. Bulletin. Science; Natural history; Natural history. 1975 ACTIVITY RHYTHM IN HORN SHARKS 21 Isthmus, Santa Catalina Island, California. The study area, in a kelp-bed community averaging 10 to 20 m deep, consisted of steep, rocky slopes with many caves in which the horn sharks spent their daylight hours. Underwater clocks (battery-powered Westclox TR-3, in waterproof housings) were used to deter- mine the time at which the sharks left their day- time refuges to begin nighttime swimming activity. A clock would be attached to a nearby resting shark's dorsal spine via a short monofilament line a
. Bulletin. Science; Natural history; Natural history. 1975 ACTIVITY RHYTHM IN HORN SHARKS 21 Isthmus, Santa Catalina Island, California. The study area, in a kelp-bed community averaging 10 to 20 m deep, consisted of steep, rocky slopes with many caves in which the horn sharks spent their daylight hours. Underwater clocks (battery-powered Westclox TR-3, in waterproof housings) were used to deter- mine the time at which the sharks left their day- time refuges to begin nighttime swimming activity. A clock would be attached to a nearby resting shark's dorsal spine via a short monofilament line and alligator clip. A pull on the line would move an external magnet, causing opening of an internal reed switch, thereby stopping the clock at the time of shark departure. Light levels were re- corded underwater by a photographic-densitomet- ric technique, using an underwater 35mm Nikonos camera and neutral-density filters. The laboratory experiments, using male sharks of 31 to 63 cm total length, were conducted in a light-proof, temperature-controlled laboratory at California State University, Long Beach. Four circular vinyl pools ( m diameter, .35 m deep) were located within the laboratory, and held one experimental shark each. The primary light source was a 300 watt projector lamp (Sylvania CRT, 3200° Kelvin) which provided a maximum light level of 8 lux in the pools. The light projector was placed on the floor at the common center of the four pools and projected to the white ceiling, thereby reflecting downward equally into each pool. An additional 300 watt incandescent bulb with reflector and rheostat control was used only when levels above 8 lux were needed. The level of the primary light source was con- trolled by neutral-density filters (Kodak Wratten , log) which were placed in a felt-lined container over the projector aperture. Each filter when placed over the aperture reduced the light level by 50%; when simulating dusk transi- tions, this was done by han
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