Background colouration influences body colour segregation in mollies

When given a choice of swimming against a black background near an empty bl ack end compartment or swimming against a white back.-round near an empty w hite compartment, black morph mollies Poecilia latipinna spent significantl y more time against the black background. The presence of a shoal (either w hite or black mollies) in the black compartment increased the preference of the test fish for the black background. However, black molly test fish spe nt significantly less time against a black back.-round if a black shoal was present in the white compartment than in assays in which a white shoal was in the white compartment. In assays where test fish were given the choice between two black shoals, test fish spent significantly more time near the black shoal on the black background. Similarly, when given the choice betwe en two white shoals, test fish spent significantly more time against the bl ack background. The results suggest that the presence of cryptic background s alter shoalmate choices in fish that assort according to body colouration . (C) 2001 The Fisheries Society of the British Isles.