Factors influencing shell growth inCerastoderma edule

Abstract

Shell increments separated by thin growth bands that are laid down at every tidal emersion allowed detailed short term measurements of growth rate to be made from sections of the shell ofCerastoderma edule. The growth rate showed a coefficient of variation of 22% between individuals from the same level and locality. This, together with the effect of season and position, accounted for 70-95% of the total variability. Of the environmental influences, tidal level was the most important, the rate of growth being proportional to the fraction of time that the animal was immersed and able to feed. Apart from this factor, growth rates remained remarkably uniform throughout the different localities and situations studied in the Menai Straits. There were small (5–10%) but significant differences in growth rate during the spring-neap lunar cycle. At mid and high tide levels and when submerged on a raft the animals grew fastest at springs and slowest during neaps. However, near low water the least growth occurred at springs and the most at neaps. This anomaly is ascribed to the reduced periods of immersion during spring tides and continuous immersion during neaps at this level. Increase in water flow is believed to account for greater growth rates at springs at mean tide level and on the raft. The diurnal variation in illumination had no significant effect on growth rate in field experiments and the beneficial effect of light observed in the laboratory is considered spurious. Abnormal shells demonstrated that apposition of the shell rims and associated mantle edges plays no part in laying down tidal growth bands.