SWIMMING DYNAMICS OF THE MESOPELAGIC VERTICALLY MIGRATING PENAEID SHRIMP SERGESTES SIMILIS: MODES AND SPEEDS OF SWIMMING

Abstract

ABSTRACT The mesopelagic vertically migrating penaeid shrimp Sergestes similis was studied in situ in Monterey Bay, California, at depths ranging from 291-569 m using a remotely operated submarine equipped with a video camera. Instances both of routine swimming and of escape behavior were observed. In routine swimming, S. similis (mean total length 43.5 mm) swims at an average of 7.4 cm/s. Propulsion is provided exclusively by the pleopods, which beat in an adlocomotory metachronous wave at approximately 4 Hz. The pereiopods are generally held close to the underside of the thorax. The long second (hereafter antennae unless first are specified) are invariably deployed in a characteristic position, with the relatively inflexible proximal portion, over 1 body length long, directed anterolaterally and angling slightly ventrally, and the flexible distal portion, over 2 body lengths long, trailing posteriorly from a distinct articulation or kink at the end of the proximal portion. The clearly respond to tactile stimuli, and perhaps have chemosensory function as well. Sergestes similis was never observed to stop swimming except for brief bouts of grooming or momentary reactions to tactile contacts on the antennae. Downward swimming was always active, and was not accomplished by passive sinking. The extremely long, setose would be expected to produce substantial drag which must be overcome by metabolically generated thrust. Thrust of the pleopods is maximized during the propulsive stroke by extending and separating the pleopods and spreading the setae, while drag is minimized on the recovery stroke by retracting the pleopods and holding them in apposition to one another and by folding the setae. The escape reaction in this species is the typical decapod tail flip.