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mise à jour du
8 janvier 2004
2002; 420; 41-42 
Suction feeding by a tiny predatory tadpole
Stephen Deban


This amphibian shoots its mouth forwards in a fish-like manner to suck in its prey
Pipid tadpoles of the African genus Hymenochirus are not only among the smallest free-swimming, feeding vertebrates, but are also predatory suction feeders unlike other tadpoles, which typically ingest a suspension of organic particles. Here we use a high-speed video system to study details of the feeding mechanics of Hymenochirus boettgeri tadpoles and find that they first track individual prey organisms visually, and chase and then capture them by mouth using suction. This feeding mechanism is unique among frogs and is strikingly convergent with that used by teleost fishes.
Frogs of the genus Hymenochirus are unique in that both adults and tadpoles are predatory suction feeders. In most species of frog, adults use their jaws, tongue or forelimbs to capture prey, and tadpoles are usually suspension-feeding detritivores that use scraping mouthparts to create the suspension. This suspension is pumped into the mouth by rhythmic movements of the hyobranchial apparatus (throat skeleton) , particles are trapped in the branchial basket, and water exits through the gill slits or a single spiracle.
Hymenochirus tadpoles, however, are morphologically divergent, lack a filter apparatus and scraping mouthparts, and have huge, frontally orientated eyes1. Also, they are obligate air breathers and do not pulse-pump to irrigate the gills like most other tadpoles. Unaided visual observation indicates that they are predatory carnivores, but their minute size (the body length is less than 1 mm at first feeding) and rapid movements make the details of their feeding mechanics difficult to determine.
Figure 1 Morphology and function of suction feeding in the tadpole Hymenochirus boettgeri(body length, 2.6 mm). a, High-speed
video sequence of feeding, showing hyobranchial depression, mouth extension and cranial elevation. Prey is engulfed within 4 ms of
the commencement of mouth opening. White arrowhead in frame 4 indicates location of prey (the brine shrimp at the nauplius stage)
in the pharynx. Scale bar, 1 mm. b, 'Yawning' tadpole, showing the hydrodynamically favourable round mouth aperture and the large,
frontally orientated eyes. c, Dynamics of the tadpole's feeding apparatus, showing the mouth extension and hyobranchial movements
that generate suction to ingest prey. Meckel's cartilage (lower jaw) and the ceratohyals are shown in light grey, the copula (basibranchial)
is shown in medium grey, and the ceratobranchials are shown in dark grey.
We used a high-speed video system (1,000 Hz) with a powerful macro lens that enabled us to follow the feeding behaviour and mechanics of H. boettgeri tadpoles (2Ð3 mm body length; Gosner stage 26). Our recordings reveal that they target each prey item visually, pursue it, then capture it by extension of a tubular mouth during an explosive buccal expansion (Fig. 1a, b) for movie, see supplementary information). Tadpoles complete their mouth extension within 2 ms and engulf the prey within 4 ms; prey travel into the mouth at 0.6 m s-1. Buccal expansion is completed within 7 ms.Comparably sized larval teleosts are slower feeders than these tadpoles, taking 4Ð12 ms to engulf their prey, which enter the mouth at 0.03Ð0.3 m s11. We calculated a Reynolds number of 300 for prey capture in tadpoles (from prey velocity and tadpole mouth diameter) compared with 5Ð70 for larval teleosts. The higher Reynolds number indicates that, although Hymenochirus is about the same size as larval teleosts, it is faster and better at overcoming the viscous drag that typically confronts small aquatic organisms.
We compared our video images of moving Hymenochirus with preserved specimens and found that the tadpole's suction action is generated by a combination of hyobranchial movements (ceratohyal depression, basibranchial retraction) and cranial elevation; downward rotation of the lower jaw unfurls the soft tissues that comprise the extensible mouth (Fig. 1c). After prey capture, the tadpoles expel water slowly (over 200 ms) through the paired gill slits of the reduced and simplified branchial basket1 by raising the ceratohyals and basibranchial and lowering the head to their resting positions. The Reynolds number drops to 50 during water expulsion (Fig. 1a), and is lower for smaller Hymenochirus tadpoles when they begin feeding. Viscosity may thus be more problematic during water expulsion, as has been proposed for larval teleosts.
The feeding mechanism of Hymenochirus is remarkably like that of teleosts, which also suction-feed by using a combination of rapid mouth protrusion, hyobranchial depression and cranial elevation, followed by slower water expulsion through the gill slits. Hymenochirus and teleosts also share a hydrodynamically advantageous round mouth opening.
Rapid mouth protrusion confers several benefits Ñ it decreases the distance to the prey, accelerates flow through the mouth, restricts flow to the area in front of the mouth, and reduces the momentum (mass * velocity) that must be imparted to the water. Suction feeders that have nonprotrusible mouths, such as some firstfeeding larval fish, must suck in more water than animals that have protrusible mouths. They therefore generate more momentum and suck themselves forwards. Hymenochirus imparts little momentum to the water, resulting in only a slight forward movement of the tadpole's body (Fig. 1a). Miniaturized Hymenochirus tadpoles begin feeding at a smaller size than larval teleosts, which are universally suction