Caenorhabditis japonica is a gonochoristic (male-female) species related to C. elegans. However -- unlike C. briggsae, C. remanei, or C. brenneri -- C. japonica is not a member of the Elegans group, but the Elegans group's closest known outgroup species. It was selected for genomic sequencing on account of its providing the best (closest) available outgroup for genomic comparisons with Elegans group members.
C. japonica has the following Web sources of information:
4. A WormBook review of Caenorhabditis ecology (including that of C. japonica).