pacificus, mouth morphology and feeding mode change depending on environmental conditions experienced during development. Our previous studies have shown that the light environment contributes to the determination of mouth morphology, and further suggest that this nematode possesses a unique light-sensing mechanism distinct from those of other animals. By combining genetic approaches such as forward genetic screening and genome editing with molecular biology, imaging, and bioinformatics, we will identify novel photoreceptors and examine their functions. We will also clarify inter-tissue communication through which information is transmitted from sensory neurons to cells forming the mouth, as well as the gene expression and epigenetic regulation that support plasticity, thereby uncovering how the environment shapes phenotype. The insights gained will contribute to an evolutionary understanding of environmental responses and may also enable the development of new experimental manipulation tools.