5 for the tips of the antennulae and antennae, respectively, during the fastest part of their power strokes.
Thus, during their power strokes, both the antennulae and antennae (and their setae) are in the high drag orientation (perpendicular to relative fluid movement) and switch to a low drag orientation during the recovery stroke.
At the low Reynolds numbers in which these animals function, the dominance of viscous forces disallows effective coasting between power strokes (Williams, 1994).
The observed coordination between limbs probably has advantages over an alternative strategy in which all four limbs start and complete power strokes at the same time.
In fact, we have observed a different, unidentified cypridoidean species in which no power strokes are coordinated.
The stroke lies within the sagittal plane, with only slight outward movement near the end of the power stroke.
As the power stroke ends and the recovery stroke begins, these long setae collapse from a fan into a tight bundle.
Its power stroke sweeps posteriorly and somewhat ventrally (Fig.
Then, the antenna moves outward, resuming its initial position at the beginning of the power stroke (Fig.
Thus the movement and coordination of the swimming limbs described here do not appear to be artifacts of tethering (which forces all the energy from the animal's power stroke to go into moving the external fluid rather than producing relative motion between the animal and the fluid; see Emlet and Strathmann, 1985; Emlet, 1990).
For net forward motion, the total forces exerted against the fluid during each power stroke must exceed the total forces exerted against the fluid during the recovery stroke.