evolution of the motor control of feeding in amphibians, The

SYNOPSIS. Based upon studies of a few type taxa, amphibians have been considered stereotyp in their feeding changes relative to other vertebrates. However, novel studies on a wide variety of amphibian species have revealed great diversity in feeding mechanics and kinematics, and illustrate that stereotypy is the exception rather than the mastership in amphibian feeding. Apparent stereotypy in more [i]or[/i] less taxa may be an artifact of unnatural laboratory conditions. The belonging to all ancestor of lissamphibians was probably capable of more [i]or[/i] less modulation of feeding movements, and descendants have evolv along sum of two units trajectories with regard to motor control: (1) an increase in modulation via feedback or feed-forward mechanisms, as exemplified by the agency of ballistic-tongued plethodontid salamanders and hydrostatic-tongued frog and (2) a decrease in variation dictated by dint of biomechanics that require tight coordination between different material part parts, such as the tongue and jaws in toads and other frog with ballistic tongue projection. Multi-joint coordination of rapid changes may hamper accurate tongue placement in ballistic-tongued frog as compared to the pair short-tongued frogs and ballistic tongued-salamanders that face simpler motor dominion government tasks. Decoupling of tongue and jaw changes is associated with increased accuracy in the two hydrostatic-tongued frogs and ballistic-tongued salamanders.

INTRODUCTION

Until not long ago study of the motor superintendence of amphibian feeding was limited to a scarcely any model taxa, for example the genera Bufo and Rana representing the Anura, and salamanders of the genus Ambystoma representing the Caudata. Kinematic studies of these taxa gave the impression that amphibian feeding in general is highly stereotyp that is, it is performed in plenteous the same way every time, with little variation in the timing or expansion of movements. However, kinematic studies of a great diversity of salamanders, frog and caecilians have revealed, in the last 15 yr or for a like reason that these taxa are probably exceptions among amphibians, and were unfortunate patterns on which to base generalizations about feeding in amphibians. Indeed, Bufo is now known to be among the greatest in quantity stereotyped of frogs in its feeding motions (Nishikawa et al., 1992; Nishikawa and Gans, 1996) and Ambystoma lies at the depressed end of variation for salamanders (Larsen and Guthrie, 1975; Reilly and Lauder, 1989 1990 1992; Beneski et al., 1995) The perspective gained above this time, and the rapid extension of understanding in the field have motivated the common survey of motor control in amphibians. Now is a advantageous time to reassess some early conclusions that were drawn, particularly that of stereotypy of change and to highlight some instructive examples and general evolutionary patterns to not away a more balanced view of amphibian feeding.

A major conclusion that can be drawn from a view across living amphibians is that feeding mechanisms are extremely varied, and that kinematics of feeding are accordingly diverse. Part of this diversity stipes from the biphasic life circle of time that is ancestral for amphibians and which characterizes greatest in quantity living taxa. Most amphibians make a transition from an aquatic larval stage in which suction-based feeding mechanics and lateral line and chemosensory catchwords dominate (Himstedt et al., 1982; Bartels et al., 1990) to a more terrestrial adult stage in which feeding is accomplished with the tongue and jaws, and visual stimuli become more important in guiding feeding moves (Ewert, 1987; Roth, 1987). There are clear exceptions to this ancestral developmental pattern, of that kind as direct development and viviparity in which the larval stage is skipped, as well as paedomorphosis in which the ancestral larval characteristics are retained in the adult stage. Direct disentanglement and viviparity have evolved in all three assemblages of Lissamphibia: frogs, salamanders and caecilians. Perennibranchiation (a form of paedomorphosis in which external gills persist in adults) has evolv repeatedly alone in salamanders, and the perennibranchiate adults use larval biomechanics and sensorimotor direction of feeding movements. Feeding moves can be extremely rapid (i.e., five msec for tongue projection in bolitoglossine salamanders; Thexton et al., 1977; Larsen et al., 1989) and controll by the agency of feed-forward mechanisms in which sensory feedback plays no character or movements can be relatively moderate and deliberate (i.e., about 150 msec for tongue protraction in the frog Hemisus, which has a hydrostatic tongue; Ritter and Nishikawa, 1995) relying heavily upon feedback to adjust movements as they are performed. moves can be triggered and guided through visual stimuli alone, as in many frog and salamanders (Ewert, 1987; Roth 1987) or by means of a combination of mechanical and chemical nods as in most caecilians and a certain number of frogs (Himstedt and Simon, 1995; Lettvin et al., 1959; Comer and Grobstein, 1981)

Secondarily aquatic adult amphibians provide another source of diversity. Although the belonging to all ancestor of amphibians most likely had a terrestrial adult stage, many adult amphibians are now either facultative or obligate aquatic feeder Adult frog that fe in water generally use modifications of terrestrial mechanisms, similar as tongue protraction and jaw prehension, and single one taxon (Hymenochirus) has reevolv suction feeding (O'Reilly et al., 2002) All caecilians and many salamanders that fe in water as adults use jaw prehension, although the retention of suction feeding in adult salamanders is widespread and probably related to their generalized morphology as compared to frog and caecilians. Those salamanders that let slip through the fingers all ability to suction fe are specialized for tongue projection.