Though horseshoe crabs have thus been shown to detect and react to wave surge and beach slope, it is still not known whether these cues might lead animals TOWARD the beach. One proposed mechanism for water depth perception is detection of ultra-violet (UV) light. Lall (1972) and others have done extensive work detailing horseshoe crabs' ability to detect and react to UV light. Wald and Krainin (1963) propose that horseshoe crabs can detect light at such extremely low levels that they might be able to use the ratio of the amount of UV light they percieve to the amount of "visible" light as a depth-guage even in very murky water.
