Scaling of ctenes and consequences for swimming performance in the ctenophore <i>Pleurobrachia bachei</i>

<jats:title>Abstract</jats:title><jats:p>Ctenophores coordinate large macrociliary structures called ctenes to propel themselves through the water. The morphology and kinematics of the ctenes mediate swimming performance. We investigated morphological and kinematic factors affecting swimming performance in free‐swimming ctenophores (<jats:italic>Pleurobrachia bachei</jats:italic>) using high speed videography. Our morphological results showed that the relationship between body size and ctene morphology and arrangement in <jats:italic>P. bachei</jats:italic> were well described using linear (i.e., isometric) relationships, which suggests functional limitations of ctenes that vary among individuals of different sizes. Our kinematic results showed that isometric constraints on swimming performance can potentially be overcome by alterations in kinematics: (a) swimming speed in <jats:italic>P. bachei</jats:italic> increased with ctene beat frequency over a range of body lengths, and (b) the separation of ctenes into clumps of cilia allowed the ctene to increase in width during the effective stroke and decrease in width during recovery. Separation increases the surface area of the ctene during the effective stroke, likely increasing the thrust produced. The finding that ctenes are not monoliths and instead are separated into clumps of cilia has not been previously described, and we subsequently observed this trait in three other ctenophore species: <jats:italic>Euplokamis dunlapae</jats:italic>, <jats:italic>Bolinopsis infundibulum,</jats:italic> and <jats:italic>Beroe mitrata</jats:italic>. Flexibility in function may be a necessary corollary to isometric development of the ctenes as propulsive structures.</jats:p>