Functional morphology of somatic muscles and anterolateral setae in Filinia novaezealandiae Shiel and Sanoamuang, 1993 (Rotifera)

The freshwater rotifer, Filinia novaezealandiae, is a planktonic species in tropical limnetic ecosystems. Like other species in the genus, F. novaezealandiae has two mobile, anterolateral setae that supposedly facilitate saltation through the water column as a means of escape from predators. However, very little is known about setal structure or the control of setal movement. Here, we use a combination of behavioral observations, scanning and transmission electron microscopy, phalloidin staining of F-actin, conventional fluorescence and confocal laser scanning microscopy, and three-dimensional (3D) analysis software to investigate saltation in F. novaezealandiae and gain insight into the contribution that both setae and muscles play in locomotion. Results from EM reveal that the paired anterolateral setae are smooth, hollow extensions of the syncytial integument and constructed of two layers: a thin internal and thick external lamina. The setal cavity is continuous with the body cavity of the trunk, and the setae articulate with the trunk at a series of flexible folds. A lack of direct muscular supply to the setae indicates that their movement is likely to result from a combination of: (1) contractions of specific muscle sets (e.g., anterior transverse muscle, post coronal ring muscle) that insert on the body wall around the setae; and (2) changes in hydrostatic pressure within the body cavity that accompany coronal withdrawal (via contraction of retractor muscles) and extension. These descriptions, together with behavioral observations, indicate that the primary function of the setae is likely to be predator deterrence and not predator evasion. Likewise, saltation is probably a result of heightened coronal activity that immediately follows cycles of setal abduction/adduction. These insights, together with new descriptions of the muscular system in F. novaezealandiae, may be of utility in understanding the evolution of locomotory mechanisms among rotifers.