The invasion of Spartina alterniflora significantly affected the local ecosystem of Western Pacific Ocean where Moerella iridescens lives. Five patches with different invasion stages of S. alterniflora were selected and the influence on distribution of
M. iridescens was studied on the coast of Wenzhou Bay, China in 2007. The aggregated distribution pattern was proved by using Taylor's power regression and Iwao's plot regression methods (p < /em><0.001). The densities were significantly affected by the factors of S. alterniflora invasion stage and season (p < /em><0.001), but no significant effect of interaction (p < /em>=0.805) occurred. M. iridescens mainly clumped in the habitats of no invasion and initial invasion of S. alterniflora was in the high tidal zone, and the lowest density was recorded where complete invasion occurred. The densities were larger in warmer than in cooler seasons. There were significant positive correlations among the average densities in seasons. Density variation must be the response of M. iridescens to the environment, including S. alterniflora invasion stage, temperate stress and interspecific associations.