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not only for errant polychaetes, but also for infaunal burrowers and tube-builders
(Smith and Brumsickle, 1989; Wilson, 1992; 1994) and for species which clearly
relied upon larval recruitment, e.g., Streblospio benedicti (Smith and Brumsickle,
1989). Smith and Brumsickle (1989) and Shull (1997) have suggested that post-larval
or adult immigration might assume a greater significance during periods of decreased
larval availability, although this was not conclusively demonstrated during this study
for P. elegans. However, the August and December experiments suggested that adult
immigration to defaunated sediments was greater in P. elegans patches than in non-
patch areas. P. elegans adults represented 40% and 45% of colonists in patches
during August and December, respectively, while only 2% and 0% in non-patch
samples.
Adult P. elegans (10-12mm long) have been found in the water column above the
tidal flats in Konigshafen (Armonies, 1994) which were assumed to be due to active
migration. Similarly, adults of other spionid polychaetes have been found in the water
column (Levin, 1984a; Armonies, 1988; Cummings et al., 1995). Adult colonisation
of azoic sediments in patches during the present two experiments may also have been
an active process involving individuals crawling across the sediment surface. Lateral
movement (crawling) was proposed to have been the mechanism by which P. elegans
colonised small patches of azoic sediment in the studies by Wilson (1992, 1994)
despite the fact that colonisation by the adults of most species in his studies was by
vertical migration and settlement. Density-dependent relocation by crawling across
the sediment surface has previously been shown for P. elegans (Wilson, 1983). It was
concluded that these individuals were able to assess worm density around their tubes
(Wilson, 1983) and subsequent relocation enabled a population to prevent over-
exploitation of its resources. Furthermore, it is conceivable that an adult worm would
be more likely to actively migrate across the sediment surface of the stabilised beds
within P. elegans patches compared to one in a non-patch area. It is not possible to
suggest whether the increased colonisation of adults to patch azoics in this study
resulted from vertical migration and subsequent settlement or increased density-
dependent relocation.
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