An overview of the systematics and biogeography of the flatfishes

Until about 25 years ago, classification of the flatfishes was based on the Regan-Norman model. Beginning with the work of Amaoka in 1969, the order started to be analysed in finer detail, and in the 1980s many questions arose concerning flatfish phylogeny as expressed in the Regan-Norman model. Two of the major questions were whether the order is monophyletic and which groups within the order are monophyletic. Recent research has clarified many of these questions. Chapleau recently redefined the order based on shared derived characters, thus supporting the hypothesis of monophyly. The most recent classifications based on phylogenetic studies have split several of the traditionally recognized families. Some flatfish groups were shown to be monophyletic once certain species were excluded. As phylogenetic studies and work on the alpha-level taxonomy continue, we are beginning to see some very interesting biogeographic patterns. Some of these distribution patterns were not previously obvious due to polyphyly of some of the flatfish groups. There is still much work to be done on the systematics and biogeography of flatfishes. Some of the more important questions that remain to be addressed in future studies are the following. (1) What is the sister group of the order? (2) What is the sister group of the Southern Ocean Achiropsettidae? (3) What are the relationships of the remaining groups in the polyphyletic Pleuronectidae? (4) What are the relationships of certain genera that are excluded from recently redefined families? (5) What are the intergeneric relationships within monophyletic groups such as the Bothidae, Achiridae and Soleidae? A great deal of work at the alpha-taxonomic level with flatfishes is still needed. Especially in tropical areas, new species and great range extensions are routinely discovered. There is also a need for more ontogenetic studies of flatfishes.