A facultative mutualism between habitat-forming species enhances the resistance of rocky shore communities to heat waves

GUTIÉRREZ, JORGE L.; BAGUR, MARÍA; LORENZO, RODRIGO A.; PALOMO, MARIA GABRIELA

Frontiers in Ecology and Evolution

Frontiers Media

Lugar: Laussane; Año: 2023 vol. 11

Heat waves have increased in frequency, duration, and magnitude in recent decades,causing mass mortality events in terrestrial and aquatic ecosystems. Arguably, massmortalities of habitat-forming organisms – i.e., dominant sessile organisms thatdefine habitats via their own physical structure – would be amongst the mostdramatic impact of heat waves because of their negative, cascading consequenceson their associated biodiversity. However, the resistance of habitat-formingorganisms to heat waves can be enhanced if they associate with secondaryhabitat formers able to tolerate and modulate extreme heat levels. Here we showthat a seaweed of the Porphyra/Pyropia (P/P) clade can shield primary habitatforming mussels, Brachidontes rodriguezii, from the impacts of extremetemperatures in a southwestern Atlantic rocky intertidal shore. By means of P/Premoval experiments and surveys, we illustrate that P/P cover (a) bufferstemperatures in the understory mussel beds during daytime air exposure periodsin the summer, (b) reduces mussel mortality and leads to increased mussel bodycondition during warm summer periods, and (c) can prevent mass mortality ofmussels during the course of a heat wave. Additionally, by means of a musselremoval experiment we illustrate that mussel cover is critical for P/P establishment,which is in consonance with the remarkably higher P/P densities and cover observedin mussel beds relative to exposed rock surfaces across a ~70 km coastal range.Collectively, these findings reveal a facultative mutualism where mussels provide afavorable substrate for P/P colonization and P/P attenuates heat mediated mortalityon mussels. The ability of P/P to enhance the resistance of mussel beds to extremeheat events and the occurrence of similar P/P-mussel associations during springsummer at globally dispersed sites suggests a widespread importance of P/P for thestability of mussel beds and their associated communities under warming climates.