The trabeculated myocardium has a distinct net‐like organization in all the specimens, differing from that described for other teleosts. The ventricle undergoes morphological changes throughout its development, from the initial saccular shape with a fully trabeculated myocardium and coronary vessel restricted to the subepicardium (Type I) (group 1) to a pyramidal shape with mixed myocardium and coronary vessels that penetrate only to the level of the compact layer (Type II) (groups 2 and 3). The hearts were collected and the anatomy analyzed with a stereomicroscope, ultrastructure with a scanning electron microscope, and histology using toluidine blue, Masson's trichrome and Sirius red stains. Specimens of A. gigas weighing between 0. g were grouped into three developmental stages. Here, we describe the heart of A. gigas, with emphasis on the ventricular anatomy and myoarchitecture. However, no information is available regarding its heart morphology. This species has a specific heart metabolism that uses fat as the main fuel when O 2 supplies are abundant but also can change to glycogen fermentation when O 2 content is limiting. Arapaima gigas, a member of the ancient teleost group Osteoglossomorpha, is one of the largest freshwater fish in the world.

In general, the Amazonian ichthyofauna undergoes constant variations in water temperature, pH and oxygen saturation, which makes these species useful for investigations of cardiac morphology. The fish heart ventricle has varied morphology and may have a specific morpho‐functional design in species adapted to extreme environmental conditions.

Anatomy of the arapaima gigas