Myliobatid Ray Gliding Dynamics: Experimental Tests of Body Shape and Tail Length on Stability.

Myliobatid stingrays (eagle, cownose, and manta rays) swim using oscillatory locomotion, flapping their pectoral fins for propulsion while relying on their elongated tails for stability. During swimming, myliobatids often exhibit gliding behavior, a passive locomotion mode when active flapping ceases and the pectoral fins are maintained in a static position. We hypothesized that different pectoral fin conformations influence body stability and that the tail plays a critical role in stabilizing the models during gliding. To test this, we designed and 3D-printed four myliobatid-inspired models with different pectoral fin conformations: three with increasing dihedral angles and one model with an anhedral configuration. Each model was tested with three tail lengths: twice the disc width, equal to disc width, and no tail. Models were tested in a flow tank at increasing flow velocities. Stability, determined by pitch, roll, sway, and ODBA (overall dynamic body acceleration), was measured using high speed video and an accelerometer embedded into each model. When the models were compared without tails, the position of the pectoral fins also affected stability. Among models with dihedral angles, stability decreased with increasing dihedral angle. The model with an anhedral conformation was the most unstable. However, all models significantly reduced pitch, roll, sway, and ODBA with the presence of the tail, indicating that the tail had a stabilizing effect in all models regardless of the pectoral fin conformation. These findings indicate that pectoral fin conformation has a substantial effect on body stability and, in combination with the tail, enables stable passive gliding. Understanding the effect of body and pectoral fin posture on stability during locomotion is important for future efforts to analyze the energetic cost of locomotion and to understand the principles of efficient underwater movement.