A new study on the Lake Tanganyikan cichlid Neolamprologus pulcher says that the fish defends its territory differently depending on the fish which encroaches its patch.
Polly Frostman and Peter Sherman of the University of Transylvania used N. pulcher to test a hypothesis of the behaviouralal response to familiar and unfamiliar neighbours, and have reported their findings in the lastest issue of the journal Icthyological Research.
Some species have been suggested to show more territorial aggression to intruders that they haven't fought with before, but less to those they fight with regularly - a behavioural response known as the "dear enemy" phenomenon.
Frostman and Sherman found that this indeed is the case with Neolamprologus pulcher. The cichlid can discriminate between familiar and unfamiliar males of the same size and responds much more aggressively to unfamilar fish than it does to those it knows.
They also spend much more time at the territorial boundary when they share it with unfamiliar neighbours than they do when with fish they already know.
For more details read the paper: Frostman, P and PT Sherman (2004) - Behavioural responses to familiar and unfamiliar neighbors in a territorial cichlid, Neolamprologus pulcher. Ichthyological Research, Vol. 51., No. 3. pp-283-285.