Meeting Abstract
77.1 Tuesday, Jan. 6 Role of chemical cues in the visual orientation of horseshoe crab larvae and juveniles. MEDINA, J.M.*; TANKERSLEY, R.A.; Florida Institute of Technology; Florida Institute of Technology jmedina@fit.edu
Adult horseshoe crabs (Limulus polyphemus) have long served as models for the study of vision in marine arthropods. Yet, little is known about the visual responses of early life history stages. We examined the visually directed movements and orientation of larvae and first stage (tail-stage) juveniles to horizons containing dark visual targets of different sizes. The study tested the hypotheses that (1) early life history stages use visual cues to avoid predators or locate potential refuge areas and (2) responses to visual targets depends upon ontogentic stage and the presence or absence of chemical cues from potential nursery habitats. Visual orientation of larval and juvenile crabs to horizontal rectangles subtending angles from 30-330 was tested in a circular arena containing water that either lacked estuarine chemical cues (offshore water) or contained odors from aquatic vegetation or predators. In offshore water, larvae oriented toward dark horizons subtending angles > 60. In contrast, juveniles moved in the opposite direction when exposed to similar horizontal rectangles. When placed in water containing chemical odors from potential predators and nursery habitats, including manatee grass Syringodium filiforme, shoal grass Haloduli wrightii, drift algae Acanthophora sp., mummichug Fundulus grandis, and blue crab Callinectes sapidus, both larvae and juveniles crabs reversed their direction of orientation (approx. 180 phase shift) relative to their responses to similar targets in offshore water. Results support the hypotheses that the visual orientation of larval and juvenile horseshoe crabs changes dramatically upon exposure to habitat and predator cues and that the direction of the response undergoes an ontogenetic shift following metamorphosis to the juvenile stage.
