While eye sensitivity in the American horseshoe crab Limulus polyphemus has long been known to be under the control
of an endogenous circadian clock, only recently has horseshoe crab locomotion been shown to be controlled by a separate
clock system. In the laboratory, this system drives clear activity rhythms throughout much of the year, not just during the mating
season when horseshoe crabs express clear tidal rhythms in the field. Water temperature is a key factor influencing the expression
of these rhythms: at 17o
C tidal rhythms are expressed by most animals, while at 11o
C expression of circatidal rhythms is rarely
seen, and at 4o
C rhythms are suppressed. Neither long (16:8 Light:Dark) nor short (8:16) photoperiods modify this behavior at
any of these temperatures. Synchronization of these circatidal rhythms can be most readily effected by water pressure cycles both
in situ and in the lab, while temperature and current cycles play lesser, but possibly contributory, roles. Interestingly, Light:Dark
cycles appear to have synchronizing as well as “masking” effects in some individuals. Evidence that each of two daily bouts of
activity are independent suggests that the Limulus circatidal rhythm of locomotion is driven by two (circalunidian) clocks, each
with a period of 24.8h. While the anatomical locations of either the circadian clock, that drives fluctuations in visual sensitivity, or
the circatidal clock, that controls tidal rhythms of locomotion, are currently unknown, preliminary molecular analyses have shown
that a 71 kD protein that reacts with antibodies directed against the Drosophila PERIOD (PER) protein is found in both the protocerebrum
and the subesophageal ganglion [Current Zoology 56 (5): 499–517, 2010].