Microbial endolithic symbionts shape the behaviour of intertidal mussels: a multispecies perspective

Symbionts play a crucial role in shaping host biology across diverse ecosystems and taxa. In coastal marine environments, microbial endoliths are known to form symbiotic relationships with intertidal mussels. While the primary effects of this symbiosis involve shell corrosion affecting the species’ survival, these alterations may indirectly influence mussel behaviour and their capacity to form aggregations. Here, we examined the behavioural responses of intertidal mussels to microbial endolithic communities within their shells, using two mussel species dominant in their respective geographical ranges, the European blue mussel, Mytilus edulis, in France and the African brown mussel, Perna perna, in South Africa. We focused on three behavioural traits known to be relevant for the formation of mussel beds: attachment strength, movement and aggregation. Our results showed that the presence of endolithic symbionts influences mussel behaviour. In P. perna , mussels with endoliths showed reduced attachment strength and movement compared with those without endoliths. Conversely, in M. edulis , individuals with endoliths showed greater attachment strength than their counterparts lacking endoliths. Furthermore, behavioural traits varied when mussels with and without endoliths coexisted. Notably, the presence of conspecifics with symbionts adversely affected the attachment and movement of individuals without symbionts in mixed beds. Reduced attachment strength heightens the risk of dislodgement and mortality, compromising aggregate integrity and mussel bed stability. Since mussel beds serve as important habitats in coastal ecosystems, such destabilization may have cascading effects on mussel populations and thus the broader ecosystem that relies on these bioengineers. • Symbionts are widespread across ecosystems and shape host biology. • We explored the role of microbial endolithic symbionts in mussel behaviour. • Symbionts alter attachment and movement, potentially impacting bed stability. • These endolith-induced effects are, however, species specific. • Mussels show altered behaviour when coexisting with endolith-hosting conspecifics.