The rowing-to-flapping transition: ontogenetic changes in gill-plate kinematics in the nymphal mayfly Centroptilum triangulifer (Ephemeroptera, Baetidae)

Comparative studies encompassing a wide range of aquatic animals have shown that rowing is exclusively used at low Reynolds numbers (Re < 1), whereas flapping is predominantly used at Re > 100, although few studies have been undertaken to document the transition in individual species that traverse the intermediate Re regime using a single set of appendages. Thus, it is not generally known whether a gradual increase in Re within a system results in a gradual or sudden shift between rowing and flapping. In the present study, we document ventilatory kinematics of a nymphal mayfly Centroptilum triangulifer that develops using a serial array of seven pairs of abdominal gill plates and operates at Reynolds numbers in the range 2-22 during ontogeny. We found that some kinematic variables (stroke frequency and metachronal phase lag) did not change during ontogeny but that others changed substantially. Specifically, gill kinematics in small instars used strokes with large pitch and stroke-plane deviations, whereas larger instars used strokes with minimal pitch and minimal stroke-plane deviation. Gills in larger instars also acquired an intrinsic hinge that allowed passive asymmetric movement between half strokes. Net flow in small animals was directed ventrally and essentially parallel to the stroke plane (i.e. rowing), whereas net flow in large animals was directed dorsally and essentially transverse to the stroke plane (i.e. flapping). The change in whole-gill kinematics from rowing to flapping occurred across a narrow Re range (3-8), which suggests a possible hydrodynamic demarcation between rowing and flapping. (C) 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 540-555.