The Overlooked Suspension Feeders: The Filtering Activity of the Bryozoans Schizoporella errata and Bugula neritina in the Northern Adriatic

Marine fouling communities, characterized by a high abundance of suspension feeders, play a crucial role in regulating ecosystem services, particularly in improving seawater quality. While not typically prioritized in conservation due to their prevalence in degraded or artificial habitats, fouling communities are important for their ecological functions under increasing urbanization and climate change. Bryozoans are an important component of these communities, although their filtering activity is less understood compared to some other groups, such as bivalves and ascidians. In this paper, we aimed to investigate the filtration activity of two widespread fouling bryozoan species, namely Schizoporella errata and Bugula neritina in the northern Adriatic (Slovenia). We measured the clearance rates (CR) of both the species when fed with microalgae to assess their filtration capacity and determine the most suitable units for quantifying the biofiltration. B. neritina exhibited a higher average CR than S. errata. The maximum CR was 32 mL/(h·cm2) for the S. errata and 52 mL/(h·cm2) for the B. neritina. Due to the morphological and growth differences between the species, the surface area was determined to be the most appropriate unit for expressing the CR. We also examined the CR of the S. errata exposed to fluorescent microplastic beads and identified active feeding areas within the colonies. Feeding zooids in S. errata were concentrated at the terminal growth margins and elevated areas of the frontal budding, as indicated by a higher fluorescence and microsphere density. These results contribute to the existing knowledge on fouling bryozoans in coastal habitats and provide further insights into their potential role as biofilters and contributors to ecosystem functioning.