Bulletin of Marine Science, 46(1): 83-94,1990 Ultrasonic Telemetry of Small-scale Movements and Microhabitat Selection by Molting Blue Crabs (call/nectes Sap/dus)

We have designed a new ultrasonic transmitter to track free-ranging premoIt blue crabs and signal the event of ecdysis. The transmitter (15 g in air, 5 g in water) operates for 6-8 weeks on a small lithium battery, producing 75-kHz tracking pulses at about I-sec intervals. It is attached to the crab's carapace, with a magnetic reed switch placed near the line where the carapace will separate from the abdomen as the molting crab backs out of its old shell. A magnet attached to the abdomen pulls away at ecdysis, signalling the event by allowing the switch to open and doubling the pulse rate of the transmitter. During field tests with II premolt male blue crabs in a small tributary of central Chesapeake Bay, the devices allowed us to assess movement patterns and selection of microhabitats for molting at scales of < I m. Even with this small sample size several trends were evident. Distance traveled per day (average 130 m, range 0-569 m) and movement patterns were highly variable in the days prior to molt. Crabs moved more during the day, and molted by day or night in roughly equal proportions. All crabs selected shallow «0.5 m) marsh-lined banks of a tidal creek for ecdysis. This study, the first in which arthropod molting has been telemetered in the field, provides a technique applicable to intraand interspecific comparisons with other large