Visual Acuity and Behavior of Juvenile Limulus

Abstract

Abstract: : Purpose: To correlate the development of the lateral eye of the horseshoe crab, Limulus polyphemus, with its visual performance observed in its natural habitat. Methods: We measured how the lateral eyes of juvenile crabs sampled visual space by mapping the direction of the optic axes of corneal cones of molts. In field experiments we videotaped the responses of swimming juvenile crabs to targets (cylinders 7.5 cm diameter by 2.5 cm high) of various contrasts inserted in their paths. Results: The larval crab emerges from an egg with about 15 photoreceptors in each lateral eye. New ommatidia are added with each molt at the front edge of the eye approximately doubling in number up to the 6th molt. After 6 molts the eye contains 260 ommatidia (25% of adult). We found that the visual field of the eye is coextensive with that of an adult, having about 80% of adult resolution in the vertical direction but only about 30% adult resolution in azimuth. A complete map of a stage 10 eye and its stage 9 molt showed that 116 new ommatidia were added at the the front edge of the lateral eye, yielding a total of 645 ommatidia. This eye has full adult resolution in elevation and 65% adult resolution in azimuth. Juvenile crabs swimming in a straight line turn to avoid visible targets in their path, but consistently bump into transparent ones. They turn away about 15 cm from a high contrast target and at about 12 cm from a low contrast one. Conclusions: Juvenile horseshoe crabs demonstrate visually-guided avoidance behavior. They can dectect objects with nearly the same visual acuity (minimum angle of a threshold stimulus) as an adult even though their eyes are not fully developed.

Keywords: visual development • animal model