Orientation of blue crab, Callinectes sapidus (Rathbun), Megalopae: Responses to visual and chemical cues

Abstract

Post-larvae (megalopae) of the blue crab Callinectes sapidus are transported from offshore areas into estuaries where they settle and metamorphose in specific areas, such as seagrass beds. The present study tested the hypothesis that intermolt and premolt megalopae had different behavioral responses to visual and chemical cues that are associated with predator avoidance and settlement. Visually directed movements to solid and striped rectangular targets subtending different visual angles (5–350°) were tested in an arena in the presence of either Offshore Water, Seagrass ( Zostera marina) Odor Water or Predator ( Fundulus heteroclitus) Odor Water. Intermolt megalopae generally swam away from 10° to 180° targets in all water types which was interpreted as a predator avoidance response. Premolt megalopae had similar behavior in Offshore and Seagrass Odor Water. However in Predator Odor water, they displayed different predator avoidance behavior. When presented with a solid target, premolt megalopae either remained motionless or swam directly away from the target. If presented with targets resembling stalks of seagrass (vertical stripes), they swam in all directions which was interpreted as a startle response. In a chemical choice chamber, both molt stages were not attracted to Seagrass Odor Water but avoided Predator Odor Water. There was no evidence that megalopae used chemical cues for orientation toward settlement sites. Thus, the hypothesis was supported and the results suggest that behavioral responses to the test chemical and visual cues are involved in predator avoidance.