Quorum sensing-controlled luminescence by Vibrio fischerii in bobtail squid is an energy intensive process, particularly when one considers 90 percent of the V. fischerii cells are expelled from the squid each day. What appears to be the selective force driving squid luminescence?

Based on the scenario being described within the question it can be said that the driving force for this would be that Luminescence provides the squid a degree of protection from predators when viewed from below against a filtered light background. Therefore it is a matter of survival that keeps this selective trait alive.