Deposition of annual growth lines in the apex of the common limpet (Patella vulgata) from Shetland Islands,UK and Norway: Evidence from field marking and shell mineral content of annual line deposition

The abundance, accessibility and value of limpets as a source of food and bait for coastal peoples have resulted in their high frequency in shell middens worldwide. The limpet Patella vulgata is found in middens from the Mediterranean to Norway, and morphometric and sclerochronological analyses of its shell can provide insight into harvesting patterns and paleoenvironmental variables valuable in reconstructing climate. Previous work with P. vulgata has relied on lines on the exterior of the shell, or on lines exiting on the shell surface in shell cross-section, as annual or sub-annual markers. Shell damage may compromise these lines and limit the use of some shells, but growth lines are also found in the better-preserved shell apex. We investigated whether the growth lines in the apex of P. vulgata from two locations in Northern Europe are annual using calcein-marking and recapture. Investigations were performed at one site in the Shetland Islands (UK) and at one site at the northern limit of P. vulgata's range in Northern Norway. We also used laser ablation to measure the concentration of minerals in the shells of two individuals from Shetland to determine if patterns of minerals suggested as bioproxies for temperature and productivity varied annually. All individuals deposited one growth line in the apex during their year in the field, and the lowest ratios of Sr/Ca and Ba/Ca and to a lesser extent Mg/Ca were coincident with annual lines on the shells from Shetland. Growth at both sites was modelled using the von Bertalanffy growth function, and apex growth was nearly five times faster in Shetland than in Norway, probably a result of differences in temperature between the two locations.     

Analysis of the Size,Shape, and Modeled Age of Common Limpets (Patella vulgata) from Late Norse Middens at Sandwick,Unst, Shetland Islands,UK: Evidence for Anthropogenic and Climatic Impacts

Zooarchaeological faunal remains are commonly examined to investigate harvesting behavior. We determined limpet (Patella vulgata) shell size and shape, and estimated shell age from several middens at the Late Norse Sandwick South Site, Unst, Shetland, UK, whose strata represent distinct occupational phases (Phase 1: AD 1100–1200, Phase 2: AD 1200–1250, Phase 3: AD 1250–1350). Our goal was to determine if the many limpets found there could provide insight into Norse harvesting behavior. Shell length, conicity, and modeled age all declined between Phases 1 and 2, suggesting intensive, size-selective harvesting of limpets and a shift to harvesting lower in the intertidal zone between phases. Length and conicity varied in Phases 2 and 3 and no major changes seem to have occurred over these periods, indicating that harvesting maintained the limpet population at an impacted level throughout the later phases. The conicity decline between Phases 1 and 2 may also have been caused by increased storminess that accompanied the onset of the Little Ice Age. The mean length of modern limpet populations near the Norse site did not differ from the archaeological phases, but did vary among collection years. Limpets were 26% larger in 2015 than in 2012 and 2013, indicating that large interannual variations in population structure can occur over short time periods. Potentially the result of extreme storms removing small limpets, this result raises the possibility that size and conicity changes during the Sandwick South Site occupation, as well as in other early populations, could also be the result of environmental factors rather than human harvesting alone. We feel, however, that the most parsimonious explanation for the patterns we document is human harvesting.     

Shellfish Exploitation in the Western Canary Islands Over the Last Two Millennia

ABSTRACT

The residents of the Canary Archipelago consumed limpets since the arrival of humans ～2500 yrs. ago, and these harvested gastropods were deposited in large coastal shell middens. This work preliminarily explores shell margin oxygen isotope composition (δ¹⁸O) and body size of the black limpet (Patella candei d’Orbigny, 1840) from archaeological sites in the Canary Islands to assess possible seasonal variability and intensity of shellfish collection throughout the late Holocene. The shell margin δ¹⁸O values of 100 shells (radiocarbon dated between ～500 and ～1800 cal. yr BP) were analysed to estimate sea surface temperature (SST) at time of death. Paleotemperature estimates suggest shellfish harvesting was not year-round, and was avoided in the cooler months (when SST?<?20°C). This pattern differs from most higher latitude Mesolithic and Neolithic human groups, which gathered shellfish year-round, targeting winter more heavily. Preliminary body-size measurements suggest shell sizes have experienced a decline from aboriginal times to the present, which possibly resulted from increasing anthropogenic pressures. During aboriginal inhabitation, maximum adult shell size remained stable, suggesting that present-day harvesting practices are more intense than harvesting from aboriginal human groups. This intensive collection has likely diminished the average adult size of limpet populations in the islands by ～27%.