Clam (Corbula modesta) as a bio-indicator of environmental condition: the case of Chabahar Bay

Clam shell can be used to reconstruct recent and past environmental variations at high-resolution time scale. During processes of growth increments, carbonate structure can record subdaily (tidally) to seasonal information of environmental variations such as water temperature, salinity and tidal change. This study aims to investigate the potential of clam shells as an environmental recorder, observe the visibility of micro-growth lines in shell cross-sections of Corbula modesta from intertidal and subtidal areas and interpret the periodicity of the shell patterns during Pre- and post-monsoon in Chabahar bay. Clams were collected on a fourteen days basis and the shells were sectioned from the umbo to the ventral margin, polishing, etched and observed micro-growth lines and increments under a light microscope. The majority of the clam shells showed that the number of growth lines in shell structures was close to the number of tidal emersions (P 0.05) and the growth in crements width in intertidal and subtidal areas were significantly different (P 0.05). Multiple regression analysis was used to assess independent associations between shell mean increment width and environmental parameters. Study model showed that 60.8% of the variation in shell growth could be explained by temperature, salinity, rainfall and tidal change. Individually, temperature and salinity made the greatest unique contribution to explain shell growth, respectively (P 0.05). Increment widths in monsoon season were much narrower and show sudden bunching of growth increments at the onset, with rapid recovery. This study shows monsoon in shell structure are considered markers for identifying the period of seasonal variation. These findings provide a basis for the interpretation of the temporal changes in shell microgrowth patterns in terms of environmental conditions of clam shells.