Page 14 - Shell_growth_2008

322 Geo-Mar Lett (2008) 28:309–325

here, with a survey interval of 3 months, any effect of the the effects considered here might not apply to shells from
notch would have been short lived and would have late Pleistocene and early Holocene archaeological contexts.
impacted more or less equally upon all specimens. Scars
showing the repair of quite severe damage, caused mostly Physiological effects on shell growth
by unsuccessful attempts at predation by crabs, can
frequently be seen on shells of O. turbinatus. Where such The effects of episodic energy-demanding processes,
damage has been particularly severe, this may lead to a notably reproduction, on patterns of shell growth should
change in the growth of the shell (as indicated by slight be considered. Schifano’s(1983) surveys of O. turbinatus
changes in shape) but, in most cases, the animal repairs its at a shore in NW Sicily linked periods of recruitment in
shell and continues growing much as before. The notches May and November to postulated spawning in spring and
cut into the shells are smaller than most predation damage, autumn. For the same population, Schifano and Censi
suggesting they have a negligible influence on shell growth. (1983) attributed periods of slow shell growth in the spring
This can be seen in the pigmentation patterns of the shell in and autumn to spawning activity. This is unlikely because it
Fig. 2, which continue in the same positions and with the is gametogenesis, rather than spawning, which is more
same periodicities in the new growth increment laid down energy demanding and, therefore, more likely to influence
in the 3 months after having been notched. growth rates. Histological studies of the gonads of O.
turbinatus in the Gulf of Trieste showed that gametogenesis
Effects of shore morphology on shell growth begins in September and continues as SSTs decline to their
yearly minimum (Valli et al. 1977; 2003). Both sexes
Although the number of shells recovered at the end of the liberate their gametes into the sea, sporadically from
summer at Mazzaforno was small, those recovered had January to March and more intensively from April to
grown by up to 4.8 mm. No shell from the analogous August, and fertilisation occurs externally. The period of
cohort at San Vito lo Capo had grown by more than sexual rest can span from August to December but is
2.3 mm. This difference may relate to the morphologies of generally from August to October. Gametogenesis in
the shores. At San Vito lo Capo, Monte Cofano and Cala females commences earlier and continues for longer than
Grande, all shores with Dendropoma (vermetid) reefs at in males, and is more energy and resource demanding.
mean low tide level, we observed that O. turbinatus rarely, Inter-individual variability in shell growth rates could
if ever, moves down onto the reef at low tide. During low reflect male–female differences but it is not possible to
tide at Mazzaforno, which lacks such a reef, numerous distinguish the sexes from their external morphology.
individuals of O. turbinatus, especially smaller/younger Although reproduction in O. turbinatus occurs over
ones, can be found on rock surfaces just below water level. much of the year, as shown by recruitment of very small
At low tide during the hot summer months, O. turbinatus at juveniles to shores, gametogenesis occurs principally in the
localities with vermetid reefs are likely to suffer heat stress autumn and winter. These are seasons during which we
because they cannot move down shore into the upper recorded maximum rates of shell growth, suggesting that
subtidal. Smaller shells, having a high surface area to reproductive effort has no significant impact on shell
volume ratio, are more likely to be affected by extremes of growth. This need not, of course, be so for all populations
ambient temperatures than larger ones and, therefore, may of O. turbinatus across its range.
actively seek less exposed situations during periods of
environmental stress. Valli et al. (2003) showed that shell Shell oxygen isotope composition
growth in O. turbinatus can differ between adjacent
18
populations in the northern Adriatic, with individuals at SSTs calculated from δ O SHELL monthly edge values are
localities subject to longer periods of submergence being highly positively correlated with monthly SSTs at the three
generally larger. A laboratory study of vertical movements study localities. This supports the results from the growth
by O. turbinatus (Chelazzi and Focardi 1982) showed that studies by showing that in most months new shell incre-
18
animals exhibit negative phototaxis and positive geotaxis ments were laid down, with δ O SHELL values reflecting
during the day and negative geotaxis at night. During low monthly changes in SSTs. Although shell-based temper-
tides in the warmer months, such behaviour could be atures track fluctuations in SSTs well at all localities, they
important for avoiding exposure to high insolation (Valli usually underestimate the values, particularly in late spring
and Vio 1975). Individuals able to move freely into the and summer. There is also variation between individual
seawater during low tide in the summer months will grow shells at particular localities and between localities, which
more than those not able to do so. Large Dendropoma reef can be attributed in part to shell size and, therefore, age.
platforms were probably unable to develop during periods Potential factors which might produce offsets between
18
of rapid sea level rise (Antonioli et al. 1999), in which case δ O SHELL temperatures and instrumentally measured SSTs