Variation in species and functional composition of octocorals and zoantharians across a tropical to temperate environmental gradient in the Indo-Pacific

Global warming causes functional shifts and reorganisation in marine communities through range shifts to high-latitude reefs and cnidarian bleaching mortality in the tropics. Such changes threaten the integrity and structure of marine communities, especially as foundational and associated species are reduced or lost. However, comparatively little is known about the extent of range shifts and their ecological consequences for the overlooked components of marine ecosystems, such as octocorals and zoantharians (O + Z) on shallow coral reefs. As these groups play a crucial part in building complexity and sustaining life in reef communities, functional shifts in these taxa may cascade through the entire ecosystem, but these processes have not been quantified. Here, we examined the environmental drivers and functional consequences of spatial variation in octocoral and zoantharian communities across 27 sites in southern Japan, spanning from tropical to warm-temperate waters. We collated a trait database for 42 entities (species, genus, and family level identifications) of octocorals and zoantharians and calculated functional diversity and functional richness to measure functional compositional change. We identified five functional groups according to their trait similarities and identified how their abundances respond to changing environmental factors with general additive models (GAMs). We found functional shifts among octocorals and zoantharians across the tropical to temperate thermal gradient, with the abundances of two functional groups best explained by gradients in minimum sea surface temperature. Non-linear relationships between the functional groups and thermal gradients imply a more intricate relationship than expected, suggesting other non-temperature-based drivers, e.g., nitrogen or pH levels might also play an important role. Only functional group richness and species richness showed significant correlations with latitude, whilst functional diversity and functional richness did not. Our results indicate that octocoral and zoantharian communities and functionality potentially undergo shifts with clear community compositional changes, influenced by climate change across environmental gradients. However, the taxonomy and identification of these taxa remain difficult, and information on functional traits is often sparse or not species-specific, indicating a clear need for further basic zoological and ecological work on octocorals and zoantharians.