Ultrasonic Biotelemetry of Muscle Activity from Free-Ranging Marine Animals: A New Method for Studying Foraging by Blue Crabs (Callinectes sapidus)

¹ Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8208
² Smithsonian Environmental Research Center, P. O. Box 28, Edgewater, Maryland 21037

The design and utilization of a small (16 g in air, 6 g in water) ultrasonic transmitter for animal tracking and bio-telemetry of muscle activity are described. To our knowledge this is the first use of such devices to telemeter a specific behavior (ingestion) from a free-ranging marine predator. The transmitter produces regularly recurring short tracking pulses, and long pulses triggered by the action potentials of a muscle. Pulses are transmitted through conductive estuarine water by a piezoelectric ring transducer at a frequency of about 75 KHz. The preparation of a subject animal, insertion of electrodes, and attachment of the transmitter are described for the telemetry of mandibular muscle contraction in the blue crab. The transmitter provides a signal that corresponds unequivocally with feeding activity and allows enumeration of bites required to consume a food item. Number of bites, and feeding time, are both positively correlated with the size of the prey specimen. As a test of the technique's feasibility, a blue crab was equipped with one of the transmitters and released in a subestuary of Chesapeake Bay. The crab was tracked continuously for 96 hours while every contraction of the mandibular muscle was recorded. The crab traveled 4000 m along the subestuary at an average speed of 12 m/h, but showed periods of rapid movement of up to 325 m/h. The crab fed 2-7 times per day, with a feeding bout comprising 15-2750 bites. The limited data did not indicate that either movement or feeding exhibit a diel or tidal cycle.