The black date mussel (Lithophaga nigra) might not look like much more than a dark, inert splodge, easily passed over by divers. However, quietly and discreetly, aggregations of these little bivalves alter the marine landscape in enormous ways, as evidenced by their scientific name, which literally means ‘rock eater’.

The species is seldom seen. When diving, individuals must search below rocky overhangs to see the tip of the black mussel in its oval shaped burrow.


1. Only 4cm long, the entire mussel looks like an elongated date pit, which is how it got its name. They are filter feeders, straining plankton from the water from their permanent perch.

2. The black date mussel is the main cause of bio-erosion in Harrington sound, burrowing in to the rock at its perimeter and creating the Harrington Sound Notch, a cleft just under the water line that can be up to 9 feet deep.

3. At the beginning of their lives, microscopic floating mussel larvae settle on to rock, and burrow into it by softening the rock with acids, and scraping the sediment away with teeth like structures on one side of their shell. As they grow, the burrow extends deeper into the rock.

4. In areas where many individual mussels live close to one another, their dozens of burrows in the rock can weaken cliffs and make them unstable, or even cause them to crumble.

5. This activity is essential for producing new sediment, as the rock the mussels scrape away is ejected into the water column. However, the burrowing also causes dangerous erosion. Not bad for a little invertebrate!