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

Post-larvae (megalopae) of the blue crab Callinectes sapidus are transporte d from offshore areas into estuaries where they settle and metamorphose in specific areas, such as seagrass beds. The present study tested the hypothe sis that intermolt and premolt megalopae had different behavioral responses to visual and chemical cues that are associated with predator avoidance an d settlement. Visually directed movements to solid and striped rectangular targets subtending different visual angles (5-350 degrees) were tested in a n arena in the presence of either Offshore Water, Seagrass (Zostera marina) Odor Water or Predator (Fundulus heteroclitus) Odor Water. Intermolt megal opae generally swam away from 10 degrees to 180 degrees targets in all wate r types which was interpreted as a predator avoidance response. Premolt meg alopae 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 moti onless or swam directly away from the target. If presented with targets res embling 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 Pred ator Odor Water. There was no evidence that megalopae used chemical cues fo r orientation toward settlement sites. Thus, the hypothesis was supported a nd the results suggest that behavioral responses to the test chemical and v isual cues are involved in predator avoidance. (C) 1999 Elsevier Science B. V All rights reserved.