Page 5 - Shell_growth_2008

Geo-Mar Lett (2008) 28:309–325 313

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adjustment of Dettman et al. (1999) for aligning δ O Mazzaforno (3% of those marked). Because of this, a
VSMOW seawater values to the VPDB scale: parallel experiment was set up at San Vito lo Capo to
examine growth patterns in the summer period. In July

SST Cð Þ ¼ 20:60 4:34
2006, 77 shells were marked, a range of shell sizes being

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* d O SHELL VPDBð Þ d O seawater VSMOWð Þ 0:27 selected to test if smaller/younger individuals grew more in
the summer than larger/older ones. This ‘summer cohort’
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The mean δ O seawater (VSMOW) values which we was left until October 2006 when 23 specimens (some 30%
determined for each locality were used to calibrate the of the individuals originally marked) were recovered. This
shell carbonate isotope values derived for the corresponding is much lower than the 60% recovery rate at the locality in
locality. The error of ±0.09‰ (one standard deviation) in January 2006 (also 3 months after marking), suggesting that
the instrumental analyses is equivalent to an error of ±0.8°C the summer is stressful for O. turbinatus, with high
(95% confidence interval) for seawater temperatures calcu- mortality.
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lated from δ O SHELL .
The oxygen isotope thermometry equation for marine Patterns of shell growth at San Vito lo Capo
aragonite of Böhm et al. (2000) was also used to calculate
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δ O SHELL temperatures; it yielded values consistently The data collected between October 2005 and October
lower than those from the Grossman and Ku equation, 2006 are summarized in Table 1. All the shell measure-
and with higher offsets from the measured SSTs. Therefore, ments (linear growth, diameter and height) show similar,
we use the latter equation in this paper. progressively declining proportions of growth from the
autumn to the summer. The seasons with the greatest
amount of linear growth were autumn and winter, with
Results 49.2% and 35.0% of the total yearly growth. Growth
progressively declined through the spring (10.9%) and
Shell growth summer (4.9%). The mean ‘seasonal’ linear growth was
7.6±0.4 mm between October 2005 and January 2006,
Here we consider growth only in shells found on at least 5.4±0.3 mm between January 2006 and April 2006, 1.7±
two consecutive visits to the shores. The numbers of 0.2 mm between April 2006 and July 2006, and 0.8±
marked individuals recovered decreased over time; at the 0.1 mm between July 2006 and October 2006. Inter-
end of the experiment, 30 shells were recovered at San Vito individual variability is noticeable, especially in the seasons
lo Capo (25% of those originally marked) and three at with greatest growth. In the spring and summer some

Table 1 Shell growth statistics from the mark-recapture experiment at San Vito lo Capo (started in October 2005 when 120 shells were marked)

Growth intervals Oct-05 to Jan-06 Jan-06 to Apr-06 Apr-06 to Jul-06 Jul-06 to Oct-06 Oct-05 to Oct-06
Marked shells recovered (specimens 70 (120) 69 (48) 48 (30) 30 (22) 30
found on previous visit)

Mean linear growth (mm) 7.6 5.4 1.7 0.8 16.2
Standard error of mean 0.40 0.25 0.17 0.14 0.71
Minimum linear growth (mm) 1.0 0.8 0.0 0.0 7.8
Maximum linear growth (mm) 14.3 9.0 3.4 2.3 21.4
Seasonal % 49.2 35.0 10.9 4.9 100.0
Mean diameter growth (mm) 1.2 0.8 0.2 0.1 2.3
Standard error of mean 0.07 0.05 0.03 0.02 0.15
Minimum diameter growth (mm) 0.0 0.0 0.0 0.0 0.5
Maximum diameter growth (mm) 2.6 1.8 0.6 0.4 3.9
Seasonal % 50.8 35.5 10.3 3.4 100.0
Mean height growth (mm) 1.5 0.9 0.3 0.2 2.9
Standard error of mean 0.07 0.07 0.04 0.04 0.15
Minimum height growth (mm) 0.0 0.1 0.0 0.0 0.9
Maximum height growth (mm) 2.8 1.9 0.8 0.6 4.5
Seasonal % 51.3 32.1 11.3 5.3 100.0
The data are the growth increments laid down in the interval between visits and show growth for the autumn/fall, winter, spring and summer
seasons, and for the whole year of the experiment