Cryptobenthic fish as bioindicators: Community shifts along urban pollution

Coastal ecosystems worldwide are increasingly impacted by urbanization, leading to habitat degradation, pollution, and shifts in biodiversity, which in turn affect overall ecosystem stability. Although the effects of urbanization have been well documented for many taxa, cryptobenthic fish-small, benthic-associated species with strong site fidelity and specialized diets-have been largely overlooked despite their abundance, critical ecological roles, and potential as indicators of anthropogenic disturbance. In this study, we investigate the drivers of cryptobenthic fish assemblages across a pollution gradient in Hong Kong, a highly urbanized coastal city, through a combination of collection-based and environmental DNA (eDNA) techniques. Cryptobenthic fishes' species richness is comparable to that observed in pristine reef sites, however, the community composition differed along the pollution gradient, with higher abundance in highly impacted sites dominated by species such as Tridentiger trigonocephalus and Gobiopsis macrostoma, which are positively with elevated nutrient levels. Conversely, species such as Gobiospsis arenaria and Parablennius Yatabei occur exclusively in low-nutrient environments. Nitrogen emerged as key environmental driver influencing species composition, suggesting cryptobenthic fish display species-specific tolerance to nutrients. Our findings offer new insights into how urbanization influences marine biodiversity and highlight the importance of cryptobenthic fishes as ecological indicators in human-impacted environments.