Circadian rhythms in locomotor activity of juvenile horseshoe crabs.

The American horseshoe crab, Limulus polyphemus, is a classic model for vision research. Not only are the lateral eyes of Limulus relatively simple, but they share many processes with the eyes of more advanced animals, including humans (1). The eyes of Limulus also exhibit a remarkable circadian rhythm: they become nearly 1,000,000-fold more sensitive at night, accommodating for the roughly 1,000,000-fold decrease in ambient light (2). Is visual sensitivity the only process controlled by a circadian oscillator in Limulus! Other marine animals have been shown to possess circadian as well as circatidal rhythms in locomotor activity (3). Does Limulus exhibit a rhythm in locomotor activity? Past attempts in this laboratory to determine the locomotor activity cycle of adult horseshoe crabs were largely unsuccessful, with only one of more than 100 crabs tested exhibiting circadian activity in darkness. Here we report a study to determine the locomotor activity rhythm of juvenile crabs. We found that most juveniles were nocturnally active under natural cyclic lighting, and that some maintain rhythmic activity in constant darkness. We assessed the locomotor activity of 10 juveniles maintained in isolation. The animals were collected in the environs of Woods Hole, Massachusetts, and were placed in a 39 X 49-cm tank (water depth: 8 cm) filled with about 2 cm of sand. The tank was placed in a lightproof box with a porthole pressed directly against a laboratory window to provide natural light. The tank was continuously illuminated with infrared light sources (Sony: Model HVL-1RC) so that we could observe the activities of the animals day and night with a video camera (SeaView Video Technology, Inc.) sensitive to infrared light. Using Snappy Video Snapshot (Play, Inc.) hardware and SnapRecorder (SB Software) software, we took one picture per minute and measured activity with the following algorithm: if an animal moved onto an imaginary line drawn across the tank, it scored a point; if the animal