Global shortfalls of knowledge on anuran tadpoles

Despite the amount of data on different aspects of biodiversity, such as species distributions, taxonomy, or phylogenetics, there are still significant gaps and biases in the available information. This is particularly true for life history traits, with fragmentary data for most taxa, especially those with complex life cycles. Anurans (frogs and toads) show larval (premetamorphic) stages that are in general radically decoupled from adult forms in most biological aspects. Our understanding of this group is highly uneven, as the main wide-scope investigations focus on adult specimens and larval stages remain unknown for a significant part of the anuran tree. The main purpose of this work was to estimate the extent of knowledge gaps regarding the diversity of tadpoles, interpret their biological and geographical patterns, and discuss possible explanations and implications for other large-scale analyses. Our findings show that more than half of the anuran species described to date still lack information on their embryonic/larval stages. Furthermore, knowledge varies among taxonomic groups, larval ecomorphological guilds, and world ecoregions. Description percentages generally decrease in lineages with a higher proportion of species known or suspected to have endotrophic development. Also, geographic areas with the highest levels of ignorance in larval biology (Tropical Andes and New Guinea) coincide with the highest diversity of endotrophic guilds. Among exotrophic larvae, generalized lentic-lotic tadpoles have the widest distribution and levels of knowledge, whereas specialized lotic, fossorial, and terrestrial forms are more taxonomically and geographically restricted. Further large-scale analyses on tadpole biology are crucial for their impact in varied scientific disciplines including anuran conservation. At a conceptual level, the discussion of the anuran biphasic life cycle is pertinent in the context of shortfalls of biodiversity knowledge and their interrelationships.