Many marine organisms, especially molluscs, produce mucus, which can be used in several physiological processes, such as locomotion and defence. In addition to utilization for themselves, mollusc mucus may influence intertidal community dynamics.Therefore, the biochemical composition of Onchidium typhae Buchannan, 1800 mucus was investigated. The effects of mucus on the microphytobenthos growth rate, representing the ecological functions of the mucus, were also tested. The results showed that the mucus was mainly composed of water (82.5%), and a large proportion of the organic content in mucus consisted of proteins (40%), followed by carbohydrates (13.33%) and lipids (0.19%). The effects of mucus on microphytobenthos growth were conditional and genus-specific. Growth of Navicular sp. and Nitzschia sp. significantly responded to different concentrations and incubation times of the slug mucus. Fresh and concentrated mucus inhibited the growth of both species; low mucus concentration and longer incubation time showed a significant increase in the growth rates of both species. However, no effect of mucus on the Thalassiosira sp. growth rate was observed. These effects of mucus on diatom growth are discussed. Moreover, our findings suggested that the intertidal slug O. typhae may act as an intertidal ecosystem engineer, which can facilitate and influence benthic phytoplankton community dynamics.