion, the team turned their attention to the gills and the plicate organ. Testing for evidence of the V-type H+-ATPase protein pump – which is involved in ammonia excretion in other aquatic organisms – Thomsen, Nick Holland, Nina Himmerkus and Markus Bleich found that the protein is produced in the plicate organ. However, when they inactivated the pump, the ammonia production rate did not drop; the pump was not contributing to ammonia excretion, suggesting that ammonia was being excreted passively through the Rhesus-like channels. Knowing that passive diffusion through protein channels only works if the external concentration of ammonia is lower than the internal concentration, and that the external concentration of the waste can only be reduced if it is carried away by water currents, Thomsen stilled the cilia on the gill and plicate organ using dopamine and this time the ammonia excretion rate fell.
‘Water pumping by ciliary beating therefore appears to be an energetically inexpensive mechanism to remove ammonia from the body because mussels constantly beat their cilia to filter water for feeding’, says Thomsen, who is now keen to know whether another protein, the ammonia transporter, recently identified in mosquitoes, might also contribute to mussel ammonia excretion. Meanwhile, it looks as though mussels are getting a cheap deal on their waste disposal in addition to the low cost of meal deliveries, thanks to water currents generated by their ever-beating cilia.