Experiments on oral suction and gill breathing in five species of Australian tadpole (Anura: Hylidae and Leptodactylidae)
Authors: Gradwell Norman 
Published: August 2009

Gill irrigation was studied in Litoria lesueuri, L. booroolongensis, L. glandulosa, L. citropa and Mixophyes balbus by monitoring hydrostatic pressure, and examining the functional anatomy and water flow.

While the sucker is engaged, the nares are the only water intakes; the nares are more important than the mouth as water intakes when the sucker is not engaged. The biconcave upper beak of L. citropa facilitates confinement of the water inflow to the nares.

The gill irrigation mechanism is similar to that of Rana catesbeiana except that M. balbus lacks a first gill cleft, and L. glandulosa lacks an auxiliary force pump behind the gill clefts. In all the five species, two alternating force pumps in front of the gill clefts drive water continuously over the gills, irrespective of the functional state of the sucker.

Except for slight differences of position, the branchiomeric muscles of the five species studied bear a close resemblance to those of R. catesbeiana.

The maximum tenacity of suctorial clinging ranged from — 36 mm H2O (L. citropa) to — 181 mm H2O (M. balbus). Average feeding pressures (4.0 to 4.5/sec) of the sucker were — 60 mm H2O for L. booroolongensis and —110 mm H2O for M. balbus. With the sucker disengaged, maximal buccal pressures for M. balbus ranged from —74 mm H2O (hyper‐inspirations) to + 240 mm H2O (hyperexpirations).

The functional anatomy of the five species of suctorial tadpole studied shows their strong dependence for respiration on narial inflow, even when their suckers are not engaged to a substratum. Their suckers are well adapted for scraping algae off rocks, which is their chief method of feeding. In the laboratory, all the species used filter feeding, but all except L. glandulosa, also fed by mastication of plants. For example, leaves are masticated with the beaks and drawn with water into the buccal cavity and pharynx. In all the species, a continuous stream of clear water emerged from the spiracle.

Tadpoles were induced to cling with their suckers over a pressure monitor embedded in a Perspex plate. This method yielded hydrostatic pressures from uninjured, unanaesthetized, and unrestrained animals. After anaesthetic implantations of cannulae into the respiratory chambers, some pressures were also recorded simultaneously with sucker pressures.

The recordings indicate that sucker pressures are generated by the buccal pump, which is continuously active while the sucker is engaged. The strength of suction varied from —36 mm H2O for L. citropa to —181 mm H2O for Mixophyes balbus. Recordings also showed that the five species have two rhythmic force pumps (buccal cavity and pharynx) in front of the gill clefts and a sporadic force pump behind the gill clefts (except L. glandulosa). In M. balbus, subambient pressures (—74 mm H2O max.) were recorded during convulsive lowering of the buccal floor in response to suspended debris pipetted into the narial inflows. This was usually followed by vigorous elevation of the buccal floor to expel the debris through the mouth. Positive buccal pressures (240 mm H2O max.) in M. balbus were recorded during the cough‐like oral expulsions of debris.

This reflex always followed a pause (0.5 to 0.75 sec) during which the mouth was opened wide.

Sucker pressures (4.0 to 4.5/sec) associated with feeding, yielded average values of — 60 mm H2O for L. booroolongensis and — 110 mm H2O for M. balbus.