Feeding performance in aquatic postmetamorphic newts (Urodela: Salamandridae): Are bidirectional flow systems necessarily inefficient?

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Abstract
There are two basic designs of suction-feeding mechanisms in aquatic salamanders. Premetamorphic salamanders use a unidirectional flow system in which water is drawn into the mouth anteriorly and expelled posteriorly through the gill slits. Postmetamorphic salamanders use a bidirectional flow system in which water enters and then exits through the mouth. It has been suggested that bidirectional flow results in an inefficient feeding mechanism. To test this hypothesis, we compared the structure of the feeding system and the proficiency of aquatic prey capture among postmetamorphic individuals from (i) predominantly terrestrial (Tylototriton verrucosus), (ii) amphibious (Pleurodeles waltl, Cynops pyrrhogaster, Notophthalmus viridescens, Taricha granulosa, and Paramesotriton hongkongensis), and (iii) aquatic (Pachytriton brevipes) species of newts (family Salamandridae). Ossification of the hyolingual skeleton, size of the labial lobes, and taper of the snout increase with increasing preference for an aquatic habitat. All species studied except T. verrucosus captured prey in water. Feeding efficiency was similar among all the amphibious species; each was highly successful at capturing immobile prey (90–98%), successful at capturing mobile prey (40–58%), and less successful at capturing evasive prey (18–23%). Fully aquatic P. brevipes was highly successful in taking immobile (100%) and mobile (90%) prey and successful at capturing evasive prey (65%). We attribute the superior feeding performance of this newt to several morphological specializations of its feeding system, including a strongly tapered snout and greatly enlarged labial lobes. We reject the hypothesis that bidirectional flow is always inefficient for prey capture by metamorphosed salamanders. The degree of feeding proficiency in water is influenced by the morphological and behavioral attributes of aquatic salamanders, rather than just by the presence or absence of gill slits and, hence, by unidirectional flow.