Influence of tides on microgrowth bands in Cerastoderma edule from Norway

Abstract

At an intertidal site on the island of Tautra in Trondheimsfjord, Norway, cockles Ceras-toderma edule L. were sampled from 121 cm (Stn A) and 66 cm (Stn B) below tidal datum. Shells were sectioned longitudinally, polished, etched and photographed under a scanning microscope to reveal microgrowth bands. Tidal heights were calibrated against a tide recorder in Trondheim harbour, thus providing immersion times and tidal heights for Stns A and B. The tidal and growth band data were subjected to time-series analysis. At Stn A, immersion time is maximal at neap tides while tidal height is maximal at spring tides, whereas at Stn B, maxima of both values occur at spring tides. Although both neap and spring tides have a cycle of 28 tidal periods, a cross-correlation analysis should show a phase shift of 14 periods at Stn A and no phase shift at Stn B. The tide recorder data supported this hypothesis. If tides are a predominant influence on microgrowth bands similar patterns should be observed in band patterns. The data showed that this was the case. After correcting the data for changes in growth with age, growth was found to increase in February-March, decrease in Apnl-May and was maximal in June. Our data clearly support the contention that it is tidal and not diurnal changes that influence microgrowth bands in intertidal populations of C. edule.