Monthly monitoring of the clam Venus verrucosa was carried out in the channel of Bizerta Lagoon (Tunisia) from September 2002 to August 2003. Approximately 100 individuals were collected on each occasion from between 7- and 9-m depth. Individual shell sizes (measured as the antero-posterior shell length) varied from 13.7–59 mm. Some specimens showed shell abnormalities that indicated a heterospecific association. The proportion of specimens affected by these abnormalities was high (38%) and a comparison of the relative growth of healthy and abnormal clams was made. The relative growth of shell thickness and weight parameters, were compared with the antero-posterior valve length measurements. We calculated regression equations for annual and seasonal changes in valve length with shell thickness (E/L) and with shell weight (W/L). Annual and seasonal assessments of relative growth (as compared with the antero-posterior valve length) in the two clam populations showed that growth was negatively affected in the abnormal population. Affected individuals contained more intravalve water than normal individuals. The different growth metrics all indicated that affected clams were less well developed than unaffected clams. The organisms living in association with Venus verrucosa have a negative impact on shell thickness, on shell dry weight and on biomass (as dry flesh). This biometric growth study indicates that hetero-specific associations stressed the studied affected clam population and retarded somatic growth. It is suggested that clam emergence, probably caused by adverse conditions, encourages the formation of unfavorable heterotrophic associations by invertebrate species.