Page 6 - Shell_growth_2008

314 Geo-Mar Lett (2008) 28:309–325

individuals grew very little, if at all. The only individual ments (linear growth, diameter and height) show a
which had not grown at all in the spring had suffered recent progressively declining proportion of growth from the
damage to the edge of its shell (probably an attempt at autumn to the spring, with an increase in growth in the
predation by a crab). This had been repaired but probably summer. Most shell growth occurred in the autumn
had a retarding effect on linear growth. Four individuals (44.5%) and winter (32.8%), the least in the spring
appear not to have grown during the summer, one having a (6.0%), while a significant amount of growth was
repaired aperture. registered in the summer (16.7%). Again, there is inter-
For the autumn, winter and spring, there are significant individual variability, especially in the seasons of greatest
negative correlations between amounts of seasonal growth growth. In the spring, six individuals appear to have grown
and original shell size, although the strength of these very little or not at all, with one having suffered damage to
decline with time (Table 2). In the autumn, smaller/younger the shell. The summer data, based on a small sample size
individuals grow significantly more than larger/older shells, (n=3), show that up to 4.8 mm of linear growth can be
while in the winter and spring there is less difference gained in this season.
between sizes. There are also significant negative relation- The regression analyses show only one significant
ships between the amount of linear growth recorded in association, between the amount of linear shell growth
April, July and October 2006, and shell diameters recorded recorded by April and the original size of the shells when
at the previous visit (Table 2), with the exception of the marked (Table 4). This shows that significantly more
summer growth sample (shells recovered in October 2006). growth was put on by smaller/younger individuals than by
Finally, there is a significant negative relationship between larger/older ones in the winter. During other seasons, the
the total amount of annual growth of shells over the quantity of growth does not appear to depend on shell
experiment and original shell size, confirming that over an size.
annual cycle, smaller/younger shells grow more than larger/ The lack of significant associations between growth
older ones. increments and size/age for the Mazzaforno cohort is
The summer cohort (marked in July 2006) had a mean probably due to the many medium to small shells in this
growth increment by October 2006 of 5.3±0.8 mm, much sample: 49% had diameters of 20.0 mm or less and 74%
higher than the 0.8±0.1 mm for the same period achieved had diameters of 21.0 mm or less. In contrast, 33% of the
by the annual cohort. The larger specimens (diameters of San Vito lo Capo cohort had diameters of 20.0 mm or less
22.0 mm or more) in the summer cohort had grown by no and 54% had diameters of 21.0 mm or less. The
more than 1.0 mm, the difference between the two cohorts experimental cohort from Mazzaforno was more homoge-
therefore being due to greater growth in smaller shells. This neous in terms of size and, therefore, age, reducing the
is confirmed by the highly significant negative relationship likelihood that significant gradients of growth relative to
(r=−0.921) between amounts of linear growth and shell size/age could be identified.
diameters at the time of marking for the summer cohort
(Table 2). Although all shells grow less in the summer Isotope analyses
months, this is particularly marked in larger/older ones.
Salinity values (psu) and oxygen isotope ratios (‰
Patterns of shell growth at Mazzaforno VSMOW) obtained from the seawater samples are given
in Table 5. The salinity values indicate the molluscs living
The data collected between October 2005 and October on the shores were exposed to fully marine conditions over
2006 are summarized in Table 3. All the shell measure- the course of the surveys.

Table 2 Analyses of shell growth patterns at San Vito lo Capo (original shell size shell diameter at the time of marking)

Correlation tested r r 2 p
Linear growth by Jan. 2006 (autumn growth)/original shell size −0.702 0.492 <0.001
Linear growth by Apr. 2006 (winter growth)/original shell size −0.398 0.158 0.01
Linear growth by Jul. 2006 (spring growth)/original shell size −0.476 0.226 0.01
Linear growth by Oct. 2006 (summer growth)/original shell size −0.292 0.085 0.19
Linear growth by Apr. 2006 (winter growth)/shell size in Jan. 2006 −0.311 0.097 0.03
Linear growth by Jul. 2006 (spring growth)/shell size in Apr. 2006 −0.449 0.202 0.01
Linear growth by Oct. 2006 (summer growth)/shell size in Jul. 2006 −0.367 0.135 0.09
Linear growth by Oct. 2006 (‘summer cohort’)/shell size in Jul. 2006 −0.921 0.847 <0.001
Annual linear growth/original shell size −0.680 0.463 <0.001