Orientation to Objects in the Sea Urchin Strongylocentrotus purpuratus depends on apparent and not actual object size.

Sea urchins are known to move toward large visual targets, indicating that they have limited visual acuity. There is a minimum target size that elicits this response. However, it is not known whether the actual or apparent size of the target is the determining factor in whether urchins will respond. The present study builds on the results of previous work by independently altering actual and apparent target sizes. Thirty-five specimens of Strongylocentrotus purpuratus (Stimpson, 1857) were presented with circular targets of 6-cm and 11-cm diameter, placed at distances such that their diameters circumscribed arcs of 7.5° or 12°. Urchins oriented significantly only toward targets with an apparent diameter of 12° (P 0.005 and P 0.02), regardless of actual target diameter. Thus, it was concluded that these urchins respond on the basis of the apparent size and not the actual size of targets. Echinoderms respond to shadows and changes in illumination (1–7), and sea urchins of at least three species visually detect objects (8, 9). When presented with a black target against a white background, these urchins will usually move directly toward the target. This response to large distant objects indicates that urchins possess rudimentary spatial vision. These targets do not have any meaningful effect on the overall level of illumination, and any such differences are almost certainly undetectable (8). Perception of these targets could not be accomplished through simple photoreception, and it requires at least some visual acuity. Urchins do not move toward targets below a critical size, but there is more than one way to define the size of an object. The absolute size remains constant, but the apparent size depends on the viewer’s distance. Thus, a small nearby object could appear the same size as a large distant object. Echinoderms likely use vision to find shelter (8, 9), so it would be maladaptive to move toward nearby objects that are too small for this purpose, or to expend energy moving toward large distant objects. Consequently, it would be advantageous for urchins to detect the distance and actual size of objects, allowing them to effectively locate suitable shelters. However, such perception would require a more advanced visual system than that required for a response based on apparent size. Although urchins lack the two eyes necessary for binocular vision, perception of distance could still be accomplished through motion parallax. With lateral movement of the observer, there is a greater apparent change in position for objects that are close than for objects that are far away. An urchin could move sideways relative to objects and use the apparent position changes of distant objects to gauge absolute size. As a viewer moves toward them, closer objects also appear to get larger faster than do more distant objects. Thus, as an urchin moves toward an object, the rate at which the object appears to grow (known as “looming”) could provide information on its distance and absolute size. It is not known if urchins can discern absolute size in either of these ways. They do move slowly, but they may nevertheless still collect the visual information needed. The aim of the present study was to determine whether sea urchins respond differently to objects of the same apparent size but different actual sizes. Previous studies have dealt only with the apparent size of targets. If urchins can determine the absolute size of objects, then it is this size measurement that will predict whether there is a response. However, if urchins do not posses this ability, then it will be the apparent (i.e., angular) size that is important. Received 24 December 2010; accepted 15 March 2011. * Current address: PO Box 14, Merrickville, ON K0G 1N0, Canada. † To whom correspondence should be addressed. E-mail: sjohnsen@ duke.edu Reference: Biol. Bull. 220: 86–88. (April 2011) © 2011 Marine Biological Laboratory