ACOUSTIC BEHAVIOR IN THE PURPLE MARSH CRAB, SESARMA RETICULATUM SAY (DECAPODA, GRAPSIDAE)

BY

RICHARD E. MULSTAY Department of Biology, Delaware Valley College, Doylestown, Pennsylvania 18901, U.S.A.

INTRODUCTION In recent years acoustic communication in the semiterrestrial fiddler and ghost crabs (family Ocypodidae) has received considerable attention (Salmon & Horch, 1972). Complex patterns of social behavior in both agonistic and sexual contexts have been described for a number of grapsids (Schone, 1968) and, though most of the emphasis has been on visual displays, it is not surprising that acoustic signals have evolved in this family. The grapsid genus Se.rarma is an extremely successful group both in terms of species and habitat diversity, and sound production has been reported for several species (Von Hagen, 1967, 1968, 1975; Mulstay, unpubl.; Tweedie, 1954; Wright, unpubl. ) . The purple marsh crab, Sesarma reticulatum, inhabits salt marshes along the east coast of the United States. These semiterrestrial brachyurans construct burrow systems with several entrances leading to a subsurface tunnel network (Crichton, 1960; Mulstay, unpubl.). As in other colonial semiterrestrial crabs, much of the agonistic behavior observed in this species is associated with burrow defense, and it is in this context that male marsh crabs produce rapping sounds. This paper describes sound production in Sesarma reticulatum and compares this behavior pattern with that found in several congenerics.

MATERIALSAND METHODS Field and laboratory observations on the social behavior of Se.rarma reticulatum, were conducted during the spring and summer months of 1971 through 1974. Field observation and collection sites were located at Flax Pond and Mount Sinai Harbor, tidal marshes on the north shore of Long Island, New York. These marsh crabs are most active at night, and nocturnal field observations were made while the tide was out using 25 watt incandescent red light bulbs in photographic reflec- tors. In order to facilitate observation the marsh grass (Spartina alterniflora Loisel.) in the study areas was periodically trimmed. In order to record the rapping sounds, crabs were returned to the laboratory and housed in glass aquariums (61 X 32 X 31 cm) containing sea water and an exposed mud bank into which the animals could burrow. Several tanks contained an equal number of males and females, while others contained only males. The mud was obtained from areas known to be inhabited by this species. The water 302 in the tanks was changed regularly, and the crabs were fed fresh lettuce daily. Animals in the laboratory were exposed to a natural photoperiod and temperature regime, and would begin to construct burrows and interact with one another shortly after being placed in the aquariums. Nocturnal laboratory observations were conducted under red light, and tape recordings of rapping sounds were obtained using a Sony tape deck (model TC- 252D) with a low impedance dynamic microphone suspended approximately 20 cm above the mud surface. Recorded sounds were analyzed from oscillographs or directly from the recording tape at reduced tape speeds using earphones and a stopwatch. RESULTS The breeding season of Se.rdrma reticulatum on Long Island, as indicated by the presence of ovigerous females in the population, extends from mid-June through August, and during the month of May there is a marked increase in male agres- siveness associated with burrow occupancy. While the intrusion of a female into a burrow that is occupied by a male does not usually elicit an aggressive response from the resident, the male's reaction toward intrasexual intrusion is generally immediate and can include chasing, threat display with raised chelae, and sound production. Though females also reside in the burrow complexes, they are charac- teristically non-aggressive and have never been observed to produce the rapping sounds.

Fig. 1. Oscillograph of a rapping sound recorded in the laboratory. The sound is composed of 15 pulses (claw contacts) and has a duration of 5.6 sec (temperature 26° C).

Since sound production frequently occurs on the surface or just within the burrow entrance, it is a readily observable motor pattern. Prior to producing a sound the animal elevates the front of the body and positions one of the chelae so that the tips of the claw are directed posteriorly. This claw is held stationary and is struck during rapping. Unless the crab's movements are restricted within a burrow, the claw usually does not touch the substrate. As this claw is being oriented, the rapping chela is moved medially and pointed toward the substrate. A series of raps is produced as the rapping claw is moved up and down repeatedly, the dorsal ridge of the dactylus striking the propodus of the stationary contra- lateral claw at the end of each stroke. While contact is typically sharp, occasionally there is some slippage in which case the sound takes on a raspy quality. Prior to actual sound production it is not uncommon for the animal to make an incom-