Boring In, Climbing Up

The free-living larva of a Mahogany Date Mussel, Leiosolenus bisulcatus (d’Orbigny, 1853), will settle on a shell or coral colony, and will bore into it away from the surface until it reaches its mature, maximum size; there it will remain until the end of its days. Mahogany Date Mussels bore by dissolving calcium carbonate using a special boring gland present in the mantle. They are also capable of coating the internal surface of their boreholes using a special lining secreted by the mantle. The Mahogany Date Mussel is the only species in the genus Leiosolenus known to bore into dead and living coral colonies.

Internal (top) and external views of a valve of Leiosolenus bisulcatus; 1 mm = 0.04 inch. Photos by José H. Leal.

When dwelling within living corals, the mussel needs to cope with the growth of the coral colony, which is faster than its own. Mussels are filter-feeders, sucking up water to filter their meals of microscopic plants. They need to stay at a constant and short distance from the surface of the growing coral, otherwise coral growth would smother them, preventing them from feeding, ultimately entombing them for eternity.

Two boreholes of the Mahogany Date Mussel on a colony of the coral Siderastrea stellata from Rio de Janeiro State, Brasil; 1 mm = 0.04 inch. Photo by José H. Leal.

The solution? After reaching maximum (final) size, the mussel will line the bottom end of the borehole with a succession of "false floors" that help prop the bivalve up in the growing coral to maintain its short distance from the surface, while the boring gland works to keep the borehole open. A Mahogany Date Mussel moves up, “outward,” keeping pace with the growth of the coral colony.

Different stages in the settlement and growth of the Mahogany Date Mussel in a living coral colony. 1: Larva settles out of the free-living stage (plankton); 2–3: Juvenile grows and bores into calcium carbonate structure, coral colony keeps growing “up”; 4: Mussel attains maximum size; 5–6: Mussel manufactures “false floors” to keep constant distance to borehole opening; 7: Mussel shell encased within coral "rock" after death.

Cross section of a Siderastrea stellata coral colony from Rio de Janeiro State, Brazil, showing a Mahogany Date Mussel borehole with a series of "false floors." The white bivalve on the right is another coral borer, the Atlantic Petricolid, Choristodon robustus; 1 mm = 0.04 inch. Photo by José H. Leal.

*In 1981, in association with two colleagues from Museu Nacional in Rio de Janeiro, Brazil, I completed a report on the abundance and mode of boring of Mahogany Date Mussels found along the coast of Rio de Janeiro State. For reasons that are now irrelevant, that report was never published. The images in this section were in the original manuscript. In 1988, a very comprehensive study on the same subject was published in the Journal of Molluscan Studies by Paula J.B. Scott of McMaster University in Canada. Her paper can be found here.