The use of pelvic fins for benthic locomotion during foraging behavior in Potamotrygon motoro (Chondrichthyes: Potamotrygonidae)

Synchronized bipedal movements of the pelvic fins provide propulsion (punting) during displacement on the substrate in batoids with benthic locomotion. In skates (Rajidae) this mechanism is mainly generated by the crural cartilages. Although lacking these anatomical structures, some stingray species show modifications of their pelvic fins to aid in benthic locomotion. This study describes the use of the pelvic fins for locomotory performance and body re-orientation in the freshwater stingray Potamotrygon motoro (Müller & Henle, 1841) during foraging. Pelvic fin movements of juvenile individuals of P. motoro were recorded in ventral view by a high-speed camera at 250-500 fields/s-1. Potamotrygon motoro presented synchronous, alternating and unilateral movements of the pelvic fins, similar to those reported in skates. Synchronous movements were employed during straightforward motion for pushing the body off the substrate as well as for strike feeding, whereas unilateral movements were used to maneuver the body to the right or left during both locomotion and prey capture. Alternating movements of the pelvic fins are similar to bipedal movements in terrestrial and semi-aquatic tetrapods. The pelvic fins showed coordinated movements during feeding even when stationary, indicating that they have an important function in maintaining body posture (station holding) during prey capture and manipulation. The use of pelvic fins during prey stalking may be advantageous because it results in less substrate disturbance when compared to movements generated by pectoral fin undulation. The range of pelvic fin movements indicates more complex control and coordination of the pelvic radial muscles.底栖运动型鳐类（batoids）依靠腹鳍（pelvic fins）的同步双足式运动在底质表面行进并获得推进力，该运动模式亦被称为「撑底推进」（punting）。在鳐科（Rajidae）物种中，该运动机制主要由小腿软骨（crural cartilages）驱动。尽管部分刺魟（stingray）类群缺失这类解剖结构，但它们的腹鳍已演化出特化形态以辅助完成底栖运动。本研究以淡水刺魟珍珠魟（Potamotrygon motoro, Müller & Henle, 1841）为研究对象，阐述了其在觅食过程中腹鳍的运动模式及其在运动表现与身体姿态调整中的作用。本研究采用250~500帧/秒的高速摄像机（high-speed camera）从腹面视角记录了幼年珍珠魟的腹鳍运动过程。珍珠魟的腹鳍运动可分为同步、交替与单侧三种模式，这与此前在鳐科物种中报道的运动模式一致。同步运动模式用于直线行进以推动身体脱离底质，同时也会在突袭捕食时启用；单侧运动模式则用于在行进或猎物捕获过程中调整身体的左右转向。腹鳍的交替运动模式与陆生及半水生四足动物的双足运动模式相似。即便处于静止状态，珍珠魟在捕食过程中也会通过腹鳍的协调运动维持身体姿态，这表明腹鳍在猎物捕获与处理过程中发挥着重要的定点维持（station holding）功能。相较于依靠胸鳍波动产生的运动，在猎物追踪过程中使用腹鳍可有效减少对底质的扰动，因此具备一定优势。腹鳍运动的多样性表明，其腹鳍辐状肌（pelvic radial muscles）的调控与协调机制更为复杂。