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touched by M. balthica siphons. C. edule has been shown to inhale settling P. elegans
larvae (Noyer, 1993; cited by Morgan, 1997). Reise (1983c; 1985), Jensen (1985) and
Flach (1996) have suggested that C. edule could affect other infaunal species by
disturbing the upper sediment layer due to its crawling (ploughing) and 'shaking'
(Flach, 1996) behaviour. In Flach's study, significant reductions in densities of most
infaunal species, including P. elegans, were found even at the lowest C. edule density,
i.e., 4-8% C. edule occupancy (area occupied by C. edule). The C. edule occupancy in
P. elegans patches in August 1998 was calculated to be 9.5%, with an appreciably
greater 'disturbance area' (i.e., total area disturbed) (Flach, 1996). Therefore, it is
likely that C. edule was having a large negative effect on P. elegans within patches in
August 1998 due to sediment disturbance.
Settling spionid larvae seem to be particularly prone to the disturbance and predation
effects of C. edule and in most studies it is the juveniles which have suffered the
greatest decrease in numbers. Size measurements taken of P. elegans from the August
1998 samples in this study showed that although the size-frequency distributions were
not quite significantly different, the proportion of juveniles was greatly reduced in P.
elegans patches compared with non-patches which had significantly lower bivalve
populations. Therefore, the decrease in numbers of P. elegans in patches at this time
was mainly due to the juveniles, suggesting that there was high larval mortality in
patches during the peak settlement period earlier in the year. However, it is not
possible to determine whether this was predominantly due to sediment disturbance or
inhalation by the bivalves.
It could be envisaged that the numerical dominance of C. edule, and to a lesser extent
M. balthica, were likely to cause further reductions in P. elegans densities from those
of August 1998 because of their negative effects on spionids. The C. edule
individuals sampled in August 1998 had reached a mean size of 2.5mm length and
hence had already reached a size refuge from predation by infaunal predators such as
Nereis spp. and epibenthic species such as Crangon crangon, 0-group Carcinus
maenas and 0- and I-group Pleuronectes spp. (Reise, 1985). C. maenas and C.
crangon have been shown to be important predators on juvenile C. edule, and C.
crangon, found in moderate abundances during sampling at Drum Sands, has been
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