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In April, when P. elegans larval availability in the water column was at its highest on
Drum Sands, significantly higher numbers of P. elegans larvae colonised azoic
sediments in patches compared with non-patch azoic sediments. Many laboratory
experimental studies have been carried out to elucidate the precise mechanisms by
which differential larval settlement of infaunal species is achieved (Scheltema, 1974;
Woodin, 1986; Butman et al., 1988a; Pawlik and Butman, 1993; Hsieh, 1994) but
such studies on polychaetes have mainly concentrated on families other than
Spionidae, e.g., Capitellidae (Butman et al., 1988b; Grassle and Butman, 1989) and
Sabellidae (e.g., Pawlik et al., 1991) and it is likely that mechanisms are species-
specific. However, settlement cues associated with adult populations were unlikely to
have been responsible for the observed differences in P. elegans larval recruitment in
the present study because colonisation occurred on totally defaunated sediments.
It is possible that larval behavioural responses to local boundary-layer flow may have
produced the observed differences for juvenile P. elegans in this study. Such
behaviour has been demonstrated for the sabellid Phragmatopoma lapidosa
califomica (Pawlik et al., 1991) and it was proposed that this enabled this species to
form large tube-beds.
Savidge and Taghon (1988) showed that in the field, colonisation was dominated by
passive advection of larvae (see review by Olafsson et al., 1994). Eckman (1983)
suggested that passive larval entrainment can be higher in areas of stabilised
sediments. Increased larval entrainment was suggested to have been a possible reason
why C. edule spat were present in higher numbers in P. elegans patches on Drum
Sands (Chapter 8). Increased passive entrainment to P. elegans patches could,
therefore, have been responsible for the increased recruitment of P. elegans larvae to
patches in this study. However, one must be cautious, apparent increased recruitment
to patches could equally have resulted from increased erosion of settled larvae in non-
patch areas. Since samples were taken some time after settlement, inferring settlement
patterns from observed recruitment patterns can be potentially misleading (Hadfield,
1986; Woodin, 1986). Woodin (1986) suggested that to differentiate between larval
settlement and recruitment in field studies is inherently very difficult. Differential
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