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CHAPTER 3 THE POPULATION STRUCTURE AND REPRODUCTIVE BIOLOGY OF PYGOSPIO ELEGANS ON DRUM SANDS INTRODUCTION Natural and anthropogenic disturbances have been shown to be important factors influencing the structure of marine soft-bottom communities (Levin, 1981; 1984a; Zajac and Whitlatch, 1982a; 1982b; Dobbs and Vozarik, 1983). The effect of a disturbance on the marine benthos is partly determined by the scale and intensity of the disturbance. A disturbance such as a natural storm, for example, can result in the large-scale defaunation of the sediment (Dobbs and Vozarik, 1983). Increased wave and current action resulting from storms erode the sediment surface causing high species mortality and/or the redistribution of infauna (Rees et al., 1977) or severe reductions in salinity and dissolved oxygen concentrations cause the mortality of many species (Boesch et al., 1976; Santos and Simon, 1980a). Such a disturbance often results in a heterogeneous environment (Levin, 1984a). A number of species have developed opportunistic life histories enabling them to exploit disturbed habitats (Grassle and Grassle, 1974). In general, opportunists are characterised as having a low reproductive age, high fecundity, strong dispersal ability and a short life span (Pianka, 1994). These characteristics enable opportunists to rapidly colonise an area under favourable conditions, although they tend to be unable to compete with non-opportunists. Many polychaete species such as Capitella capitata (Warren, 1976) and the spionids Pygospio elegans (Muus, 1967; Anger, 1977), Polydora cornuta (formerly P. /igni)(Zajac, 1991) and Streblospio benedicti (Levin and Creed, 1986) have been shown to rapidly increase in numbers following a disturbance (Grassle and Grassle, 1974; Pearson and Rosenberg, 1978). These polychaetes, with the exception of P. elegans, have been particularly well studied in relation to their opportunistic nature. 59
Pygospio elegans has been shown to be polytelic (Gudmundsson, 1985; Anger et al., 1986) with a flexible reproductive strategy. This species normally reproduces sexually and, depending on the degree of adelphophagy (whereby the developing larvae consume 'nurse eggs'), planktotrophic or benthic larvae may be produced (Thorson, 1946; Smidt, 1951; Mileikovsky, 1971; Rasmussen, 1973; Anger, 1984; Levin et al., 1991). The latter larval type was termed as 'non-pelagic lecithotrophic' larvae by Vance (1973). The existence of more than one mode of reproduction within a single species, or `poecilogony' (Giard, 1904, cited by Hoagland and Robertson, 1988) is especially common in Spionidae where the presence of both brood protection and adelphagia allows considerable flexibility in the duration of the planktonic larval phase and in the mode of nutrition (Levin, 1984b). The existence of two different larval types within a species has important demographic implications (Levin, 1987). After release, the eggs are retained in capsules attached to both the female and to the inside of her tube. As many as 19-34 capsules have been found in the deepest part of the female tube (Rasmussen, 1973). The numbers of eggs per capsule have been shown to vary greatly, but in general, far fewer eggs are laid down in capsules producing benthic larvae. Non-brooded larvae hatch from 3 setigers (Anger et al., 1986) to 6 setigers (Hannerz, 1956) while brooded ones hatch later at 12-16 setigers (Hannerz, 1956; Rasmussen, 1973. At the 12 to 14 setiger stage the larvae are ready to metamorphose (Hannerz, 1956). However, metamorphosis to the juvenile stage does not occur until a suitable sediment type is encountered, the particle size distribution and organic content of the sediments being important factors (Smidt, 1951; Hannerz, 1956). Newly settled individuals grow rapidly and reach maturity within a few months (Smidt, 1951), thus populations of P. elegans can reach very high densities in a relatively short period of time. 60
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Pygospio elegans has been shown to be polytelic (Gudmundsson, 1985; Anger et al.,<br />
1986) with a flexible reproductive strategy. This species normally reproduces<br />
sexually and, depending on the degree of adelphophagy (whereby the developing<br />
larvae consume 'nurse eggs'), planktotrophic or benthic larvae may be produced<br />
(Thorson, 1946; Smidt, 1951; Mileikovsky, 1971; Rasmussen, 1973; Anger, 1984;<br />
Levin et al., 1991). The latter larval type was termed as 'non-pelagic lecithotrophic'<br />
larvae by Vance (1973). The existence of more than one mode of reproduction within<br />
a single species, or `poecilogony' (Giard, 1904, cited by Hoagland and Robertson,<br />
1988) is especially common in Spionidae where the presence of both brood protection<br />
and adelphagia allows considerable flexibility in the duration of the planktonic larval<br />
phase and in the mode of nutrition (Levin, 1984b). The existence of two different<br />
larval types within a species has important demographic implications (Levin, 1987).<br />
After release, the eggs are retained in capsules attached to both the female and to the<br />
inside of her tube. As many as 19-34 capsules have been found in the deepest part of<br />
the female tube (Rasmussen, 1973). The numbers of eggs per capsule have been<br />
shown to vary greatly, but in general, far fewer eggs are laid down in capsules<br />
producing benthic larvae. Non-brooded larvae hatch from 3 setigers (Anger et al.,<br />
1986) to 6 setigers (Hannerz, 1956) while brooded ones hatch later at 12-16 setigers<br />
(Hannerz, 1956; Rasmussen, 1973.<br />
At the 12 to 14 setiger stage the larvae are ready to metamorphose (Hannerz, 1956).<br />
However, metamorphosis to the juvenile stage does not occur until a suitable sediment<br />
type is encountered, the particle size distribution and organic content of the sediments<br />
being important factors (Smidt, 1951; Hannerz, 1956). Newly settled individuals<br />
grow rapidly and reach maturity within a few months (Smidt, 1951), thus populations<br />
of P. elegans can reach very high densities in a relatively short period of time.<br />
60
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