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Changes in the filtering activity during the two periods were compared using Wilcoxon signed-rank tests.
2.5.1.2. Experiment 2 Experiments were run during June 1997 on worms from the bay `
of Banyuls-sur-Mer and during October 1997 on worms collected off of Argeles-sur-Mer to assess the effect of food concentration on filtering activity. Batches of 12 worms were
successively exposed to increasing concentrations of D . tertiolecta i.e. 0, 1000, 5000,
21
and 10 000 cells ml . During June, worms were exposed to each concentration for 8
versus 4 h in October. Friedman two-way analysis of variance was performed to detect significant changes in the filtering activity with increasing food concentrations. Signifi-
cant differences between matched samples were tested using Wilcoxon signed-rank tests.
3. Results
3.1. Description of the filtering activity The filtering activity of Ditrupa arietina is presented in Fig. 2 for 15 worms collected
` in February 1998 at the site off Argeles-sur-Mer. This example illustrates what was
observed in experiments conducted to describe the standard filtering activity i.e. in absence of any stimulus of D
. arietina. The basic pattern of filtration in D. arietina Fig. 2A consisted of a succession of active periods i.e. gill-crown openings and rest i.e.
gill-crown retracted inside the tube. Such patterns are described by the following three variables: i the total filtration duration i.e. total time during which the gill-crown was
wide-open, ii the number of filtering events i.e. number of times the gill-crown went out, and iii the average duration of each filtering event Fig. 2B. During the
time-course of the survey, gill-crowns stayed widely opened for 3 to 21 h i.e. 12.5 and 87.5 of the total experiment time, and worms filtered between 23 and 135 times of
which the average duration ranged from 2 to 48 min. The number of simultaneously filtering worms Fig. 2C changed very frequently but was usually comprised between
between six and nine and never exceeded thirteen. Spectral analysis see corresponding periodogram; Fig. 3 did not reveal any clear periodicity in temporal changes of the
number of filtering worms. The cumulative filtration time Fig. 2D increased steadily during experiment. These results suggest that, on a daily basis, there was no particular
rhythm in the filtering activity of D
. arietina. 3.2. Monitoring of filtration activity
3.2.1. Comparison between populations The time series of total duration of filtration for worms originating from the two study
sites Fig. 4 sites showed a significant date but not location effect two-way ANOVA, P ,0.001 and P 50.281, respectively. There was also a significant interaction between
the two factors P ,0.001 which means that the date effect differed at the two sites. From January to April the filtration durations were rather high around 60 of the
experiment time and constant for both populations. The lowest values were observed in
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` Fig. 2. D
. arietina. Standard filtering activity in 15 worms collected in February 1998 off Argeles-sur-Mer. A Filtering pattern for a single worm dark bars account for filtering. B Box plots showing total filtration
durations, the number of filtering events and the average duration of the filtering events run by the batch of worms boxes encompass 25 and 75 quartiles; the central line represents the median and bars encompass
95 limits of the obtainable values. C Temporal changes in the number of worms filtering. D Cumulated filtration time per worm.
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Fig. 3. D . arietina. Periodogram corresponding to the temporal changes in the number of worms filtering.
` May, with average filtration durations of only 23 and 12 for the Argeles-sur-Mer and
Banyuls-sur-Mer populations, respectively. Between June and December, the time dedicated to filtration was more variable and often accounted for less than 50 of the
experiment time. For both of the time series Fig. 5A, total filtration duration correlated
` positively with average duration of the filtering events for Argeles-sur-Mer and bay of
Banyuls-sur-Mer populations, r 50.955, n 512, P ,0.001 and r 50.795, n 510, P 5 0.006, respectively. No correlation was observed between the total filtration duration
and the total number of filtering events Fig. 5B. During the year, the average near-bottom water temperature ranged from 10.8 February to 22.08C September.
Fig. 4. Comparison between populations. Seasonal changes in the total filtration duration means6S.D.; n 515of worms originating from the two study sites together with the monthly averages of near-bottom
temperature. The double arrow indicates the spawning period.
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Fig. 5. Comparison between populations. Relationships between total filtration durations and A average durations of each filtering event, and B numbers of filtering events, for the populations of Banyuls-sur-Mer
` j and Argeles-sur-Mer h.
There was no correlation between near-bottom temperature and the activity of the gill-crown of D
. arietina P.0.05 for both population. Temporal changes in the proportion of worms carrying oocytes Fig. 6 shows that, in
January 1998, the number of gravid females accounted for 20 of the total population of the bay of Banyuls-sur-Mer. This proportion increased to 50 at the beginning of
March, suggesting that the whole population was sexually mature at this time sex ratio in D
. arietina is 1:1. Release of oocytes started during April and ended in July; most of the oocytes depletion occurred in April and May. Spawning period coincided with the
period of decrease total filtration duration Fig. 4.
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Fig. 6. Monitoring of the filtration activity. Temporal changes in the proportion of individuals carrying oocytes in the coelomic cavity. Values correspond to samples of 150 worms collected every 2 weeks from January to
September 1999 within the bay of Banyuls-sur-Mer population. The region in grey indicates the period of spawning.
