"A. fischeri c"olonization occurs in juvenile squids and induces morphological changes the squids light organ. Interestingly, certain morphological changes made by "A. fischeri" do not occur when the microbe cannot luminescence, indicating that bioluminescence (described below) is truly essential for symbiosis. In the process of colonization, ciliated cells within the animals' photophores (light-producing organs) selectively draw in the symbiotic bacteria. These cells promote the growth of the symbionts and actively reject any competitors. The bacteria cause these cells to die off once the light organ is sufficiently colonized.