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324 Geo-Mar Lett (2008) 28:309–325

cessation of growth, at a number of localities across the ing of isotope values over periods of slow or no
western Mediterranean basin. Our monthly surveys in NW growth.
Sicily show that growth lines appear just behind the – Smaller/younger shells have significantly larger growth
apertures of shells in the late summer to early autumn increments than larger/older ones at the same locality
period, suggesting a period of growth cessation in the and selection for isotope analysis should favour smaller
summer months probably in response to environmental shells.
(thermal) stress. Schifano (1983) showed that shells grow – Growth checks, when widely spaced, can be useful
proportionately more in thickness during this period than at markers of annual shell growth increments when
other times of the year. Alyakrinskaya (2004) suggested sampling for isotope palaeotemperature sequences. It
that the solidity and weight of the shell of O. turbinatus is unlikely that growth checks in O. turbinatus can be
protects the animal not only from mechanical damage but used to establish the ontogenetic age of specimens
also from excess water loss, which can be particularly because there is variability in their expression (espe-
significant when air temperatures are high. Houlihan and cially in young shells or in the earlier growth phases of
Innes (1982) showed that although O. turbinatus is capable older shells).
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of respiring and moving in both air and seawater, the – δ O values in shell aragonite samples track annual
energetic costs of movement are about double in air and variations in SSTs, indicating that the species has good
rise with increasing temperature. The thermal ecology of potential for investigating seasonal patterns of prehis-
this species clearly depends on a range of morphological, toric hunter–gatherer shellfish foraging.
18
physiological and behavioural factors but thickening of the – The offsets between calculated δ O SHELL and mea-
nacreous layer in the summer could be an adaptive feature sured SSTs indicate a closer approximation in the
18
during periods of high insolation. If energy and materials cooler months of the year, with δ O SHELL SSTs
are being sequestered for shell thickening, this could either significantly underestimating measured SSTs in the
reduce or halt linear shell growth. hotter months when SSTs higher than 25°C are not
Menzies et al. (1992) showed there was significant recorded. Shells from late Pleistocene and early
mortality in O. turbinatus exposed to atmospheric temper- Holocene sites might more faithfully record annual
atures of 35°C. Air temperatures above 35°C are frequently temperature ranges, given that summer SSTs would
recorded between late spring and early autumn in NW have been generally lower.
Sicily, with temperatures often reaching 40°C between June – O. turbinatus shows complex adaptive variation in
and August (Gianguzzi and La Mantia 2000). Through the growth, physiology and behaviour leading to variability
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summer, atmospheric temperatures in NW Sicily rise to or in shell growth and δ O SHELL values between indi-
above the upper thermal tolerance of O. turbinatus, viduals at and between localities, as well as in
probably causing inactivity, especially in larger/older successive years.
individuals which tend to occupy the higher intertidal zone.
Growth checks probably arise during prolonged spells of Acknowledgements KDT and MAM are grateful to the Leverhulme
18
hot temperatures, as shown by the growth lines and δ O Trust, UK, for Research Grants F/07196/B and F/07196/D; KDT
values in the marked individual from Cala Grande. The thanks the Natural Environment Research Council, UK, for Grants IP/
degree of expression of growth checks in shells is variable 780/0503 and IP/854/0505 for stable isotope analyses at NIGL (NERC
Isotope Geosciences Laboratory, Keyworth). Invaluable technical
and related to age (being more pronounced in larger/older assistance was provided by Carol Arrowsmith and Joanne Green of
shells), limiting their usefulness for determining age NIGL. We thank Enrico Ryunosuke Crema for preparing the figure
structures of populations or archaeological assemblages. showing the study localities, and C. Fred T. Andrus, David P. Gillikin,
Bernd R. Schöne and an anonymous reviewer for their thoughtful and
constructive comments on this paper.

Conclusions
References
The results from the studies at localities in NW Sicily have
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Bailey GN, Deith MR, Shackleton NJ (1983) Oxygen isotope analysis
selected carefully, avoiding those with many evident and seasonality determinations: limits and potential of a new
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