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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and are therefore different between valves. This
more consistent commissural orientation is probably
why the commissural dimensions correlate
more precisely.
(3): Dimensions in the plane of commissure are
identical for both valves of the same oyster, but
(because oysters are inequivalve) Hmax and Lmax
can only be measured on the larger (left) valve.
Therefore the sample sizes for shell shape in
archaeology will be larger if the commissure is measured,
since the more numerous type of either valve
in any archaeological deposit can be measured.
(4): Dimensions are better preserved in the
plane of commissure than in the whole shell. Hc
and Lc are taken on features that are inset from the
shell edge (slightly in the right valve, distinctly in the
left). Hmax and Lmax are measured on the shell
edge, so the margins and umbones must be intact.
Erosion of the fragile shell margins and loss of the
umbones is common in archaeological oysters,
and can be severe. Umbones and margins of
modern oysters can be worn away or broken
during life or recovery. The sample sizes for shell
shape in archaeology will be larger if the commissure
is employed, since the shells retaining the
commissural shelf and inner edge of the hinge will
always be more numerous than those in which the
margins and umbones are intact. Also, smaller
shells tend to be thinner and more fragile than larger
shells, and therefore more likely to have damaged
umbones and margins, so employing Hmax
and Lmax biases the analysis towards larger shells.
Another aim was to determine whether it is likely
that there are consistent relationships between shell
features which survive well in archaeological oysters
and overall shell size, so that archaeological oyster
shell sizes can be estimated. Well-preserved
dimensions have been used previously to reconstruct
shell size distributions of other archaeologically
important bivalves that preserve poorly (e.g.
Buchanan 1985). Unfortunately, shell-to-body
height ratio in the archaeological and modern samples
varied widely (Table 1). Size and shape of the
visceral cavity has little predictive value for shell
size and shape in archaeological material. The
inter-relationship between other features measured
in this study (height, length, hinge width) also
varied significantly between samples. Therefore it is
unlikely that original shell size can be estimated
well from a badly preserved shell.
Applying the methodology developed here to
the two morphotypes thought to exist in deposit
2239 confirmed the distinction. The relationship
between height and length was distinct for the two
morphs, and the average proportions were different:
length typically was greater than height in the
round morph, but not in the oval morph. The average
proportions of the hinge to width were also
different, the round morph tended to have smaller
hinges. Of course the intention was not to show the
shells in the round morph were ‘round’ internally or
externally, but to see whether the morphs were significantly
different according to a method useful for
studying ecophenotypic variation. Since it does
seem probable that oyster shape varies with conditions
in the bed, it seems probable that the two
morphs were growing in different beds. The round
morph had shell shape like modern harbour-offshore
oysters, while the oval morph had more oval
and wider-hinged shells than any modern form.
The archaeological shells in the oval morph
changed from a range of shapes to one typical
shape, and from a range of hinge morphologies to
a single morphology. Since it does seem probable
that oyster shape varies with conditions in which
the oyster grows, it seems probable that these oysters
have moved from a range of growth conditions
to a single one. This suggests intentional human
intervention, such as the re-laying of dredged oysters
to a single site for ‘fattening’. If so, it may be
the first direct evidence for oyster management in
British archaeology.
The archaeological shells were larger on average
than the modern samples (Table 1). It is therefore
possible that the difference in shape between
modern and ancient oysters is due to exponential
allometry during growth. However, the oval
archaeological morphotype had a height-length
ratio and hinge-width ratio much greater than any
of the modern samples (Table 1). Also, almost all
the oval oysters had taken on a similar shape from
a range of shapes following a growth step, suggesting
the oysters grew relatively quickly into the oval
shape. It is more likely that the oval morphotype
grew in conditions that were different from those of
any of the modern samples of this present study.
6. CONCLUSIONS
Since this small study identified trends, and
detected one which appears to be statistically significant,
it is probable that there is ecophenotypic
variation in O. edulis. This variation may be due to
hydrodynamic differences between beds.
Further more detailed research to elucidate the
relationship between shape and conditions of
growth is likely to be worthwhile.
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián