Snakes evolved from lizards but have dramatically different eyes. These differences are cited widely as compelling evidence that snakes had fossorial and nocturnal ancestors. Their eyes, however, also exhibit similarities to those of aquatic vertebrates. We used a comparative analysis of ophthalmic data among vertebrate taxa to evaluate alternative hypotheses concerning the ecological origin of the distinctive features of the eyes of snakes. In parsimony and phenetic analyses, eye and orbital characters retrieved groupings more consistent with ecological adaptation rather than accepted phylogenetic relationships. Fossorial lizards and mammals cluster together, whereas snakes are widely separated from these taxa and instead cluster with primitively aquatic vertebrates. This indicates that the eyes of snakes most closely resemble those of aquatic vertebrates, and suggests that the early evolution of snakes occurred in aquatic environments.

Abstract. 1. Although tadpoles and mosquito larvae may compete for scarce resources in natural freshwater systems, the mechanisms involved in such competition remain largely unstudied.2. Replicated artificial ponds were set up to examine the role of pathogenic interference (water-borne growth inhibitors) in two tadpole±mosquito systems from south-eastern Australia. One system comprised taxa that are commonly sympatric in freshwater ponds (tadpoles of Limnodynastes peronii and larvae of Culex quinquefasciatus) while the other comprised species that co-occur in brackish water ponds (tadpoles of Crinia signifera and larvae of Ochlerotatus australis).3. Water that had previously contained tadpoles suppressed the rates of survival and pupation of mosquito larvae in both systems. Fungicide reduced or eliminated this effect, suggesting that the growth inhibitors may be fungal organisms (possibly the yeast Rhodotorula glutinis) from tadpole faeces. Fungicide also enhanced growth rates of tadpoles.4. These results suggest that interference competition between tadpoles and mosquito larvae is mediated by other organisms in some ecological systems.

Tadpoles and mosquito larvae often coexist in natural freshwater bodies. We studied competitive interactions between: (i) tadpoles of the striped marsh frog (Limnodynastes peronii) and larvae of the mosquito Culex quinquefasciatus; and (ii) tadpoles of the common eastern froglet (Crinia signifera) and larvae of the mosquito Aedes australis. These two sets of taxa occur in natural water bodies in the Sydney region. Laboratory trials revealed competition between mosquito larvae and tadpoles in both systems. For example, mosquitoes displayed reduced rates of survival, growth and development, and smaller size at metamorphosis, when they were raised with tadpoles. The intensity of competitive suppression was influenced by attributes such as pond size (and hence, larval density), the location of food (on the water surface vs the substrate), and the extent of opportunities for direct physical interactions between the two competing organisms. These effects differed between the two study systems, suggesting that the mechanisms of suppression also differed. Limnodynastes peronii tadpoles suppressed C. quinquefasciatus even when the two types of organisms were separated by a physical partition, suggesting that chemical or microbiological cues may be responsible. Pond attributes also affected the impact of C. signifera tadpoles on Aedes larvae, but (unlike the Limnodynastes–Culex system) these effects disappeared when densities were lowered or when the tadpoles and mosquito larvae were physically separated. Thus, direct physical interactions may suppress mosquitoes in the Crinia–Aedes system. Our results suggest that tadpoles suppress the viability of larval mosquitoes by multiple pathways.

Tadpoles and mosquito larvae often coexist in natural freshwater bodies. We studied competitive interactions between: (i) tadpoles of the striped marsh frog ( Limnodynastes peronii ) and larvae of the mosquito Culex quinquefasciatus ; and (ii) tadpoles of the common eastern froglet ( Crinia signifera ) and larvae of the mosquito Aedes australis . These two sets of taxa occur in natural water bodies in the Sydney region. Laboratory trials revealed competition between mosquito larvae and tadpoles in both systems. For example, mosquitoes displayed reduced rates of survival, growth and development, and smaller size at metamorphosis, when they were raised with tadpoles. The intensity of competitive suppression was influenced by attributes such as pond size (and hence, larval density), the location of food (on the water surface vs the substrate), and the extent of opportunities for direct physical interactions between the two competing organisms. These effects differed between the two study systems, suggesting that the mechanisms of suppression also differed. Limnodynastes peronii tadpoles suppressed C. quinquefasciatus even when the two types of organisms were separated by a physical partition, suggesting that chemical or microbiological cues may be responsible. Pond attributes also affected the impact of C. signifera tadpoles on Aedes larvae, but (unlike the Limnodynastes-Culex system) these effects disappeared when densities were lowered or when the tadpoles and mosquito larvae were physically separated. Thus, direct physical interactions may suppress mosquitoes in the Crinia-Aedes system. Our results suggest that tadpoles suppress the viability of larval mosquitoes by multiple pathways.

Tadpoles and mosquito larvae compete for scarce resources in natural freshwater ecosystems, but factors determining the extent of competition between these two groups of organisms remain largely unstudied. Natural ponds display great variation both in the densities of larvae, and in the body sizes of tadpoles. We set up replicated artificial pond experiments to examine the effects of population density and tadpole size on interactions between tadpoles and mosquito larvae. We examined the effects of larval density in two systems of co-occurring tadpoles and mosquito larvae, one from brackish-water ephemeral ponds (Crinia signifera with Ochlerotatus australis) and one from permanent freshwater ponds (Limnodynastes peronii with Culex quinquefasciatus). In both systems, increasing densities of larvae suppressed growth and development both of conspecifics and of the competing taxon. In the C. quinquefasciatus–L. peronii system, larger tadpoles exerted more powerful suppression. Our results suggest that mosquito developmental rates and adult body sizes (and thus, the danger which mosquitoes pose to public health) may be reduced if natural water-bodies contain dense populations of large tadpoles.