Abstract: In order to determine the interactions among the biofilm characteristics, culture media and settlement of marine invertebrate, the present study investigated the effects of the biofilm formation in two types of culture media, and the subsequent effects of bacterial biofilms on the settlement and metamorphosis of Mytilus coruscus larvae. The biofilm inducing activities on larval settlement and metamorphosis were correlated to the bacterial density with exception of one and four species in culture media Zobell 2216E and Seawater Luria-bertani (SWLB), respectively. The inducing activity of the bacterial biofilms was correlated to the culture medium type and initial cell density of bacteria. Scanning electron microscope showed that a thick Staphylococcus sp. 3 biofilm comprising of a stack of bacterial cells under the condition of 2216E compared to SWLB. In contrast, patchy monolayer was found under 2216E in Pseudoalteromonas sp. 8 biofilm, and short rod was observed under SWLB. SDS-PAGE showed that protein profiles of both Staphylococcus sp. 3 and Pseudoalteromonas sp. 8 biofilms changed distinctly in two different culture media. In Staphylococcus sp. 3 biofilm, eight protein bands decreased significantly and two protein bands disappeared under SWLB compared to 2216E. In Pseudoalteromonas sp. 8 biofilms, five protein bands appeared under SWLB. Thus, the culture media impacted the development of bacterial biofilms such as distribution, morphology and phenotypes, and changes in biofilm characteristics may contribute to the variability in the inducing activity on larval settlement and metamorphosis.