3.2.2. Effect of worm age The time series to assess changes in filtering activity in a declining population Fig.
22
7A showed a worm density close to 1500 ind m in May, that regularly decreased
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during the survey to less than 200 ind m in December. Mean individual dry weights
confirmed that worms were old adults as the individual biomass remained constant ´
Medernach and Gremare, 1998. Simultaneously Fig. 7B, the total time spent by the worms in suspension feeding decreased dramatically one-way ANOVA, P ,0.001. By
the end of the survey worms were filtering on average less than 7 of the total experiment time. The number as well as the average duration of the filtering events also
changed over time one-way ANOVA, P ,0.001, for each variable. They were drastic decreases in total filtration durations resulting from the joint reduction of the number and
the average duration of the events of filtration. In December, worms had an average of two filtering events of which the mean duration lasted only 2 min.
3.3. Effects of environmental factors 3.3.1. Effect of the presence of food
3.3.1.1. Experiment 1 To determine effect of food on filtration activity, we first had to evaluate a potential date effect. Temporal changes in filtering activity Fig. 8 showed
that the total filtration duration in the absence of food was significantly affected by the
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Fig. 7. Monitoring of the filtration activity. Decreasing filtering activity in a declining population. A Temporal changes in density
j and individual dry weight h. B Effect of worm age on filtering activity. Box plots showing total filtration durations, the number of filtering events, and the average duration of the
filtering events run by the batch of worms boxes encompass 25 and 75 quartiles; the central line represents the median and bars encompass 95 limits of the obtainable values. Stars and dots indicate observations,
respectively, outside the 95 limits and outside the 99 limits.
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Fig. 8. Effect of the presence of food. Temporal changes in the effect of the presence of food on the total duration filtration for 12 batches of randomly selected worms. Values correspond to mean6S.D. n 512.
experiment date n 5141, df511, P ,0.001. For both of the treatments, the lowest values were measured 35 days after the worm collection i.e. on 20th May.
The characteristics of the 141 filtration record pairs i.e. absence vs. presence of food; three records were unexploitable Fig. 9A showed that the presence of food enhanced
the total time dedicated by the worms to filtration Wilcoxon signed ranks test, n 5141, P ,0.001. In addition, the total duration of filtration following food addition correlated
2
positively with values measured in filtered seawater n 5141, r 50.679, P ,0.001. The enhancement of the total duration of the filtration could reflect two kinds of changes in
the filtering behaviour of the worms: either an increase in the frequency of the filtering events Fig. 9B, or an increase in the average duration of filtration events Fig. 9C.
Pairwise comparison tests indicated that the presence of food led to the lengthening of the duration of individual events of filtration Wilcoxon signed-ranks test, n 5141,
P 50.001 rather than to an increase of the number of filtration events Wilcoxon signed-ranks test, n 5141, P 50.339.
Individual spectral analysis of filtering behaviour of the worms did showed neither a particular rhythm in the filtering activity nor synchronicity in the activity between
worms. 3.3.1.2. Experiment 2 In June, the average time spent filtering by the worms under each
of the food concentrations did not vary significantly Fig. 10; Friedman two-way
21
ANOVA, P 50.296. Times ranged from 241 no food to 341 min 1000 cell ml .
The mean number of filtration events, as well as the average duration of these events remained unchanged for all cell concentrations Friedman two-way ANOVAs, P 50.139
and, P 50.331, respectively. In October, the mean standard filtering activity i.e. in the absence of food was lower
than in June Fig. 10. The mean duration of the total filtration time ranged from 80 no
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Fig. 9. Effect of the presence of food. Individual effect of the presence of food on A the total duration of filtration, B the total number of filtering events, C the average duration of the filtering events. Straight lines
correspond to the first bisecting line i.e. no changes in the filtering parameters.
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Fig. 10. Effect of food availability. Changes in the filtering activity of worms successively exposed to increasing phytoplanktonic cell concentrations. Box plots showing total filtration durations, the number of
filtering events and the average duration of the filtering events run by the two batch of worms boxes encompass 25 and 75 quartiles; the central line represents the median and bars encompass 95 limits of the
obtainable values. Stars and dots indicate observations, respectively, outside and far outside the 95 limits.
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food to 173 min 10 000 cells ml . Four worms did not start or stopped filtering for
several hours, explaining the large dispersion of the data. In spite of such variability, there was a significant effect of food particle concentration on filtration duration
Friedman two-way ANOVA, P 50.021. The average duration of the filtering events was also significantly affected by the concentration of food Friedman two-way
ANOVA, P 50.024. Wilcoxon signed rank tests indicated that filtering events lasted longer at the highest concentration than at the two lowest concentrations Wilcoxon
signed-rank tests, P 50.008 and P 50.009 but that the number of filtering events remained unchanged Wilcoxon signed-rank tests, P 50.084 and P 50.099.
4. Discussion
