How a Frog, Pipa pipa, Succeeds or Fails in Catching Fish
We quantified factors contributing to failed or successful prey captures by Pipa pipa, a permanently aquatic, tongueless frog widely distributed in Amazonian South America. Pipa catches fish by entraining and ingesting large volumes of water and by limiting fish escape with its fingers. Based on analysis of high-speed video (250 and 500 fps), feeding attempts appeared superficially stereotyped, but many features were modulated and slower than in other suction feeders. For both successful and unsuccessful capture attempts, the entire frog might or might not move and fingers might or might not contact the prey. Mouth opening generated initial suction but continued movement of entrained water depended on actively enlarging the volume of the anterior trunk by depressing the ventral pectoral girdle. Although captured fish varied in size and position relative to the frog, both fish size and the distance of the fish from the frog's mouth at the initiation of mouth opening were significantly greater for unsuccessful attempts. Our data suggest that capture success depends partly on sensory evaluation of prey size and distance that initiate capture movements and partly on the independent probabilities of rapidly moving parts in two different organisms favoring the predator or its prey.
(A) Oblique ventrolateral view of a CT scan of Pipa pipa, MCZ A7646, showing positions of the larynx, clavicles, and coracoids of the ventral pectoral girdle. (B) A drawing of the same view of the skeleton showing the positions of the cartilaginous epicoracoids (from Trueb et al., 2000 and AMNH 58075) and the hyoid and hyoglossal foramen lying dorsal to the epicoracoids along the dorsolateral edge of the larynx (stippled), as determined from dissections and Ridewood (1897).
Six frames from high-speed video of an anterolateral view of a lunging capture of a Carassius auratus by Pipa pipa, showing, at 0 ms, beginning of toe movement; 2 ms, mandible depression; 8 ms, middle of mandible depression, beginning of intraoral suction and forward lunge; 48 ms, beginning of pectoral girdle depression and trunk negative pressure, movement of fish into frog's mouth; 110 ms, dorsal bending of the trunk, continued trunk negative pressure, beginning of reversal of body movement and peak shoulder girdle depression, entry of front feet into mouth, although the fingers did not contact the prey; and 180 ms, inflation of trunk shortly following the end of trunk negative pressure and removal of front feet from mouth.
Six frames of anterior and anterolateral views of a stationary capture event by Pipa pipa with finger movement but with no contact of fingers with the prey (Carassius auratus). The final frame shows asymmetric jaw position with prey in the oral cavity during expulsion of water. 0 ms, beginning of finger movement; 4 ms, 1 frame (2 ms) after beginning of mouth opening; 12 ms, beginning of clavicle and hyoid retraction and depression and beginning of fish (Carassius auratus) movement into oral cavity; 28 ms, fish (except tail) within oral cavity, middle of pectoral depression; 44 ms, fish in caudal region of buccopharyngeal cavity, peak pectoral depression; 488 ms, middle of water expulsion from buccopharyngeal cavity that combined with slight rearward movement of the whole frog.
Clavicle and hyoid movement: 0 ms, beginning of finger movement by Pipa pipa; 12 ms, mouth partially open, clavicle and hyoid pressed against roof of oral cavity, but beginning of clavicle and hyoid retraction and depression; 18 ms, clavicle and hyoid in process of depression and retraction; 24 ms, clavicle and hyoid no longer visible, one finger almost in contact with fish (Carassius auratus). The fish was caught.
Fish (Carassius auratus) in body of Pipa pipa: 0 ms, beginning of capture; 80 ms, peak gape and pectoral girdle depression; 204 ms, inflation of buccopharyngeal cavity with fish near its caudal border; 216 ms, fish eye visible through body wall of frog (Pipa pipa). This sequence also shows the approximate caudal limit of water inflow, which matches the border of the inflated buccopharyngeal cavity. Fish movement inside the frog is clearly evident in the video although fish position is difficult to see in single frames.
Eleven views (five anterolateral, five dorsal from the same video frames, and one of the frog [Pipa pipa] at peak inflation) of an unsuccessful capture attempt on a fish (Carassius auratus: arrows) too far from the frog at the initiation of the event and during which the frog delayed pectoral girdle depression by more than 50 ms.
Eight views of an unsuccessful attempt to capture a large fish (Carassius auratus) during which the right manus of Pipa pipa is repositioned (52–76 ms) after the fish initiates a C-start (34–76 ms) that drives its head below the major volume of water entrained by suction.
Four video frames showing asymmetric mouth opening by Pipa pipa and the behavior of the mandibular symphysis during capture of Carassius auratus. 0 ms is the frame before the first frame showing movement of the frog's right mandible, which is depressing at 8 ms, prior to depression of the left mandible, which began at 12 ms and shows clearly at 14 ms. At 40 ms the mouth has reached peak gape, the relative cross-sectional area of which is increased by flexion around the symphysis. The frog's right manus is adducted during suction but its fingers never contact the fish.
Contributor Notes
Associate Editor: J. D. Litzgus.