Abstract
Depth plays a crucial role in shaping marine biodiversity patterns, with species richness typically decreasing with depth. Despite extensive documentation of this pattern, the drivers of richness changes across depth remain unclear. Richness–depth patterns can arise from multiple proximate mechanisms acting simultaneously, such as changes in abundance, evenness, and the spatial aggregations of individuals. Using a unified framework, we examined variation in richness patterns and their underlying mechanisms in the northern Red Sea for fishes (down to ~ 150 m), Scleractinia, Octocorallia, and Porifera (down to ~ 45–70 m). Our results reveal taxa-specific richness responses. Overall, abundance changes emerged as the most consistent driver of richness variation with depth, although its direction and magnitude differed among taxa. Fish richness declined with depth, primarily driven by reduced abundance and lower evenness. In contrast, Porifera exhibited increased richness with depth, largely explained by higher abundances. Scleractinia richness peaked at 10 m and slowly declined with depth, similarly linked to changes in abundance. In contrast, Octocorallia showed a slight richness increase with depth, mainly due to higher evenness at greater depths, although gaps in observations at intermediate depths may have obscured fine-scale peaks within the studied range. Increasing sampling scale (by combining sampling units) amplified these patterns. The results highlight the importance of understanding abundance gradients for richness, which may be overlooked when relying solely on cover-based estimates. The taxon-specific variability in richness–depth patterns challenges the conventional expectation of declining richness with depth, yet uncovers important commonality in the underlying mechanisms.