Flat-tip paddock (Penitella penita) are clams that slowly bore into rocks, clay or firm mud over the course of their lifetime (1). Their shells are white with a brown periostracum, a thin organic coating, and can grow to lengths of 7.6 cm (1). P. penita shells are elongated with rounded ends, with the anterior more rounded than the posterior (1). The roughened anterior end is used for boring into their substrates, up to depths of 15 cm (1). Upon settling on a rock P. penita have two distinct life phases; a boring phase with an apparent external muscular foot and a non-boring phase where the foot is reabsorbed and the anterior ended is closed with a callum (2). Once they stop boring, P. penita become sexually mature (1). The age of maturation depends heavily on the hardness of the substrate, ranging from 3 years in shale and 20 years on harder rock (1)(2). They are filter feeders, extending their siphon during high tide (2).
Cowles, D. (2006). Penitella penita (Conrad, 1837). https://inverts.wallawalla.edu/Mollusca/Bivalvia/Myoida/Pholadidae/Penit...
Evans, J. W. (1968). FACTORS MODIFYING THE MORPHOLOGY OF THE ROCK-BORING CLAM, PENITELLA PENITA (CONRAD, 1837). Journal of Molluscan Studies, 38(2), 111-119.
- rocky intertidal