Changing form and function during development in rowing hydromedusae
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Bell morphology, propulsion and foraging appear to be interdependent traits among medusan lineages. In general, taxa that possess large oblate bells swim via rowing propulsion and forage as cruising predators, and taxa with small prolate bells swim via jet propulsion and forage as ambush predators. However, hydromedusae from the taxa Leptothecata (referred to here as leptomedusae) experience large changes in bell size and shape during their development. We used video analysis to evaluate changes in bell morphology and kinematics, fluid interactions and behavior of 2 leptomedusan species, Aequorea victoria and Eutonina indicans, during these developmental transitions. Both species begin as small, prolate, jetting juveniles (<1 mm diameter) and develop into large, oblate, rowing adults (> 0.5 cm diameter). In addition to morphological and propulsive alterations, we observed changes in behavior, whereby as jetters they rarely swam and as rowers they swam nearly continuously. These findings demonstrate that bell fineness, velar aperture ratio and bell kinematics interact to determine fluid interactions and thus determine propulsive mode.