Size-dependent effects of an epibiotic seaweed on the probability of dislodgement of an intertidal mussel

Epibiosis can have beneficial and detrimental consequences both for the basibiont and the epibiont. Here we investigated bidirectional effects in a seaweed-mussel, epibiont-basibiont system in a southwestern Atlantic rocky shore, focusing on variables such as mussel attachment strength, mussel body condition and seaweed and mussel dislodgement. Neither seaweed, Porphyra sp., weight nor seaweed presence accounted for variation in mussel, Brachidontes rodriguezii, attachment strength and body condition. Mussel attachment strength was, instead, positively related with mussel size (i.e., irrespective of seaweed presence and size). Additionally, we found that Porphyra weight was positively related with mussel size, suggesting that the smaller and relatively weakly attaching mussels are unable to withstand the drag imposed by relatively large Porphyra thalli. The latter was demonstrated with an experiment where large Porphyra thalli transplanted into large and small mussels caused increased dislodgement of small mussels. These findings illustrate a size-dependent, bidirectional negative effect on the interaction between mussels and their epibiotic seaweeds where (a) the seaweeds induce a proportionally higher dislodgement of small mussels and, (b) small mussels are low-quality substrates for seaweeds as they weaker attachment imposes limits to their growth and survival. [Display omitted] •Nor seaweed presence or seaweed weight accounted for mussel attachment strength and body condition.•Mussel attachment strength and seaweed weight were positively related with mussel size.•Seaweeds induce a proportionally higher dislodgement of small mussels.•Small mussels are low-quality substrates for seaweeds as they weaker attachment imposes limits to their growth and survival.