Meeting Abstract
Giant clams (genus Tridacna) are the largest living bivalves and are ecologically important members of Indo-Pacific reefs. Several species exhibit the remarkable ability to bore fully into coral skeletons and are major agents in reef bioerosion. The mechanisms facilitating boring in these species however, have remained unresolved. Although acid secretion has been implicated, early failures to detect acidification signals led to hypotheses involving purely mechanical or non-acidic chemical mechanisms such as shell rasping and calcium-chelation. Here, we definitively addressed the question of acid secretion in the boring giant clam, T. crocea. Using novel 2D-imaging optode technology and manipulating clams so that they press their pedal mantle against pH-sensitive foils, we show this organ is able to acidify the contact surface to at least 3 pH units below seawater. Further, we demonstrate that vacuolar-type H+-ATPase (VHA), a H+-transporter implicated in acid secretion in other epithelia, is highly abundant within T. crocea pedal mantle and localized in the apical membrane of cells in contact with coral skeleton. Similar localization patterns of VHA have been demonstrated in human osteoclasts and in Osedax worms suggesting that active H+-secretion by VHA may be a common mechanism for dissolving carbonate substrates, an exciting example of convergent evolution. Our identification of VHA in giant clams and demonstration that bored surfaces are chemically acidified rather than mechanically rasped solves a decades old mystery and sets the stage for a greatly improved understanding of both the cause and fate of eroded reef carbonates.
