Projected warming disrupts temperate serpulid communities, key engineers of benthic ecosystems: evidence from in situ thermal experiments

Global ocean warming is reshaping marine communities, with consequences for species- and community-level functioning. Serpulid polychaetes are a key component of benthic ecosystems that may be particularly vulnerable to rising temperatures, yet their responses remain poorly studied. Using in situ bespoke heated recruitment panels, we investigated the effects of warming (mid- and end-of-century temperature projections, i.e. +1 and +2°C above ambient temperature, as well as short-term warming heatwave events) on the abundance, diversity, recruitment, survival, percentage cover, and growth rate of 11 morphologically distinct serpulid taxa in southern New Zealand. Temporal variability across experimental repetitions strongly influenced serpulid responses. No differences in community structure or growth rate were detected under warming treatments during the winter deployment. In contrast, negative responses became more pronounced at later deployment times, when ambient temperatures were higher. While diversity remained stable across treatments, the most significant changes occurred under +2°C during the late-summer deployment: abundance decreased 10-fold, recruitment 6-fold, survival and cover 3-fold, and growth rate was halved, resulting in smaller individuals. The +1°C and heatwave treatments had intermediate effects, indicating a progressive response to warming. While some serpulids exhibited greater short-term resilience, our observations suggest that warming may favour thermally tolerant taxa, including invasive species. This work is the first in situ experimental study investigating the response of temperate serpulid communities to ocean warming. Our findings highlight their vulnerability to climate-driven changes and underscore the need for further research into their ecological role in a rapidly warming ocean.