Zebrafish (Danio rerio) heart. 3D computer illustration of the heart of a zebrafish. The atrium (top right) is above the ventricle (bulbous, lower right). The arteriosus (blue, centre) connects the ventricle to the aorta (long, thin, left). Behind the atrium is the sinus venosum with cardinal veins draining into it. The zebrafish is a tropical freshwater fish belonging to the minnow family.