The Relationship Between Small- and Large-Scale Movements of Horseshoe Crabs in the Great Bay Estuary and Limulus Behavior in the Laboratory

Abstract

The overall goal of our research program is to determine the short- and long-term patterns of horseshoe crab (Limulus polyphemus) movements in the Great Bay estuary and then seek an understanding of the endogenous and exogenous processes that give rise to these patterns. Small- and large-scale movement data were obtained from 27 horseshoe crabs tracked using ultrasonic telemetry for at least a year. During mating season animals were most active during high tides, but they did not increase their activity or approach mating beaches during every high tide. During the remainder of the year tidal or daily patterns of activity were less evident, and the extent of their movements gradually decreased as water temperatures dropped in the late fall and winter. During the spring, when water temperatures exceeded 10°C, tagged animals moved several km up into the estuary into shallow water (< 4 m) 1 month prior to spawning. A similar temperature threshold was also evident in laboratory experiments, with little rhythmic behavior expressed at temperatures below 11°C. Mating activity lasted approximately 1 month and was followed by a period of high activity. In the fall, most animals moved downriver into deeper water, where they remained during the colder months. Thus, the majority of Limulus exhibited a seasonal pattern of movement, remaining within a 3 km stretch of the estuary. In the laboratory, animals expressed both daily and tidal rhythms of locomotion. Those with daily rhythms were more likely to be diurnal than nocturnal, but both tendencies were evident. The clock involved in modulating their locomotory activity appears to be separate from the clock controlling their circadian rhythm of visual sensitivity. When animals were exposed to “artificial tides”, created by changing water depth every 12.4 hours, they expressed clear tidal rhythms of activity that were synchronized to the imposed tides. Similar data were obtained from horseshoe crabs in running wheels placed in the estuary. However, if the running wheels were attached to a floating dock, so water depth did not change with the tides, the horseshoe crabs were primarily diurnal. Thus, while endogenous biological clocks are capable of controlling many aspects of horseshoe crab locomotion, the actual patterns manifested in the field are strongly influenced by the water depth changes associated with the tides, as well as light levels and seasonal changes in water temperature.