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Asexual reproduction by fragmentation has also been reported for P. elegans
(Rasmussen, 1953; Wilson, 1985) and has been found to be more important for the
maintenance of some local populations than sexual reproduction (Hobson and Green,
1968).
According to Anger et al. (1986) the normal life span of P. elegans is between one
and two years and it is likely that a female worm is able to produce two generations
per lifetime (Hannerz, 1956; Rasmussen, 1973). The species is dioecious and the ratio
of males to females has been shown to vary from month to month (Rasmussen, 1973).
In addition, male worms have been shown to change sex in relation to season
(Rasmussen, 1973), an increase in temperature causes the male sexual characteristics
to degenerate. The male, therefore, appears to be a sexually immature female during
the summer months which alters the male to female sex ratio.
The aim of this study was to provide information on the population structure and
reproductive biology of P. elegans, both of which are important factors affecting the
spatial distribution of a population. Knowing the timing of reproductive activity and
the method of reproduction, for example, are necessary prerequisites in an
examination into how the population is likely to respond to both physical and
biological changes on Drum Sands. Furthermore, the ability of a population to
undergo asexual fragmentation and/or production of benthic larvae has important
implications for small-scale distribution. This is particularly pertinent for a species
such as P. elegans whose method and timing of reproduction has been shown to vary
between locations (Smidt, 1951; Gudmundsson, 1985).
The P. elegans population on Drum Sands, Firth of Forth, Scotland, was studied in
terms of its size distribution, annual proportions of adults and juveniles, sex ratio of
adults and reproductive activity of adult females from March 1997 to February 1998.
The following two aims were specifically addressed in this study:
1) to investigate the population structure and reproductive mode of development of P.
elegans on Drum Sands, Firth of Forth;
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