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The American horseshoe crab Limulus polyphemus expresses both tidal and daily rhythms of locomotion in the laboratory and the tidal rhythms can be entrained to artificial tides. The main purpose of this study was to determine the types of rhythms horseshoe crabs express when freely moving in their natural habitat where they are exposed to natural light:dark and tidal cycles. A secondary goal was to determine if their overall activity patterns and depth preferences changed during the year. In 2010 and 2011, 20 adult horseshoe crabs (11 males, 9 females) were fitted with ultrasonic tags and released in the Great Bay Estuary, NH, USA. The tags transmitted acceleration and depth data every 3 to 5 min from June until December during the year in which they were tagged, and from March to May of the following year. Acoustic transmissions from the tags were detected and logged by a series of VR2W receivers moored throughout the estuary. Accelerometer data were used to assess when animals were active and to determine (1) whether they were expressing tidal or daily rhythms and (2) their overall activity level each month. We discovered that horseshoe crabs were just as likely to express tidal rhythms as daily rhythms, despite being continuously exposed to natural tide cycles. In addition, there was a tendency to move into deeper water and become less active as water temperatures cooled in the fall, and then to move up into the estuary and become more active as water temperatures warmed in the spring.

Rhythms of locomotion and seasonal changes in activity expressed by horseshoe crabs in their natural habitat