Santander, February 19th-22nd 2008 - Aranzadi

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GREG CAMPBELL
range and more obviously normally distributed.
Closure height was therefore used in shape comparison
throughout this paper.
Height-length allometry was very similar in all
the modern samples, and was well-approximated
by a linear relationship (isometry); positive or
negative allometry could not be distinguished in
these small samples with their restricted size ranges.
Even in the modern sample with the fullest
range of sizes and ages (Southsea Beach) the
allometry was very well approximated by a linear
relationship (Fig. 5b; r² = 0.919); fitting to an exponential
relationship generated an allometry (Hc =
(1.246)Lc 0.918 ) which did not significantly improve
the regression (r² = 0.947). Shell sizes in the other
modern oyster samples forming a transect across
the Solent were not significantly different and had
similar standard deviations (Table 1). Therefore
shell shape was compared via the ratios.
There was a significant variation between samples
in the shell-to-body height ratio (Hc/Hb) (Table
1). Closure HLR and closure eccentricity (He/Hc)
did tend to vary with conditions, but the variations
were not statistically significant. However, hingewidth
ratio (Wh/Lc) varied across the transect, and
this variation was very significant. There was also
some correlation between closure HLR and hingewidth
ratio, with harbour oysters having the smallest
values of both ratios, near-shore oysters
having larger ratios, and deeper water oysters
having greater hinge width ratios.
Modern harbour oysters were distinctive (Fig.
6): as well as being thin-shelled and frilly-edged,
about a third had extensions of the shell around the
hinge. These were not auricles (anterior and pos-
terior extensions of the hinge, as in some scallops),
but lobe-like extensions of the margin. Usually
these lobes were posterior to the hinge (Fig. 6a),
but some shells had lobes both anterior and posterior
to the hinge, sometimes weak (Fig. 6b), often
clear (Fig. 6c) and sometimes spectacular (Fig.
6d). The average shell shapes in the samples showed
a gradation with conditions (Fig. 7): rounded,
frilly, occasionally lobed shells in harbours give
way to smooth-edged more oval forms as sample
position moved further into the Solent.
4.3. Archaeological oysters
The round and oval archaeological morphotypes
in deposit 2239 were significantly different in size
according to the t-test, but were effectively the same
according to the Mann-Whitney U test (Table 1).
Height-length allometry was not similar for the two
archaeological morphotypes (Fig. 8a), but positive
or negative allometry could not be distinguished
due to the restricted size ranges and broad spreads.
In the round morphotype the allometry was as
well approximated by a linear relationship (r² =
0.618) as an exponential one (r² = 0.619). In the
oval morphotype the linear relationship (r² = 0.625)
was marginally better than an exponential one (r² =
0.614). Therefore shell shape was compared via
the ratios.
The shell-to-body height ratio was not significantly
different between morphotypes (Table 1);
closure HLR and hinge-width ratios were very significantly
greater in the oval form, and closure
eccentricity was significantly larger in the oval form
according to the distribution-free test. The round
morphotype had closure HLR values similar to
modern near-shore oysters, and hinge-width ratios
similar to deeper-water oysters. The oval morphotype
had closure HLR, eccentricity and hingewidth
ratios much larger than any of the modern
samples. The closure HLR showed a weak positive
correlation with hinge-width ratio (r 2 = 0.22), and
this relationship produced a good separation of
the two morphs (Fig. 8b); ‘round’ in this case seem
to be defined principally as a shell with a closure
HLR of less than 0.97, provided its hinge-width
ratio was also less than 0.34.
Figure 6. Inside of modern oyster (O. edulis) left (lower) valves from Chichester
Harbour lakes which were (a) lobate (Hmax: 75mm); (b) weakly bi-lobate (Hmax:
77 mm); (c) bi-lobate (Hmax: 79mm); and (d) strongly bi-lobate (Hmax: 80mm).
5. DISCUSSION
The chief aim of this study was to determine
whether O. edulis shape is likely to vary significantly
with habitat. Visual examination of shape of
modern samples was encouraging: shells in har-
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