Lateral line system of flatfishes (Pleuronectiformes): Diversity and taxonomic distribution of its characters

An analysis of lateral line system (LLS) morphology in 304 species (39% of those presently known) from 121 (92%) genera belonging to all 14 families of Pleuronectiformes is presented. The main feature of the LLS of flatfishes is bilateral asymmetry in the course of the cranial lateral line canals, but also in the reduction of canals or portions of canals, which occurs independently on the eyed and blind sides. The number of reductions is used to define two patterns for comparative analysis. A complete pattern is defined by the presence of all main canals, connected with each other, and the canaliculi that end in pores are usually highly branched. The reduced pattern is defined by the presence of more than two reductions. Each family is characterized by the occurrence of a complete pattern, a reduced pattern or both patterns. The combination of reductions varies considerably and characterizes taxa at the generic level. A reduced pattern appears independently several times and has a complex evolutionary history with reversals to a complete pattern. This study has shown that LLS characters are very informative for phylogeny and taxonomy and should be an important component of the description and revision of pleuronectiform taxa and their relationships.