Biofilm and substrate complexity affects recruitment of a functionally extinct native oyster

Oyster recruitment can be limited by small‐scale characteristics of their settlement substrate, including changes in biofilm and fine‐scale (i.e., sub‐millimeter to centimeter) complexity. Understanding how these factors affect recruitment is critical for improving oyster population recovery and guiding restoration practices. In our study, we investigated the role of biofilm development and substrate complexity affecting recruitment of the subtidal flat oyster ( Ostrea angasi ), a native shellfish species largely functionally extinct in Australia. In natural habitats, remaining O. angasi individuals and reefs can be shaded by canopy‐forming macroalgae, potentially altering biofilm formation on settlement surfaces. However, the implications of these shade‐affected biofilms, particularly in combination with substrate complexity, are unclear. We field‐conditioned two types of settlement substrata, that is, sandstone and oyster shells, with varying levels of fine‐scale complexity and applied three biofilm treatments: ambient light (biofilm‐promoting), shaded (reduced light, potentially altering biofilm), and cleaned (biofilm removed), after which O. angasi recruitment was assessed ex situ. Biofilm treatments were not observed to affect recruitment on sandstone substrates, but on shells, recruitment declined with removed biofilm. Recruitment also increased with increasing complexity of the shell substrate. Our findings suggest that the effects of biofilm on O. angasi recruitment are dependent on substrate type, but recruitment can increase on substrates with greater complexity. Future restoration efforts involving seeding of O . angasi on settlement substrates may therefore benefit from managing biofilm development and enhancing substrate complexity.