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Reproductive activity of Pygospio elegans - Approximately 50% of the sexually mature females sampled between March and June 1997 contained embryos and/or larvae in their tubes (Figure 3.4). Reproductive activity decreased during the summer and by October very few female tubes contained embryos or larvae. However, reproductive activity increased again over the winter months and by February 1998, the majority (81.2±12.5%) of female tubes again contained embryos or larvae. These two increases in the proportions of mature females brooding larvae in their tubes potentially explain the timings of larval recruitment. The high numbers of female tubes containing larvae during the spring (Figure 3.4), approximately 50%, may have been responsible for the cohort observed settling during May (Figure 3.2(iii)). It is not possible to account for the December cohort in this way without sampling during November when a high percentage of tubes containing larvae would have been expected. The high percentage observed during February 1998 would presumably have been responsible for a spring recruitment during 1998. 11-). ,cs 60 — 50 — 40 — g 30 — Et -I- 20 — 10 — Mar Apr May Jun I Jul Aug Oct Dec MONTH -r Feb — 100 — 90 —80 —70 czt —60 rfi —50 —40 a.) ba ez: —30 Figure 3.4 : Mean number per core (bar graph) and percentage (line graph) of female tubes containing embryos or larvae from March 1997 to February 1998 (± S.E.mean, n=3). 71 —20 — /0 —0 -10
The larvae found within the female tubes were all of the 3-5 setiger stage and had long natatory bristles. These resembled the P. elegans planktonic larvae described by other authors, e.g., Smidt (1951). There is strong evidence to suggest that this larval type was the only type produced by P. elegans at Drum Sands within the study period. Measurements of larval body length (Figures 3.5(i-vi)) showed that the majority of the individuals ranged in size from 0.15 to 0.20mm between March to July 1997 and February 1998, with a mean length of 0.17mm, and there were no significant differences in their size distributions between these months (Kolmogorov-Smimov test with a Bonferroni correction). Furthermore, the maximum lengths of the larvae did not increase during the summer months when some P. elegans populations have previously been shown to produce benthic larvae. The mean larval length (0.17mm) was far smaller than the average lengths of benthic larvae (0.65mm) observed emerging from egg capsules in the study by Hannerz (1956). Muus (1967) observed 20 setiger benthic larvae still inside egg capsules, although most tend to hatch at the 12-14 setiger stages (Hannerz, 1956). This is in contrast to the 3-5 setiger larvae continually observed in the parental tubes in this study. Further evidence for the production of planktonic larvae only was the complete absence of 'hunchback larvae' (Thorson, 1946; Rasmussen, 1973). These are 3 setiger larvae distended in shape due to the large amount of yolk ingested. These larvae are destined to hatch at a later stage of development to become benthic larvae. It is possible that the larvae produced by the P. elegans population at Drum Sands would have remained in the plankton for a relatively long period of time since Hannerz (1956) found that the lengths of the larvae found in the plankton ranged between 0.33-0.42mm; the maximum length obtained in this study being noticeably shorter, 0.225mm. However, the results here support the findings of Anger et al. (1986) who suggested that the larvae reared by laboratory cultured P. elegans hatch anywhere within the size range 250-330m. There is likely to be some size discrepancy between the results obtained by Hannerz and this study since the larvae measured in this study were taken from within the parental tubes rather than from the plankton, and larval body length, rather than actual length (including natatory bristles) was measured. The significant differences between the sizes perhaps suggest that the 72
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- Page 56 and 57: 8m survey - spatial patterns Figure
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- Page 78 and 79: METHODS Survey design - It has been
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The larvae found within the female tubes were all of the 3-5 setiger stage and had long<br />
natatory bristles. These resembled the P. elegans planktonic larvae described by other<br />
authors, e.g., Smidt (1951). There is strong evidence to suggest that this larval type<br />
was the only type produced by P. elegans at Drum Sands within the study period.<br />
Measurements of larval body length (Figures 3.5(i-vi)) showed that the majority of the<br />
individuals ranged in size from 0.15 to 0.20mm between March to July 1997 and<br />
February 1998, with a mean length of 0.17mm, and there were no significant<br />
differences in their size distributions between these months (Kolmogorov-Smimov<br />
test with a Bonferroni correction). Furthermore, the maximum lengths of the larvae<br />
did not increase during the summer months when some P. elegans populations have<br />
previously been shown to produce benthic larvae. The mean larval length (0.17mm)<br />
was far smaller than the average lengths of benthic larvae (0.65mm) observed<br />
emerging from egg capsules in the study by Hannerz (1956). Muus (1967) observed<br />
20 setiger benthic larvae still inside egg capsules, although most tend to hatch at the<br />
12-14 setiger stages (Hannerz, 1956). This is in contrast to the 3-5 setiger larvae<br />
continually observed in the parental tubes in this study. Further evidence for the<br />
production of planktonic larvae only was the complete absence of 'hunchback larvae'<br />
(Thorson, 1946; Rasmussen, 1973). These are 3 setiger larvae distended in shape due<br />
to the large amount of yolk ingested. These larvae are destined to hatch at a later stage<br />
of development to become benthic larvae.<br />
It is possible that the larvae produced by the P. elegans population at Drum Sands<br />
would have remained in the plankton for a relatively long period of time since<br />
Hannerz (1956) found that the lengths of the larvae found in the plankton ranged<br />
between 0.33-0.42mm; the maximum length obtained in this study being noticeably<br />
shorter, 0.225mm. However, the results here support the findings of Anger et al.<br />
(1986) who suggested that the larvae reared by laboratory cultured P. elegans hatch<br />
anywhere within the size range 250-330m. There is likely to be some size<br />
discrepancy between the results obtained by Hannerz and this study since the larvae<br />
measured in this study were taken from within the parental tubes rather than from the<br />
plankton, and larval body length, rather than actual length (including natatory bristles)<br />
was measured. The significant differences between the sizes perhaps suggest that the<br />
72
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