Santander, February 19th-22nd 2008 - Aranzadi

Oysters ancient and modern: potential shape variation with habitat in flat oysters (Ostrea edulis L.), and its possible use in archaeology
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shapes would probably require analysis by ANCO-
VA on the log-transformed data (Campbell 2008:
115-116). Shell shapes in the modern samples forming
a transect across the Solent were compared
by ANOVAs on the shape ratios, and shell shapes
in the two archaeological morphotypes from deposit
2239 were compared by t-tests on the shape
ratios (Sokal and Rolph 1995: 223-225). Since
sample sizes were small and the distributions of
the ratios may not have been normal, distributionfree
tests were also employed: shell shape ratios in
the modern transect samples were subjected to
Kruskal-Wallis tests (Sokal and Rolph 1995: 423-
427), and the two archaeological morphotypes
were compared by Mann-Whitney U tests (Sokal
and Rolph 1995: 427-430). A difference was said
to be significant if its probability was less than 0.05.
4. RESULTS
4.1. Relationship between dimensions
The lines of maximum shell height (Hmax) and
length (Lmax) were not consistently related to features
of the shell. The point on the umbo which
defined the dorsal point of maximum height could
be above the chondrophore or either bourrelet,
depending on the extent of curvature in the growth
of the hinge. The point on the ventral edge defining
Hmax could be anterior, directly ventrally or posterior
to the adductor scar. However, the maximum
dimensions in the plane of commissure did have
consistent relationships with other shell features in
both modern and archaeological shells, regardless
of size or shape (Fig. 4). The line of maximum
closure height (Hc) passed through the centre
of the adductor muscle scar, usually within
±1mm of the ventral edge of the scar; closure shell
height and maximum body height are on the same
alignment. Also, closure length Lc was parallel to
the hinge axis, regardless of the relative orientation
of Lc and Hc. These relationships held true even in
one- or two-year-old shells, despite their xenomorphism
(the strong trend for young shells to fit the
shape of their substrates) (Stenzel 1971: 1021).
The only shells which did not conform were those
which bore clear evidence of trauma or physical
growth restriction.
4.2. Modern Oysters
In the modern samples, the scatter in the relationship
of height with other variables such as
length (Fig. 5) was consistently broader with maximum
shell height Hmax (Fig. 5a) than with closure
height Hc (Fig. 5b). Regression coefficients (r 2 )
were consistently lower with Hmax than with Hc for
all shape ratios in all modern samples. For example,
the relationship between height and length in
the sample with the largest size range (Southsea
Beach) had a greater scatter and larger regression
coefficient with Hmax (Fig. 5a) than with Hc (Fig.
5b). Distributions of shape ratios were also consistently
broader in range and more poly-modal when
generated using Hmax than when Hc was employed.
For example, the distribution of shell HLR for
Southsea Beach (Fig. 5c) was probably positively
skewed and possibly poly-modal, while closure
HLR (Fig. 5d) in the same sample was narrower in
Figure 4. Inside of L. valve of O. edulis, showing measurements taken.
Figure 5. Modern oyster O. edulis sample from Southsea Beach: height as a
function of shell length for (a): shell height Hmax; (b): closure height Hc; and
distribution of height-length ratio HLR using (c): shell height Hmax; (d): closure
height Hc. Regression coefficient ‘ r2 ‘ for least-squares fitting to straight line.
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián