Contributions to Zoology, 74 (1/2) (2005)S. Grosjean: The choice of external morphological characters and developmental stages for tadpole-based anuran taxonomy: a case study in Rana (Sylvirana) nigrovittata(Blyth, 1855) (Amphibia, Anura, Ranidae)

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Introduction

Biodiversity studies of anurans suggest that a great number of new taxa remains to be discovered in this group (Glaw and Köhler, 1998; Glaw et al., 1998). Species complexes, in which individuals are morphologically similar, are of particular interest in this context. In such complexes, data sets other that those scored from adult morphology (e.g. molecular biology, advertisement calls, reproductive biology and ecology, and larval morphology) are often needed to fully resolve taxonomic and systematic issues. Numerous anuran families (e.g. Ranidae) require thorough systematic revision using new approaches. The increasing number of tadpole descriptions and the 'discovery' of new characters (e.g. buccopharyngeal features) proved to be relevant in resolving taxonomic problems (e.g. Wassersug, 1980; Lannoo et al., 1987; Grillitsch et al., 1993; Chou and Lin, 1997; Altig and McDiarmid, 1999a; d’Heursel and de Sá, 1999). The morphology of anuran larvae was thought to be highly adaptive to their environment and thus only poorly reflect phylogenetic relationships between them. However, several recent papers have shown, on the contrary, that tadpole characters contain phylogenetic signals (Haas, 1996, 1997, 2003; Larson and de Sá, 1998; de Sá and Swart, 1999; Maglia et al., 2001). Due to the entirely different organisation of anuran larvae in comparison with adults, the characters of tadpoles are complementary to those of adults and this set of new characters could help to resolve taxonomic and phylogenetic problems where adult characters alone have been inadequate. Furthermore, the increasingly reliable identification of larval stages, thanks to the DNA bar-coding method, should promote interest in taxonomic descriptions of tadpoles and their use in ecological studies. More generally, the importance of larval stages in biological studies is likely to increase with the availability of new molecular identification methods (Hebert et al., 2004; Thomas et al.,2005; Vences et al., 2005).

The use of tadpole morphology in taxonomic and systematic studies requires full comprehension of the events that punctuate tadpole development, e.g. time of appearance, development, and regression of larval structures. Various authors have used a developmental 'climax' period, on which they based their description of 'mature' tadpoles (e.g. Boulenger, 1897-1898; Wright, 1924; Nichols, 1937; Orton, 1946, 1947; Gosner and Black, 1954, 1957; Gosner, 1959; Inger, 1956; Hampton and Volpe, 1963; Wassersug and Duellman, 1984; Wassersug, 1989). However, most researchers have not clearly defined the stages of the larval ontogeny corresponding to the 'climax', with the exception of Wassersug (1973), who 'framed' this concept between stages 26-36. In any case, the term 'mature' should be avoided for tadpoles and its use restricted to its proper sense (i.e. attainment of sexual maturity).

A number of previous studies have focused on variations of tadpole characters during development (Nichols, 1937; Bresler and Bragg, 1954; Gosner and Black, 1954; Limbaugh and Volpe, 1957; Gosner and Rossman, 1960; Zweifel, 1961, 1970; Gaudin, 1965; Agarwal and Niazi, 1980; Dutta and Mohanty-Hejmadi, 1984; Tubbs et al., 1993; Hall et al., 1997). Ontogenetic variation has important implications for the use of tadpole characters in taxonomy. For example, should only animals at an identical stage of development be compared? Do the characters continue to vary throughout the larval period, or do they stabilize at a given point of ontogeny? If a developmental 'climax' actually exists, which stages of development does it encompass? Are all the morphometric measurements that can be taken on a tadpole relevant for comparisons?

Herein, I use tadpoles of Rana nigrovittata to assess ontogenetic character variation. This study integrates, for the first time, the variation of the morphological characters commonly used in comparative studies (general colour, oral disk features) with morphometric characters and buccopharyngeal morphology.