Periodicity of growth marks in operculum of snail Astrea turbanica of Isla Natividad, BCS, Mexico

To support the use of opercula for age determination of the waby turban snail Astrea turbanica, results concerning features of its growth marks, its recognizing criteria and a test to show its formation periodicity, are presented. The opercula were obtained from samples of A. turbanica collected in Natividad Island, BCS, which is the principal commercial fishing zone of this species. The presence of one to three rings near the spiral was a useful criterion for recognizing the first annulus. One age group was formed by the total of one broad white band and one dark and thin band. The qualitative method of operculum edge analysis described a sinusoidal cycle with a frequency of one year in true annulus when plotted as a function of month, showing a periodicity in the annual marks formation. The white band of faster growth was formed from summer to autumn and was associated with a warm period.