The effects of dredging on shell formation in the razor clamEnsis siliquafrom Barrinha, southern Portugal

Shell growth of the razor clam Ensis siliqua (Mollusca: Bivalvia) from southern Portugal has been analysed using both surface growth rings and internal shell microgrowth patterns. The growth rate estimated from an analysis of the growth rings is slower (von Bertalanffy growth, constant K=0·27) than that determined from the annual narrowing of the internal microgrowth patterns present in shell sections (K=0·65), although both methods predict a similar asymptotic length, L∞, of 144·8 and 139·6 mm, respectively. The Barrinha razor clam population occurs in a heavily dredged area and an analysis of shell sections reveals the presence of a series of shell margin breaks consisting of deep clefts in the outer shell layer in which sand grains are embedded. It is suggested that these disturbances to shell growth are the result of repeated dredge damage. The frequency of the clefts increases with the size and age of the razor clams, and thus the shells provide a record of the intensity and frequency of unsuccessful capture or retrieval attempts. Cleft formation also occurred seasonally with the deposition of a small cleft during June, but these annual clefts were much less pronounced than those caused by dredge damage.