Consistency in growth line counts in bivalve specimens

Use of growth increments in invertebrates as tools in paleoecology, archaeology and geophysics depends in part on the consistency with which the increments are formed and the consistency with which they are recognized by the researcher. Regarding this consistency, our research in effect tested the hypotheses that: (1) growth line counts are consistent between specimens (a) of different size or age, (b) from different tidal heights, and (c) of different taxa; and (2) multiple, independent counters obtain consistent results in line counts made on the same sample. The bivalves Chione fluctifraga (Sowerby, 1853) and Protothaca staminea (Conrad, 1837) were used. The field location for specimen collection and growth was upper Newport Bay, southern California. Notched specimens of various sizes were grown for 47 days at different intertidal locations. Line counts were made from photomicrographs of acetate peels of shell cross-sections. In C. fluctifraga, significantly fewer lines were counted both in larger specimens (P<0.001) and in older specimens (P<0.001). Also in C. fluctifraga, intertidal location had no significant effect (P>0.50) on line counts. There was no significant difference (P>0.10) in counts between C. fluctifraga and P. staminea. Although the six counters in the multiple counter study disagreed significantly (P<0.001) in absolute counts they were in excellent agreement in their ranking of specimens by counts. Such a ranking of counts may be useful in some growth increment applications.