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SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
Tabic 3. Results of 2 two-way analyse- ol variance (AM >YAi to Ic-l effect ol siphon weighl on burying depth of individual- within a similar
size class (same data us in Figs. 7-8).
S/>r c 1, | Siphon Season Siphon x Season r.
l< . ', P fP.lb P K '• P
Macoma balihica 18.0 0.001 12.8 0.001 0.5 0.034 2077
Scrobicularia plana 5.9 0.001 55.1 0.001 0.5 0.001 4313
mg siphon mass increases its depth 1 cm for each extra
mg in summer and 2 cm per extra mg in winter (Fig. 7,
Table 3). so depth and siphon weight are proportional
within that range. If the siphon weight, however, is
more than 5 mg. a depth of 6 to 7 cm in summer and 10
to 12 cm in winter is maintained. There is scarcely any
increase in burying depth if the siphon is 10 or 20 mg.
The relationship between burying depth and siphon
weight in Macoma (Fig. 8) is the same as in Scrobicularia
(Fig. 7). with one remarkable difference. All
large Scrobicularia having no extremely low siphon
weight, live at the same depth in summer as well as in
winter (Figs. 5 and 7). This is also the case with large
Macoma during Ihe vv inlet, but not in summer. Most
Fig. 7. Scrobicularia plana 135-39 mm). Burying depth (cm ± SE)
in w inier (•. n = 1253) and in summer (O: n = 3060) as a function
of weighl of inhalant siphon. Table 3 give- statistical analysis.
103
0.5 1.0 15
siphon weight (mg AFDW)
Fig. 8. Macorna Imlihica (10-22 mm). Burying depth (cm ± SE) in
winter (•: n = 225) and in summer (O: n = 1852) as a function of
weight of inhalant siphon. Siphon weights increase w uh size within
the range selected. Thai is why all siphon weights are transformed
into ihe siphon weighl of a 15-mm individual. Table 3 gives statistical
analysis.
large Macoma then live at 2 cm (Fig. 4), but individuals
with a heavy siphon enlarge their depth to nearly 5
cm. which is their usual depth in winter (Fig. 8).
It was possible to select three size classes with sufficient
data to analyse the relation between siphon
weight and depth in Mya (Fig. 9). Depth increases
within each size class if the siphon is heavier, but size
classes with a similar siphon weight are compared, it is
found that smaller individuals bury deeper than larger
ones. The explanation for this is indicated in Fig. 10:
the siphon diameter of Mya increases with shell size;