EFFECTS OF LUNG REDUCTION ON RHEOTACTIC PERFORMANCE IN AMPHIBIAN LARVAE

In this study we test the hypothesis that lung reduction in amphibians is a larval adaptation to reduce buoyancy and facilitate rheotaxis in lotic environments. Larvae of the spotted salamander Ambystoma macula tum (family Ambystomatidae) were raised from eggs with (= control) and without (= experimental) access to the air-water interface. The exper imental larvae developed significantly smaller and more solidified lun gs than the controls. Although mean snout-vent length did not differ b etween the control and experimental groups, the latter had a significa ntly greater mean body mass, indicating greater specific gravity in ex perimentals versus controls. To evaluate rheotactic behavior, larvae w ere subjected to a water current in an artificial stream. The distance and time required to establish a purchase on the stream bottom were r ecorded. Although the times did not differ, the distances displaced do wnstream were significantly less in experimental than in control larva e. We conclude that lung reduction and solidification reduce buoyancy and enhance rheotactic ability in salamander larvae. These findings su ggest that lunglessness in plethodontid salamanders evolved as an adap tation to stream environments. (C) 1994 Wiley-Liss, Inc.