Modes of algal capture by the freshwater copepod Diaptomus sicilis and their relation to food‐size selection 1, 2

High‐speed motion pictures (500 frames s− 1 ) of tethered Diaptomus sicilis feeding in suspensions of Chlamydomonas spp. of three different sizes at three different concentrations (0.3, 1.0, and 3.0 mm 3 liter −1 ), showed two modes of feeding: a passive mode in which algae flowed into the space between the left and right second maxillae and an active mode in which the second maxillae or second maxillae plus maxilliped made large amplitude flaps to bring in the cell. Cells ≤4 µ m in diameter were always captured passively, and cells ≥6 µ m were captured both actively and passively. Active captures were 18% of total captures for the medium‐sized (6 µ m) algae and 36% for the large (12 µ m) algae. During passive captures, algae usually flowed through the space between the left and right second maxillae rather than being caught on the setules; thus, the leaky‐sieve model of particle retention does not apply for passive captures by D. sicilis. Except for brief interruptions to actively capture a large alga nearby, the second maxillae of D. sicilis oscillated continuously at low amplitude at all times during feeding, in marked contrast to the marine copepods Eucalanus and Paracalanus that vibrated their second maxillae to enhance capture in monocultures of small algae but not large algae. These observations explained both the shape of the curve of selectivity vs. particle size for D. sicilis and its relative invariance. Analyses of feeding mechanisms and the water currents around Diaptomus showed that, in contrast to Eucalanus and Paracalanus, Diaptomus is specialized for capturing small particles, which may be a more important food in freshwater than in marine environments.