Aspects of the functional morphology of cirrate octopods: Locomotion and feeding
Cirrate octopods swim by a combination of fin action and medusoid propulsion by the arm/web complex. The fins of cirrate octopods are associated with a unique cartilage-like shell in a shell sac. In cross-section, the fins have distinct proximal and distal regions, both of which are covered by a thin surface sheath of muscle. The distal region is characterized by dorsal and ventral layers of muscle somewhat similar to a typical decapod fin. In the proximal region, the fin cartilage forms a flat central core within the fin and provides skeletal support for attachment of densely packed muscle. Whereas Stauroteuthis maneuvers slowly by sculling with the fins, Grimpoteuthis swims primarily using powerful fin strokes. In Stauroteuthis, the mantle is extensively modified. The mantle opening closely surrounds the funnel, and the posterior mantle muscle is thickened and probably controls water flow for respiration. The "secondary web" in some cirrate species results from a modification of the way the web muscles attach to the arms. The more benthic opisthoteuthids lack this modification. The secondary web enables larger volumes of water to be trapped in the web in some postures. The entrapment of water resulting in a bell-shaped posture in Stauroteuthis could be related to predator defense or to feeding. Buccal secretory glands found in Stauroteuthis and the presence of small copepods in the digestive tract, suggest that this benthopelagic species feeds by entrapping planktonic prey in mucus.