Group sweeping: Feeding activity and filtration rate in the tubiculous amphipod Haploops nirae (Kaim-Malka, 1976)
Résumé
Haploops nirae is a gregarious tubiculous amphipod that uses its two pairs of antennae to filter particles from the water column. Recent sonar mappings of Haploops seabeds revealed that thousands of hectares of softbottom muddy sediments in South Brittany coastal ecosystems are now colonized by this amphipod. The present study investigates (1) the structure of the feeding organs and the mechanisms involved in particle capture, (2) the effect of environmental parameters (current velocity, turbidity and food concentration) on the feeding activity of H. nirae, quantified by the percentage of active individuals, the frequency of antennae beats and the number and duration of feeding events and (3) the retention efficiency and clearance rate of H. nirae individuals, so extrapolation to colonized areas and grazing impact of this species can be estimated. Staining techniques and scanning electron microscopy revealed that Haploops antennae have a more complex structure than congeneric Ampelisca species, involving long setae bearing short setules and that mucus is involved in particle capture. An increase in current velocity seemed to have very little effect on feeding activity but unexpectedly slightly increased the frequency of antennae beats. However, an increase in both organic and inorganic matter both exponentially increased the number of filtering individuals and showed that Haploops are well adapted to feed in turbid conditions: the total time individuals spent filtering remained constant while inorganic particulate matter increased from 10.7 to 85.6 mg.L-1. Using a flow-through system, evaluation of retention efficiency showed that only particles larger than 20 mu m equivalent spherical diameter were 100% retained by feeding structures, which corresponds to the mean distance between two setules. Clearance rate was estimated at 14.6 +/- 0.4 mL.h(-1).ind(-1) or standardized to 25.2 +/- 0.7 L.h(-1).g(-1) dry weight. Those values are discussed in light of values obtained in other suspension-feeding species and also extrapolated to the whole Haploops community. We estimated that Haploops have the capacity to filter the entire water column in 4-5 days and a volume of water equivalent to the whole bay in 29-30 days. This study provides evidence that changes in the spatial extent of Haploops communities may have serious impacts within coastal ecosystem they have colonized.