Adult and young individuals of the shrimp Palaemonetes antennarius (Milne Edwards) collected in the field (experienced), and laboratory-born (inexperienced) young were tested in a transparent bowl screened to hide the surrounding landscape from view. Field individuals were collected in South Tuscany, Italy, and testing was carried out in spring-summer 1987 and 1988. Results confirm that adults are able to assume an ecologically efficient escape direction (towards deep-water) by using celestial cues. They also show that: (1) orienting factors consist of the sun and polarized light in the sky; (2) the compass mechanism is time-compensated; (3) escape direction is not genetically fixed but (4) is learnt during the shrimp's lifetime and allows ample modification. From the results, it is also hypothesized that genetic determination of the escape direction is related to the type of larval development (with or without planktonic phases) as well as to the vagility of the species.