Functional morphology of the muscles in Philodina sp (Rotifera : Bdelloidea)

Whole-mounts of Philodina sp., a bdelloid rotifer, were stained with fluore scent-labeled phalloidin to visualize the musculature. Several different mu scle types were identified including incomplete circular bands, coronal ret ractors and foot retractors. Based on the position of the larger muscle ban ds in the body wall, their function during creeping locomotion and tun form ation was inferred. Bdelloid creeping begins with the contraction of incomp lete circular muscle bands against the hydrostatic pseudocoel, resulting in an anterior elongation of the body. One or more sets of ventral longitudin al muscles then contract bringing the rostrum into contact with the substra te, where it presumably attaches via adhesive glands. Different sets of ven tral longitudinal muscles, foot and trunk retractors, function to pull the body forward. These same longitudinal muscle sets are also used in `tun' fo rmation, in which the head and foot are withdrawn into the body. Three sets of longitudinal muscles supply the head region (anterior head segments) an d function in withdrawal of the corona and rostrum. Two additional pairs of longitudinal muscles function to retract the anterior trunk segments immed iately behind the head, and approximately five sets of longitudinal retract ors are involved in the withdrawal of the foot and posterior toes. To achie ve a greater understanding of rotifer behavior, it is important to elucidat e the structural complexity of body wall muscles in rotifers. The utility o f fluorescently-labeled phalloidin for the visualization of these muscles i s discussed and placed in the context of rotifer functional morphology.