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shown to consume up to 68% of a 0-group C. edule population (Pihl and Rosenberg,
1984). Reise (1985) suggested that once C. edule have managed to escape their
predators in their first summer, they are capable of establishing dense populations
lasting about 5 years, unless they are killed by the next severe winter.
Chapter 3 has shown that the P. elegans population on Drum Sands did not produce
benthic larvae nor reproduce asexually, but instead had a dispersal mode of
reproduction with planktotrophic larvae. Consequently, this population was
particularly susceptible to adult-juvenile interactions and therefore likely to be
replaced by suspension-feeding bivalves. For example, Commito (1987) and
Commito and Boncavage (1989) suggested that only those tube-building or deposit-
feeding species which produce cocoons or benthic larvae that are too large for
siphonal inhalation are relatively immune from adult-larval interactions with
suspension-feeding bivalves.
It is not possible to suggest why 1998 saw the start of the interaction between the two
species at Drum Sands and the onset of P. elegans patch decline. It was possibly due
to a high C. edule spat-fall, low adult bivalve densities (facilitating successful juvenile
C. edule settlement) or the relative timing of bivalve and spionid larval settlement.
Since there were low adult C. edule densities at this time it is possible that adult
densities may have been regulating C. edule recruitment. Successful C. edule spat
settlement has been recorded during summers following severe winters which
decimated adult populations (Smidt, 1944; Reise, 1985) and Kristensen (1957)
concluded that high adult C. edule densities may prevent a successful settlement of
larvae. C. edule has been shown to ingest its own larvae, in addition to other bivalve
species (Jensen, 1985; Andre and Rosenberg, 1991). The almost complete absence of
adult C. edule in P. elegans patches could have therefore been responsible for the
increased number of juvenile C. edule observed in patches compared to non-patches in
this study.
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