Life History Traits of the Bivalve Mollusk Saxidomus Gigantea from the Coast of British Columbia: Insights for Paleoclimate and Archaeological Applications

The bivalve Saxidomus gigantea (butter clam) is the most commonly recovered shellfish species from archaeological midden deposits on the coast of British Columbia (BC). Shell middens containing this bivalve have been dated from ca. 7800 BP to the contact era, and their presence has traditionally been used to interpret paleo-diet and the utilization of shellfish resources in this region. However the potential for identifying the season of shellfish collection, and by proxy the season of site occupation as well as paleotemperature has been limited by (1) The uncertainty of the timing of growth band formation; (2) The ability to reliably identify annual increments; (3) An understanding of the influences of freshwater on growth pattern formation.

Modern shells collected alive on a monthly basis for a period of one year from Pender Island, southern BC, were analyzed to determine the life history traits of this species (timing of growth line formation, duration of growing season, growth rate etc.). Oxygen isotope analysis confirms the seasonal variation in growth increment formation, with the most positive δ¹⁸O values of -0.37‰ associated with winter line formation (annual growth lines), and negative values (on average, -3.13‰) with more rapid warm-temperature growth. Intra-annual increments show clear seasonal oscillations with broadest increments (age four: 137µm) in summer and very narrow increments (age four: 6µm) or a growth cessation during winter months. A fortnightly pattern of microincrement width identified by frequency analysis suggests a diurnal periodicity in formation of intra-annual increments.

This research provides new insights into the biology and seasonal growth cycles of the butter clam, which is crucial information for interpreting the season of shellfish collection and paleotemperature. Biological and local environmental factors must be taken into consideration when using this species to determine patterns of seasonal shellfish collection in the archaeological record.