The distribution of the recessus orbitalis across flatfishes (order: Pleuronectiformes).

The recessus orbitalis is an accessory organ of flatfishes functioning in the protrusion of the eyes. This character, along with cranial asymmetry and a forward insertion of the dorsal fin, have been considered synapomorphies for the Pleuronectiformes. New dissections and examination of images taken in the wild show that the recessus orbitalis is present in all representatives of Pleuronectoidei examined but is absent in the single species of Psettoidei dissected. Psettoidei, the most primitive pleuronectiform lineage, contains three recognized species; thus, the absence of the recessus orbitalis in this whole lineage is unclear without further dissections. Ancestral character estimation at the family level for the recessus orbitalis indicates that the recessus orbitalis was likely absent in the common ancestor of Pleuronectiformes but was most likely present in the common ancestor of the Pleuronectoidei. Given that so few species of flatfishes have been assessed for the recessus orbitalis to date, additional characterization of the distribution of the recessus orbitalis across flatfishes will further inform what states this character may have and if it is a synapomorphy of Pleuronectiformes or simply a derived character state of Pleuronectoidei.