Specifically, we hypothesise that parasites lower host body temperatures during low tide aerial exposure by either enhancing solar reflectivity as a result of the whitening of the shell or by dissipating heat through photosynthetic activity. In this study, we consider a situation in which shell-degrading endoliths could benefit their hosts. Because of their boring activity, these parasites cause a distinctive discoloration of the outer layer of the mussel shell 11. In extreme cases, endoliths cause the shell to collapse and can be responsible for 50% of total mortality in highly infested mussel populations 7, 8, 9.Įndoliths are metabolically dependent to their host excavation into the host shell is through chemical dissolution, with the carbonate ions released being converted from calcite into CO 2 that is then used for photosynthesis 10. Phototrophic shell-degrading endoliths (mainly cyanobacteria) bore into mussel shells, weakening shell strength the demand for shell repair depletes energy resources at the expenses of reproduction, byssal attachment strength, general condition and growth 7, 8. Here we address the neglected question of whether endolithic infestation can provide a previously unrecognised protective role for mussels and increase their survival during periods of intense heat stress. Population extinctions and mass mortalities have been reported as a result of continuous, gradual temperature increase over relatively long temporal scales or extreme events 4, 5, 6. Significant effects of raising temperatures on populations, communities and ecosystems have been observed across many regions and in a wide variety of marine and terrestrial taxa 2, 3. Parasites that are otherwise detrimental may become beneficial, improving host’s performance under specific environmental conditions while maintaining an overall negative effect under others (‘conditionally helpful parasites’ as defined in Fellous and Salvaudon 1). However, the drawbacks of being parasitized may be context-dependent. Experimental and observational studies have consistently shown that parasites reduce host survival and fecundity causing a decrease in host population growth and density that can even drive the population to extinction.
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