Effects of low pH and high temperature on two Palythoa spp. and predator-prey interactions in the subtropical eastern Atlantic

In the current context of climate change, benthic cnidarians of the genus Palythoa have been suggested to be resistant owing to their intrinsic biological characteristics. In tropical regions, some species are currently proliferating in areas where environmental conditions are less suitable for other organisms, even replacing hard coral ecosystems. Considering their tropical affinities, phase-shifts towards Palythoa-dominated areas could become more frequent in future climate change scenarios, leading to changes in ecosystem organization. The aim of this study was to evaluate the effect of climate change stressors in two common Palythoa spp. with different habitat affinities within a subtropical region, and the effect upon their predator-prey interactions. The results of this experimental study demonstrated that colonies of P. aff. clavata and P. caribaeorum were significantly affected by exposure to temperature and pH conditions predicted for 2100 in the Canary Islands, during 62 days. Despite zoantharians' lack of carbonate in their body wall, Palythoa spp. were most affected in their growth rates by lowered pH, and colonies significantly decreased in weight and size. Although all colonies exhibited symptoms of bleaching at high temperature, a reduction in chlorophyll content was also observed at low pH. Predation by Platypodiella picta crabs decreased on P. aff. clavata exposed to acidic conditions, which may compensate for the lowered ecological performance of the species in these climate change conditions. In contrast, P. picta was able to actively feed on P. caribaeorum colonies regardless of the experimental conditions. Despite being suggested as winner species in a climate change scenario, our study demonstrated that low pH negatively impacted Palythoa spp. survival. If the species are not able to acclimatize to the new conditions, changes in their populations may be expected, although their magnitude could be ameliorated by means of a decrease in predation rates.

Automated summary

In a subtropical region, Palythoa colonies of different habitation affinities were significantly affected by predicted temperature and pH conditions for the year 2100, with growth rates being most affected by lowered pH. Despite exhibiting bleaching symptoms under high temperature, a decrease in chlorophyll content was also observed at low pH conditions. Predation rates by Platypodiella picta crabs decreased on P. aff. clavata colonies exposed to acidic conditions, compensating for the species' lowered ecological performance, while P. picta continued to actively feed on P. caribaeorum colonies regardless of the experimental conditions. This study demonstrated that Palythoa species' survival is negatively impacted by low pH, suggesting potential changes in their populations if they are unable to acclimatize to new conditions, although decreased predation rates could help mitigate these effects.