Growth Lines in CHIONE FLUCTIFRAGA and PROTHACA STAMINEA (Bivalvia): Precision of Counting and Comparisons of Size, Age, Intertidal Location and Taxa

Because formation of growth lines in the shells of bivalve molluscs is affected by environmental parameters, the growth lines should contain much information about the environments in which the animals live. However, the use of growth lines in ecology· and paleontology is limited by ambiguity in their identification. Chione fluctifraga and Protothaca staminea were collected, marked, and replanted. C. fluctifraga were grown at three intertidal levels over a vertical range of one meter. In addition, P. staminea were grown at the middle level. All specimens were re-collected after 47 days. Acetate peels of shell cross sections were use for growth line counting. Precision of counting was analyzed·by having six persons count lines on the same specimens. There was a significant difference between counters on number of lines per specimen, but excellent agreement in ranking of specimens by counts. This makes relative comparisons possible although absolute values may not be obtainable. Younger (usually, smaller) specimens grew faster and formed more growth lines. Size was not always a good indicator of age. • This makes it important to use equivalent age classes for studies involving growth line count- There was an unexpected difference in growth rate between specimens grown at different intertidal heights. Those at the lower intertidal level grew less than those at the highest level. There was no difference between species in number of growth lines formed