Phallus morphology in caecilians (Amphibia, Gymnophiona) and its systematic utility

Introduction 143 Abbreviation used in text 144 Abbreviations used in figures 144 Morphology 144 Disposition of the cloaca 144 Divisions of the cloaca 146 Urodeum 146 Blind sacs 146 Anterior phallodeum 147 Posterior phallodeum 148 Phallodeal ornamentation 148 Composition of phallodeal structures 148 Relationship between the uneverted cloaca and the phallus 149 Systematics 149 Is phallus morphology species specific? 149 Species differentiation and generic identity 152 Discussion 152 Acknowledgements 153 References 153 The cloaca of male caecilian amphibians (Gymnophiona) is a tube that comprises an anterior urodeum and a posterior phallodeum. The phallodeum everts (with the urodeum lying inside it) to form a phallus used for direct sperm transfer in copulation. Phallodeal morphology is rich in detail and variation, and has therefore been considered a potentially useful and much needed tool for caecilian phylogenetics and species-level taxonomy. Despite this, it has been almost entirely ignored in caecilian systematics, there is confusion regarding some aspects of morphology, and variation within and among species is poorly understood. A short review and reconsideration of phallus morphology is presented, and the systematic potential assessed. The anterior part of the phallodeum appears to offer the most obvious systematic potential, and the morphology of longitudinal ridges and their ornamentation here seem to have diagnostic and/or phylogenetic value for some taxa. Although there is evidence of intraspecific variation, at least some of which is associated with ontogeny and reproductive condition, individuals of the same species generally have a common pattern of phallodeal ridges and ornamentation, and congeners often share a similar pattern. However, these patterns are not universally species specific, at least among uraeotyphlids. Although variation needs to be better understood, the male cloaca offers great potential for caecilian systematics.