Download (20MB) - Repository@Napier

fragmentation at any one time. It is more likely that this apparent fragmentation of P.
elegans individuals observed at Drum Sands resulted from predation (Muus, 1967)
rather than as a reproductive strategy. This is supported by the fact that those seen
regenerating in this study were in the process of growing new anterior regions only. If
asexual reproduction had been occurring, individuals growing new posterior regions,
as well as individuals present in all stages of regeneration, would have been observed.
Implications of planktotrophic larval development for the population
maintenance and spatial distribution of P. elegans on Drum Sands.
In view of the relatively high densities and the spatial pattern (Chapter 2) exhibited by
P. elegans at Drum Sands, it was expected that the species reproduced with the
production of benthic larvae and/or asexual proliferation, at least at certain times of
the year. This would have helped to explain the high local densities and its patchy
distribution. However, this study has revealed that the population here, in contrast to
what was initially thought, reproduced entirely by the production of planktotrophic,
pelagic larvae.
Studies on P. elegans and other spionids have suggested that differences in
reproductive mode are not due to adaptations to suit its environment, as shown for
other polychaetes, but to non-poecilogonous behaviour and genetic divergence
between populations (Anger, 1984; Levin and Creed, 1986; Hoagland and Robertson,
1988; Chia et al., 1996). For example, Anger (1984) conducted laboratory studies of
seasonal reproduction in P. elegans and found that temperature and salinity changes
did not trigger shifts to or from planktotrophy, lecithotrophy or asexual reproduction.
Furthermore, Levin (1984a) stated that studies on many species have never shown a
single female to produce more than one larval type, and many apparently
poecilogonous polychaetes have actually been sibling species. This implies therefore,
that the P. elegans population at Drum Sands has explicitly evolved this method of
reproduction and is possibly not capable of producing other larval types or
reproducing asexually. The spatial pattern exhibited by P. elegans at Drum Sands
(Chapter 2) has therefore been established by processes acting on planktotrophic
larvae and/or on post-metamorphic individuals. Some of these processes will be
addressed in Chapters 4-8.
78