Suprabranchial Chamber of Secrets: Comparative Morphology of the Air‐Breathing Organ in Anabantoid Fishes

While the two most common vertebrate organs for gas exchange are gills and lungs, some actinopterygian (ray-finned fish) lineages supplement oxygen demand with various accessory respiratory organs. For instance, walking catfishes have arborescent organs, gymnotiforms use a vascularized oropharynx, and anabantoids use an elaborate skeletal labyrinth housed in a suprabranchial chamber. The gouramis, climbing perches, and kissing gourami - the so-called "labyrinth fishes" (Anabantoidei) - have been recognized since the 1700 s for their ability to survive in low-oxygen environments, with some species (e.g., Anabas) even able to make forays overland in search of better conditions and food. The utility of the respiratory labyrinth organ has long been associated with the prodigious dispersal ability and invasivity of anabantoids; however, our understanding of the diversity of these organs within a phylogenetic context is lacking. This study compared the microstructural (histological) and mesostructural (morphological) features of labyrinth organs across Anabantoidei. Combining histology and contrast agents coupled with computed tomography, we explore tissue-level differences among vascularized tissues and render the suprabranchial chamber in three dimensions for the first time. We also use phylogenetic comparative methods to explore the nature of labyrinth organ reductions and potential ecological correlates. We document vascularized epithelial tissue lining both the suprabranchial chamber and labyrinth organs, as well as regional variability in vascularization within the labyrinth. Finally, we corroborate anecdotal labyrinth organ reductions in taxa like pike gouramis (Luciocephalus) and kurpers (Sandelia) and identify putative respiratory organ hypertrophy in Anabas and Helostoma.