Old Dirt – New Thoughts

March 6, 2007

From Snails to Storms

Filed under: Aniakchak,Shellfish,Zooarchaeology @ 11:58 am and

Orange snail with blue mussel and barnacle Can this snail tell us about climate change? We think it can. We think studying Aniakchak Bay’s modern snails and those we recovered archaeologically will tell us something about the intensity of wave action (or storminess) 1500 years ago compared to today. Storminess and exposure to waves would have been an environmental condition important to the Aniakchak Bay villagers since their economy was so heavily focused on the harvest of maritime resources. We hope that our analysis of snail biomechanics will give us insight into the climatic conditions faced by the Aniakchak villagers during their occupation and especially during the abandonment of their home on the bay.

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.

Nucella measurmentsPaleoclimatologists 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 N449 Test (Fall 2006 Results) 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.


  1.   Patrick Saltonstall — March 6, 2007 @ 12:34 pm    

    Cool stuff!

    Just some thoughts from me. Have you considered that ‘storminess’ might also reflect changes in coastal geomorphology? Perhaps a higher or lower sea level exposed a shelf that waves are more likely to crash onto? (ie effected the slope angle of the shoreline). Also maybe the creation of an offshore sandy beach decreased ‘storminess’? Just some ideas.


  2.   Brian — March 6, 2007 @ 3:22 pm    

    Wow Patrick, you are quick to respond!

    You are correct about the potential meaning of our results (assuming our study provides any useful information). What we are measuring is Nucella’s response to wave action. You’re right that we should be cautious about immediately attributing a change in wave energy on Aniakchak’s beaches to climate change. What we’ll probably need is some additional studies. Do you have Nucella in your Kodiak middens? May be we could get someone to expand this study across the Gulf of Alaska. That could be interesting.

  3.   Shelly Love — March 6, 2007 @ 8:23 pm    

    I like the idea of incorporateing a geological survey of the Aniakchak shoreline. Brian, don’t you know a geologist who has studied the formation of the Alaskan Peninsula? Maybe I could ask him for some suggestions?I also think it would be of interest to archaeologists and biologists to expand the study (Nucella in the Alaskan Peninsula and Aleutian Islands are poorly understood). Patrick, thank you for your thoughtful reflection.


  4.   Patrick Saltonstall — March 7, 2007 @ 2:55 pm    

    My shellfish ID is pretty pitiful (basically ‘clams’, ‘mussles’, and ‘whelks’). But if you have Nucella doen on the Lower Alaska Peninsula then I’m sure we have them on Kodiak. Plenty of clam midden samples from lots of sites to look through too!

    I do know that James Taylor at University of Washington is examining shells from the Karluk one site (he’s interested in isotopes, cold water warm water stuff). Ben fitzhugh (a prof at UW) also examined seasonality of site use by looking at clamshells from the Uyak site. Might give Ben a call at the University of Washington. Most of the Kodiak fauna is there anyway – and Ben might have a student interested in measuring Nucella shells for you.

    as I said – cool stuff and I am interested in what you turn up.


  5.   David Gilbertson — March 29, 2008 @ 10:39 am    

    David Gilbertson writes – When we carried out the original study of morphological variation in the common dogwhelk Nucella lapillus in the early 80s, we went to great lengths to establish that the changes that were being observed in the ancient Mesolithic coastal shell middens were NOT due to changes in coastal configuration compared to the preent day. The ancient (~5000) year old coastal geography of the inner Hebridean island of Oronsay (the west coast of Scotland – find Oban and go south west; and then immediately south of the island of Colonsay: there are a number of Scottish islands named Oronsay) was reconstructed from geomorphological evidence. This coastal reconstruction served to re-inforce the conclusion that the overall mesolithic shoreline was less stormy than today. (see also Paul Mellars (1987)Prehistoric Human Ecology on a Small Island. Edinburgh University Press). The initial investigation was fortunate in the shape and situation of the island of Oronsay; and in the other information available about it (search under Oronsay Fieller, Gilbertson, Olbricht, Jardine). As the commentator above suggests – in this type of study it is vital to establish the coastal geomorphology of the time from independent evidence other than that of the dogwhelk shape, and for this to be used as part of the evidence base used to reconstruct the palaeo-oceanography. It could be that other molluscan taxa might also be sensitive to details of the wave climate – e.g. netted dogwhelks. The subject of palaeostorminess at the coast might also be addressed more directly by investigating the remains of crabs if they occur in number in coastal middens.

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