E . Jordana et al. J. Exp. Mar. Biol. Ecol. 252 2000 57 –74
71
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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
Results of the present study showed that there was a strong inter-individual variability in the filtering activity of Ditrupa arietina. Worms filtered neither continuously nor
according to a particular rhythm. Moreover, there was no synchrony in the filtering activity between individuals. This trend was illustrated by the large dispersion of the
data around the average values.
4.1. Effects of life history There was a seasonal pattern of variation in the filtering activity of D
. arietina. Three main periods were characterised in this study. In the first period, lasting from January to
April, total filtration durations were high and relatively constant. In the second period, in May and June, there was a dramatic drop in the filtering activity. Lastly, in the third
period, from June to December, the total time dedicated by the worms to filtration was highly variable. Such seasonal changes in the feeding activity already have been reported
in bivalves, and were related to changes either in the food availability Newell and Bayne, 1980 or in the water temperatures Kamermans, 1994. In the present case, the
activity of the gill-crowns of D
. arietina was independent of the water temperature, but not of food availability see paragraph below. During the first two periods, worms of
both populations behaved similarly. Thus, filtering activity may be controlled by one strong factor or even by a combination of several factors. Through the year excepted in
February, the phytoplanktonic content of the waters of the bay of Banyuls-sur-Mer is rather low and constant Jacques, 1970; Jordana, personal communication. This suggests
that food availability alone can not entirely explain the seasonal variations in the filtration activity of D
. arietina. So, other factors must affect the filtering activity such as endogenous factors like
reproductive activity. The filtering activity would first be high to insure gonad growth and then would drop drastically in May after the spawning period. At that time of the
year, worms appeared exhausted. They were thin, and lot of them had regenerating gill filaments Charles, personal observation. The drastic drop of the filtering activity in
` May was previously observed during 1997 on worms of the population of Argeles-sur-
Mer Jordana, unpublished data. The experiments run here to determine the effect of the presence of food on these worms was to ascertain the cause of the occurrence of a slack
72 E
. Jordana et al. J. Exp. Mar. Biol. Ecol. 252 2000 57 –74
period of the filtering activity in May. During the third period from June to December, the total duration of filtration varied with time but it also seemed to change according to
population. Such differences can be related to local environmental conditions which may be specific for both populations, but also to population aging. Ditrupa arietina has a
2-year lifespan Medernach et al., 2000. At the beginning of the study the population of
` Argeles-sur-Mer was mainly composed of 2-year-old worms 1,age2 years old. This
population aged during the course of the study. In June, their filtering activity was still fairly low in comparison with that of younger worms ,1 year of the bay of
Banyuls-sur-Mer. This difference in the filtering activity Fig. 6 may be explained by the effect of the senescence of one part of the population. In this case changes in filtering
activity can be related to the physiological state of the worms. As proportionally more worms aged, the number of filtering events decreased and the duration of these events
was reduced.
4.2. Effects of environmental factors The experiments dealing with presence of food showed that the filtering activity of D
. arietina was indeed affected by variations in external factors. The results of these
experiments also showed that recent physiological state had a significant effect on the response of worms facing changes in their environment. Thus, according to the level of
the standard filtering activity i.e. in absence of stimulus, total filtration durations increased or remained stable.
The range of food concentrations tested here was similar to those experienced by the worms in the field Jacques, 1970, Jordana et al. submitted. Our experiments showed
that worms kept their gill-crowns wide-open even when food concentration reached
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1310 cells ml . However, no relationship was noticed between filtration activity and
food availability. When presence of food enhanced filtering activity, in most of the cases this increase was associated with a lengthening of the average duration of the filtering
events. In suspension feeding polychaetes, filtration rate is assumed to be constant when gill-crown is wide-open Riisgard and Ivarsson, 1990, suggesting that for the range of
food concentrations tested in the present study, a decrease in the clearance rate with increasing food concentration would be due to an overloading of the feeding system and
not to a reduction of the activity of the gill-crown. Davies et al. 1989 showed, for another serpulid Ficopomatus enigmaticus, that ingestion rates increased with particle
21
concentrations ranged from 1 to 10 mg ml . These results suggest that, during the
present study in which particle concentrations were lower, clearance rates probably remained unchanged and so ingestion rate of D
. arietina increased steadily with food concentrations.
5. Conclusion
