To understand the ecological significance of diversity in the buccal pump morphology of anuran larvae we have developed a descriptive geometric model of the tadpole buccal pump. The model allows comparison ofthe buccal pumping mechanism ofdifferent species.The reliability of the model was established by comparing buccal volumes predicted by the model with buccal volumes reported in the literature for R a m sylvatica and Rana catesbeiana as well as those obtained from our own feeding experiments with Xenopus laevis larvae.The model was used to examine the following features for the larvae of 40 species from 1 1 anuran tainilies: ( 1 ) the amount of dellection in the buccal floor with each stroke of the buccal pump (angle of rotation of the ceratohyal); (2) the mechanical advantage of the pump (the relative length of the lever arm on the ceratohyal); and (3) buccal volume. We identify patterns in these three features that correlate with the feeding ecology of a variety of tadpoles. Macrophagous larvae tend to have a long lateral lever arm on the ceratohyal (high mechanical advantage), and produce a large buccal volume by deHecting a large buccal floor area through a shallow arc (i.e. a large bore, short stroke system). Midwater, niicrophagous larvae tend to have poor mechanical advantage, but nevertheless achieve a large buccal volume; in these forms the large buccal volume is obtained by deflecting a smaller buccal floor area through a larger arc (i.e. a small bore, long stroke system). Benthic larvae, such as stream forms with suctorial mouths, tend to have high mechanical advantage, but only modest buccal voluine.Larvae can regulate their feeding rate by varying either the pumpingrate (frequency modulation) or the buccal Hoor displacement (amplitude modulation). The anatomy of the buccal pump may determine not only the feeding ecology of a larva, but the predominant behavioral pattern it uses to regulate its feedingrate.Buccal volume has a negative allometric relationship to body length both intraspecifically and interspeci tically.KEY WORDS:-Anuratadpolesanuran larvaebuccal pumptadpole ecologytadpole morphologytadpole behaviorfeeding mechanismmorphological modellingallornetry.
Larvae of the South African clawed frog, Xenopus laevis (Daudin), are efficient, obligate suspension feeders . We examine the relationship between the ambient particle concentration offered these larvae as food and their filtering, ingestion, and buccal pumping rates . We demonstrate that : (i) the larvae can sense and respond to a broad range of particle concentrations, down to 0 .2 mg 1-1 (dry weight) ; (ii) their metabolic needs theoretically can be met by particle concentrations as low as 5 mg 1 -1 ; and (iii) their patterns of regulation of filtering and ingestion fit predictions from certain models used to describe zooplankton feeding dynamics . Two such models are discussed : the modified Monod (Michaelis-Menten) model, with a lower threshold below which the tadpoles do not feed, and an energy optimization model . Both the models and the observed behavior of the tadpoles allow for stability of populations of food organisms . Tadpole feeding dynamics apparently are compatible with both the predictions and assumptions of these models, suggesting similar regulation of feeding by tadpoles and zooplankton . However, the size, morphology, and behavior of X. laevis larvae make their feeding regulation uniquely accessible to direct observation .
Terrestrial amphibians take up water by abducting the hind limbs and pressing a specialized portion of the ventral skin to a moist surface, using a characteristic behavior called the water absorption response. An assay of the water absorption response was used to quantify physiological factors associated with thirst and water uptake. Dramatic changes in the water absorption response resulted from subtle changes in hydration state and from altering the reserve water supply in the urinary bladder. The water absorption response could be induced by intraperitoneal and intracerebroventricular injection of angiotensin II, demonstrating that components of the renin-angiotensin system on both sides of the blood-brain barrier have a dipsogenic function in amphibians. These experiments also demonstrated that the water absorption response could be influenced by changes in barometric pressure. Toads avoided the water absorption response on hyperosmotic substrates, and behavioral experiments showed that the amphibian skin served a sensory function similar to that of the lingual epithelium of mammals. The water absorption response assay has enormous potential as a tool for the investigation of physiological processes and sensory capabilities of amphibians.
Toads obtain water by absorption across their skin. When dehydrated, desert toads exhibit stereotyped hydration behavior in which they press their ventral skin onto a moist surface. However, dehydrated toads avoid surfaces moistened with hyperosmotic NaCl and KCl solutions (Hoff KvS, Hillyard SD. 1993. J. Exp. Biol. 183:347–351). We have studied neural mechanisms for this avoidance with physiologic, behavioral, and morphologic approaches. Spinal nerves innervating the ventral skin could be stimulated by exposure to a hyperosmotic NaCl solution applied to the outer surface of the skin. This neural response occurred with much longer latency than to mechanical stimulation and could be reduced by amiloride, a blocker for Na+ channels known to be responsible for epithelial ion transport and salt taste transduction. In behavioral experiments, avoidance of a NaCl solution was also reduced by adding amiloride to the solution, suggesting involvement of amiloride‐sensitive Na+ channels for detecting the hyperosmotic salt solution. Neural tracing with fluorescent dye revealed spinal nerve endings and connections to putative receptor cells, both located in the deeper layer of the epidermis. Either of these or both may be associated with the transduction of Na+ flowing into the skin. The ability of toads to detect hyperosmotic salt solutions in their environment reveals a previously unknown chemosensory function for spinal nerves in anuran amphibians. J. Comp. Neurol. 408:125–136, 1999. © 1999 Wiley‐Liss, Inc.
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