Since shape (Barlow et al.1982), size and contrast (Herzog, et al.1996) were shown to be likely cues involved in mate selection, a more basic question as to how the horseshoe crab visual system detects these differences was raised. Cole (1923) showed that adult horseshoe crabs are positively phototactic and that increasing light level increased the tropic response.
However, is it simply the size of an object and therefore the amount of light it reflects (or blocks) that attracts an animal, or is it some characteristic of its movement? Von Campenhausen (1967) demonstrated through a series of experiements that horseshoe crab eyes are designed to identify shapes and not necessarily to detect movement. Therefore, an object moving past a stationary horseshoe crab would look the same as if the horseshoe crab was walking past it; and the characteristics of the object itself would have attractive qualities rather than the movement of the object relative to the horseshoe crab. A further set of experiments by Passaglia et al. (1996a) and Passaglia et al. (1996b) showed that, in fact, the horseshoe crab eye is particularly adapted to see "horseshoe crab sized objects" particualry well.
