Santander, February 19th-22nd 2008 - Aranzadi

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GREG CAMPBELL
Figure 9. A possible explanation for oyster shell shape variation between beds.
soft muddy beds where the main risk to oysters is
sinking, so the high surface area of broad, round,
and occasionally lobate shells is advantageous. In
stronger more turbulent tidal flows off-shore, the
risk of sinking falls but the risk of transportation
increases, so heavier and more streamlined shells
are advantageous. Also, oysters in harbours are
subjected to low and steady currents which generate
little torsional force on the upper valves, so the
hinges can be quite small. The faster turbulent
currents off-shore generate greater twisting forces
on the upper valve, so oysters tend to grow broader
stronger hinges off-shore. The oyster hinge
has the tensilium both anterior and posterior to the
resilium (Trueman 1951: 138), on the bourrelets
(Stenzel 1971: 974), so the oyster hinge is particularly
effective against torsional forces (Hautmann
2004: 168).
Another aim of this study was to determine
whether it is likely that there is a simple way to
measure O. edulis shells to make shell shape
objectively comparable between samples, both
biological and archaeological. Some morphometric
studies of marine shells employ over a dozen
measurements (e.g. McDonald et al. 1991: 325) or
examine the interrelationships of dozens of features
(e.g. Carvajal-Rodriguez et al 2005: 314).
Fortunately, significant differences in shape were
discerned in this study by employing only four
measurements, a feasible number to take when
faced with potentially thousands of oysters that
some excavations produce.
Oyster ecophenotypic variation is likely to be more
successfully studied using dimensions in the plane of
commissure rather than maximum overall size:
(1): The correlations with maximum dimensions in
the plane of commissure (Hc, Lc) are more precise
than those based on maximum shell dimensions
(Hmax, Lmax). In the modern material studied here, r²
was always greater for shape ratios based on Hc than
the analogous ratios based on Hmax. If there is ecophenotypic
shape variation, it will be more detectable
in the plane of commissure than in the overall shell.
(2): Maximum dimensions in the plane of commissure
are orientated precisely with features in
the valve, making them simple and quick to locate
on a valve, and consistent between valves.
Maximum shell dimensions are not so consistently
positioned with respect to the hinge or margins,
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián