Telemetered cephalopod energetics: swimming, soaring, and blimping.

Cephalopods are uniquely suited to field energetic studies. Their hollow mantles that pump water for respiration and jetting also can accommodate differential transducer-transmitters. These transmitters indicate pressure-flow power output, which can be calibrated against oxygen consumption by swim-tunnel respirometry. Radio-acoustic positioning telemetry (RAPT) records pressure-flow power and animal movements with meter accuracy in nature. Despite inherent inefficiencies, jetting is the primary mode of locomotion for both primitive nautilus and powerful, migratory oceanic squids. In between, large-finned squid and cuttlefish mix jetting with fin undulation in complex gaits that increase locomotor efficiency. Our studies show that the complex nervous systems cephalopods evolved to control mixed gaits are also sensitive to flow and density fields in nature and that they use these to further reduce locomotion costs. Buoyed up by evacuated shells, nautilus and cuttlefish live in boundary layers and navigate cheaply through them like balloonists. Large-finned, negatively buoyant squid soar like eagles in rising currents, but lose control in currents above one body length per second. Many muscular squids have life histories linked to current systems. Neutrally buoyant ammoniacal cephalopods in the mesopelagic are a limiting case in need of study. The small density differential between seawater and isotonic ammonium chloride trebles their volume, making them blimp-like with very low power densities. Some species live entirely in this restricted habitat, but most become ammoniacal late in ontogeny, as they approach semelparous reproduction. Ammonium retained for buoyancy as carbon is terminally mobilized from muscle protein for gametes and energy, compensates for lost muscle power.