The gastropod Nucella (dogwhelk) is a carnivorous snail found in rocky intertidal zones where it feeds on barnacles and blue mussel. Its a tough life. Nucella living on exposed coastlines are hammered by heavy surf. During low tide the snails face danger from the dry air and burning sun. Returning ocean waters at high tide bring other invertebrate carnivores, ones that consider Nucella a fine meal. These various threats put conflicting demands on Nucella. A larger opening (aperture) enables the snail to better withstand heavy surf by hanging on to its rocky home with a larger “foot”, but the opening is also where the snail is most vulnerable to attack from other carnivores. Evolutionary forces over time have shaped the snail’s shell to meet these different threats.

Zoologists have noticed an interesting fact about Nucella. At least some populations have morphological plasticity (Palmer 1992). That is over time these populations can adjust their shell’s shape to better protect against the specific threats found in their portion of the coast, so different populations of the same species of Nucella have different shaped shells depending on their particular environmental location. This morphological plasticity also allows these populations to adjust when the local conditions change over time. They respond, for example, to climatic shifts that result in increased or decreased storminess by shrinking or enlarging the size of their aperture.

Paleoclimatologists and archaeologists have analyzed Nucella assemblages dating to different time periods to help reconstruct climate change (Andrews et al. 1985). The basic method is fairly simple. They use the ratio of the aperture height to total shell length. Relatively large apertures are associated with high wave energy environments and small apertures with low wave energy environments. They use the ratios recorded for modern Nucella from different wave environments to interpret any archaeologically documented variations.

To my knowledge this approach has not been attempted by any Alaskan archaeologists. So last fall I asked one of my lab students, Shelly Love, to conduct a pilot study of the Aniakchak Nucella from one set of stratified midden samples. She found 42 shells complete enough to measure from 7 shell midden lenses. Although based on a small sample size, her results were very intriguing. In the Aniakchak Wiki she wrote,

The Nucella ratios, averaged for each level, suggest that Aniakchak Bay was initially inhabited at a time when storms were fairly severe (higher ratios equate to longer shells and less turbulant waters, lower ratios represent short shells and turbulant waves). Wave action decreases as storms decrease in severity, so the analysis suggests that the weather grew increasingly calm for the people of Aniakchak Bay.

_{Nucella ratios (aperture size to total length). The longer bars suggest less stormy conditions. Level 10 was the deepest stratigraphic level excavated in Unit N449 E454.}

Given these intriguing results we decided to pursue this analysis. Shelly will be coming to Aniakchak this summer to measure modern Nucella in the intertidal zones around the site. We hope that we can study both sheltered and exposed beaches (although everything close to the site is pretty exposed). Shelly has contacted some of the zoology experts in invertebrate biomechanics, especially Dr. Richard Palmer at the University of Alberta. Palmer warns us that some four species of Nucella inhabit the north Pacific coast. The degree of morphological plasticity for each of these species, especially at the population level, is poorly known. He will be assisting Shelly in her species identification and in assessing the modern data.

Our ultimate goal is to better understand why people largely abandoned Aniakchak around 1300 years ago after having lived successfully in the bay for over 300 years. One set of hypotheses we are testing has to do with changing environmental conditions, habitat degradation, and resource depletion. The Nucella ratios should help us better understand this complex environmental situation.

Andrews, M. V.,
D. D. Gilbertson, M. Kent and P. A. Mellars 1985. Biometric Studies of Morphological Variation in the Intertidal Gastropod Nucella lapillus (L): Environmental and Palaeoeconomic Significance. Journal of Biogeography 12:71-87.

Palmer, A. R. 1992. Intraspecific variation in three species of rocky shore gastropods from Hong Kong: Correlations among habitats and a comparison with temperate species. Pp. 649-691 in B. Morton, ed. Proceedings of the Fourth International Marine Biological Workshop. Vol. 1. General Proceedings. Hong Kong University Press, Hong Kong.