MICROSTRUCTURE OF VALVES AND BORING LININGS OF THE ENDOLITHIC “ROCK-EATER”, LITHOPHAGA SPATIOSA (BIVALVIA)

Endolithic bivalves are common bioeroders in hard carbonate substrates. The mytilid genus Lithophaga (literally “rock-eater”) is one of the longest-lived and most diverse members of this group. Evidence of a full endolithic life mode by such bivalves appears as early as the Late Triassic. It is generally accepted that the Lithophaga animal is a biocorroder, utilizing mainly chemical means to excavate a clavate shaped chamber in a rock or shell substrate. After excavating its boring, the bivalve secretes a carbonate lining that thins towards the base of the bored chamber. Hypotheses regarding the mechanism(s) of boring activity include (1) secretion of a mucous able to dissolve and break apart carbonate material; (2) a discharge of metabolically derived CO2 built up in the animal’s tissues; or (3) a specialized behavior associated with shell resorption, which is an ability general to all molluscs. How these potential mechanism(s) of boring interact with secretion of the animal’s carbonate lining is not fully understood, although they likely are intimately connected. To better understand this connection, valves and borings of modern Lithophaga spatiosa were collected, and their microstructure was observed using scanning electron microscopy (SEM). The specimens were collected at Cholla Bay and Las Conchas Beach in Puerto Peñasco, Sonora, Mexico. SEM examination revealed visible differences in the microstructure of the animal’s valve and carbonate lining of the boring. The valve microstructure is consistent with that of bivalves in general. The lining of the boring differs in consisting of tiny elongate prisms. Aggregates of these prisms are homogenous with no consistent arrangement or pattern. In thin sections, a distinct stratification is apparent in the carbonate lining. Difference in microstructures of the shell and boring lining suggests differing levels of biologic control on carbonate precipitation.