Individuals of Idotea resecata and I. wosnesenskii were videotaped at 200 frames s−1. while swimming freely. Propulsion is provided by the first three pairs of abdominal appendages (pleopods), which may also function as gills. Unlike typical crustacean metachronal beating, in Idotea all three pairs of propulsive pleopods begin their recovery strokes simultaneously. Each pair then carries out its power stroke in sequence: third pleopods have a short power stroke, then second pleopods have an intermediate power stroke, finally first pleopods have a long power stroke. After these power strokes, there is a pause before the next recovery stroke begins. The duration of the power stroke of any pair of pleopods, as well as the overlap with other pleopods' power stroke, is variable and is not directly related to swimming speed. Stroke amplitude is approximately constant, but stroke frequency is significantly correlated with swimming speed. Other stroke variables which could affect swimming speed are also loosely correlated with frequency, but it appears that frequency is the most important determinant of swimming speed. The unusual stroke pattern in Idotea may be related to the respiratory function of the pleopods.

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