The decay and learning of an y axis orientation behavior: the offshore escape response of the shrimp Palaemonetes vulgaris (Say)

Abstract

The shrimp Palaemonetes vulgaris (Say) lives along shorelines and, when frightened, uses solar cues to orient in an offshore direction. The decay and learning of this behavior were studied. The persistence of this behavior in the absence of daytime celestial cues was investigated both by holding the shrimp in the laboratory under constant conditions for 2–24 h, and by collecting the shrimp before sunrise, and then, in both cases, testing them in an outdoor arena. This elevated arena allowed the shrimp to view celestial cues but removed slope and landmarks as orientation cues. The behavior is a time-compensated menotactic orientation that decays after 7–24 h under constant conditions and overnight. Therefore, after extended periods of complete cloud cover and each morning after sunrise, the shrimp must relearn the relationship between the skylight cues and the offshore direction. The shrimps' ability to learn new offshore directions was demonstrated by placing shrimp in a cage on an unfamiliar shoreline for various periods of time before testing. After 2.5–4 h in the cage, the animals learned to orient in the new offshore direction, using celestial cues. Both the decay and learning of this orientation behavior appear rapid relative to similar behaviors of other arthropods. Since Palaemonetes live along convoluted shorelines, it is likely that they will encounter new offshore directions to learn on a daily or even hourly basis. Their rapid learning ability then may be necessary for efficient avoidance of shoreline predators.