The simple morphology of the sunfish heart.

The evolutionary conservation of the building plan of the heart suggests this organ is under substantial form-function constraints. Its form varies to such a degree, however, that it questions whether we understand the form-function relations of the heart. A previously published image of the heart of the sunfish (Mola mola, Tetraodontiformes) indicates the presence of an exceptionally simple topology of the chambers. Here, we describe the gross morphology of the sunfish heart (N = 7) because, we assess, this expands the known spectrum of cardiac form that is compatible with function. The usual teleost piscine compartments were present, guarded by valved junctions: sinus venosus, atrium, ventricle, and bulbus arteriosus. Especially, the ventricle was much displaced ventrally and the atrium was dorso-ventrally elongate. The ventricular central cavity exhibited a simple 90-degree curve, or a-quarter circle, and opened into an almost vertical bulbus arteriosus. Overall, the normal piscine S-topology of the cardiac chambers (when seen from the left) appeared distorted to a simpler J-shape. When compared to other fish, including a tetraodontiform boxfish, these highly unusual characters appeared to have evolved recently. The distantly related spotted tinselfish (Xenolepidichthys dalgleishi) resembles sunfish in shape, its heart is almost J-shaped, but the ventricle was sac-like and typically piscine. Surprisingly, the ventricular wall had a comparatively high proportion of compact myocardium, approximately 34%. The relative mass of the sunfish heart was typically piscine, approximately 0.08% of body mass. In conclusion, the sunfish heart may be the least curved cardiac structure described for any fully formed vertebrate.