Description:
Squids rely on an ancestral molluscan circulatory geometry; a single‐chambered systemic heart and a simple unidirectional flow in which gill and muscle capillaries are in series. However, a fast predatory lifestyle required evolution of traits to meet higher metabolic demands and ensure adequate oxygenation; a closed, high‐ pressure circulatory system and peripheral branchial hearts to boost gill blood pressure. Previous studies have described branchial heart evolution from veins, their use in producing ultrafiltrate, the neural control of contractions, and pulse pressure measurements. However, few studies have investigated branchial heart morphology with a focus on developing functional hypotheses that include considerations of flow and material properties. In this study, we use histology and scanning electron microscopy of corrosion casts to visualize the three‐dimensional orientations of the contractile, tensile, and elastic elements in branchial hearts of Dosidicus gigas (Humboldt squids). The thin‐walled branchial hearts have complex orientations of trabeculae that extend into the lumen. It is thought that the muscle and connective tissue trabeculae are organized to maximize contractile force, to store elastic energy between contractions, to increase luminal surface area, and to decrease heart wall thickness. Study funded by an NAU GBI/TRIF Grant 2008–09.