Phylogenetic assessment and taxonomic description of the first scale-feeding candiru from west of the Andes (Siluriformes, Trichomycteridae)

Abstract Among vertebrates, specialized scale-feeding occurs almost exclusively in tropical freshwater fishes, with the Amazon basin having the richest regional assemblage of such specialists. With 28 valid species and 11 valid genera, 10 of which are ectoparasitic scale and mucus-feeders, the Neotropical trichomycterid subfamily Stegophilinae is the only fish lineage to have significantly diversified within this niche. Stegophilines are widespread and ubiquitous throughout the lowland freshwaters of South America, east of the Andes. We describe Arhinoglanis gen. nov. and Arhinoglanis relictus sp. nov., the first Stegophilinae from west of the Andes, from the upper Cauca River drainage, Colombia. At 26.4 mm maximum standard length, the new genus and species is also the first miniature Stegophilinae. Using diaphanization, light microscopy, and micro-computed tomography imagery, we coded 534 morphological characters for 50 terminal taxa to recover the new taxon as sister to the exclusively cis-Andean clade of Homodiaetus + Schultzichthys. A single unique synapomorphy unites these genera: proximal tip of ceratobranchial 5 wider than proximal tip of ceratobranchial 4. Five autapomorphies diagnose the new genus, the most notable of which is a paedomorphically unossified dorsal lamina of the mesethmoid. The scarcity and highly restricted distribution of the new species within the Magdalena basin has led to its evaluation as Critically Endangered.