Aspiratory respiration in Arapaima gigas (Teleostei, Osteoglossomorpha): A reappraisal

Recently it has been hypothesized that the osteoglossoid fish Arapaima gigas is unique amongst air‐breathing bony fishes in that it ventilates its respiratory swimbladder by aspiration. Aspiration, it was suggested, is effected by a diaphragm‐like septum lying below the swimbladder, and activated through lateral movement of the ribs. By using both high‐speed X‐ray and light cinematography this hypothesis can be refuted. Furthermore, dissection shows that the presumed anatomical basis for aspiration stems from a misinterpretation of the usual retroperitoneal position of the swimbladder, and that the ribs are capable of only a very restricted lateral movement. Arapaima gigas, like other bony fishes with a respiratory swimbladder, ventilates that organ by means of a buccopharyngeal pump. The principal afferent vascular supply to the swimbladder is, however, unusual since it stems from the caudal vein via the renal‐portal system. This and other aspects of swimbladder structure and anatomy may throw further light on the interrelationships of osteoglossoid fishes.