Page 3 - Shell_growth_2008

Geo-Mar Lett (2008) 28:309–325 311

the shores were surveyed every 3 months on a broadly
‘seasonal’ basis.
The growth experiments at San Vito lo Capo and
Mazzaforno began in early October 2005, when 120 and
100 individuals respectively were marked at the two shores,
using a battery-powered hand drill. After being carefully
measured with callipers to the nearest 0.1 mm (with an
accuracy of±0.1 mm), a notch about 1.0 mm deep was cut
into the outer lip of each shell, to mark the position of the
aperture at the beginning of the experiment (Figs. 2 and 3).
Each shell was marked with a pattern of small drill holes on
the underside (Fig. 4), allowing the shells to be identified
individually. The animals were carefully returned to the
area of the shore where they had been found. At each
subsequent visit, marked individuals were searched for,
identified, measured and then returned to the specific area
of shore where they had been found. The shell parameters
measured were height, aperture height and diameter. In Fig. 3 Lateral view of a shell of O. turbinatus from Cala Grande
addition, the ‘linear shell growth’ was measured between showing the notch made when it was initially marked for the growth
study and the growth cessation lines laid down prior to marking and
the notch and the growing edge of the shell (Fig. 2), using prior to collection. This shell was sampled for a sequence of oxygen
the fine points on the callipers. In those shells which grew a isotope determinations
lot during the experiment, the ‘linear growth’ measurement
became less accurate because it does not measure along the Oxygen isotopes in shell carbonates of O. turbinatus
curvature of the shell (i.e. it becomes a measurement of the
chord). Despite this imprecision, this measure is a more The principal aims were to investigate oxygen isotopes
accurate way of tracking shell growth than is the case for 18
(δ O SHELL ) in:
diameter or height, because it estimates growth increments
along the axis of growth. – single samples from the growing edges of shells
collected at monthly intervals, to find if they reflect
The two shores were visited early in January, April, July
and October 2006 to record shell growth which had the recorded mean sea surface temperatures in those
months;
occurred in the autumn/fall, winter, spring and summer
seasons respectively. – a series of shell carbonate samples taken through one full
year of annual growth in a shell, to see how well they
reflect the SSTs which the animal had experienced.

Fig. 2 A shell of Osilinus turbinatus from San Vito lo Capo, showing
the notch made at the time of marking (October 2005) for the growth Fig. 4 Ventral view of the same shell as in Fig. 3 showing the
experiment and the measurement of ‘linear growth’ made in January summer growth cessation lines and individual pattern of marks (drill
2006 (being the autumn growth increment). This individual was found holes) used to identify it. After 1 year these holes had become eroded,
on all subsequent visits to this shore over the duration of the although the pattern is distinct enough to permit individual identifi-
experiment cation of specimens