Host performance
To obtain measures of host performance, we recorded the final wet weight at the end of the experiment (t12), summed this with the weight of the fragments harvested for the sampling of epibiota, divided this by the initial wet weight, and multiplied it by 100 to express relative growth rate (RGR) in average percentual growth per day. In addition, we measured the maximum quantum yield of photosystem II using a Diving-PAM fluorometer (Heinz Walz GmbH, Effeltrich/Germany). For this purpose, algae were dark-adapted for 15 min after the final sampling. During the experiment, we also observed the development of two types of disease or stress symptoms. The most prevalent of those symptoms was an increased brittleness of the thallus. We also observed thallus bleaching and decay (similar to (Saha & Weinberger, 2019)) which was often combined with fragmentation. Both thallus brittleness and decay were binary scored (0 or 1 for absence or presence of symptoms; see also Krueger‐Hadfield & Ryan, 2020; Sotka et al., 2018). To investigate correlations, we calculated Spearman rank coefficients among all four performance measures.