The photosymbiotic acoel Convolutriloba retrogemma (Xenacoelomorpha) is sensitive to thermal stress

Multiple marine invertebrate species have been utilized as model organisms for investigations on the effects of climate change and bleaching on animal-algal symbioses. However, one potential model host that has not been addressed is the acoel Convolutriloba retrogemma, which associates with chlorophytes. Therefore, this study assessed whether thermal stress reduces survival, size, symbiont density and chlorophyll-a concentration for C. retrogemma. Acoels were subjected to an experiment to examine the combined effects of temperature (26, 28 and 30 °C) and exposure (1, 2 and 4 weeks) on each of the four variables. The population size and total length of acoels were monitored, followed by sampling and maceration for quantifying symbionts and extracting chlorophyll-a, which was measured using a fluorometer. Results showed significant acoel mortality at both 28 and 30 °C, with populations decreasing by 50 % and 100 %, respectively, after four weeks. Length did not differ among conditions. Acoels underwent bleaching as symbiont density decreased at 30 °C, and chlorophyll-a content decreased at both 28 and 30 °C. In the current context of climate change, the association between C. retrogemma and Tetraselmis sp. appears to be considerably fragile compared to other model organisms. With the predicted increase in marine heatwaves, this holobiont may be severely threatened.