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Cap itella capitata has planktonic larvae giving widespread dispersal but also
possesses demersal larvae which are completely benthic (Warren, 1976). Shull (1997)
demonstrated that this polychaete was capable of colonising azoic sediments by larval
settlement, bedload transport and by burrowing. C. capitata has a short life cycle, is
capable of reproducing all year round and one female may produce up to 10,000 eggs.
These characteristics allow this species to quickly colonise suitable unoccupied areas
of sediments in vast numbers (Rosenberg, 1973; Grassle and Grassle, 1974). The
size-frequency differences between patch and non-patch colonisers for C. capitata was
only observed during December. It is difficult to suggest the possible reason why
those colonising P. elegans patches were predominantly adults while those colonising
non-patches were mainly larvae but a comparison with the situation in the ambient
sediments reveals a possible explanation. Although not significant, there were higher
densities of C. capitata in patches than in non-patches at this time (23.0±2.9 and
20.1±4.1 respectively, Figure 8.1 (iv)). Furthermore, the size-frequency distributions
were significantly different at this time (Table 8.2) due to a higher proportion of adults
in patch populations. Therefore, the increased proportion of adults colonising azoic
sediments in P. elegans patches in the December experiment may have been a direct
result of the differences in the ambient populations between patches and non-patches
at that time. More adults may have colonised via lateral migration through and/or on
the sediments in patches.
Can sediment disturbance lead to P. elegans patch formation on Drum Sands?
In addition to giving an important insight into differential colonisation of patches
compared with non-patch areas and into to the mode of colonisation of P. elegans in
response to small-scale disturbances, this study has suggested that P. elegans patch
formation on Drum Sands could possibly have resulted from sediment disturbance.
However, since the population relies on a highly seasonal input of planktonic larvae
(Chapter 3) the timing of the disturbance is important. Patch formation could only
have, therefore, resulted from disturbances occurring during the summer or the winter.
It is not always possible to scale-up the results of small-scale disturbance experiments
such as this one to those of larger disturbances (Thrush et al., 1997a, 1997b).
However, it is likely that the mode of P. elegans colonisation following larger-scale
disturbances and consequent successional dynamics would be the same as those
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