Microhabitat associated variation in energetic trade-offs, oxidative balance and shell morphology in an intertidal mussel.

Fine-scale environmental heterogeneity can drive intraspecific functional divergence, but the mechanisms linking environmental variability to organismal responses are unclear. In this study, we examined how three contrasting intertidal microhabitats where P. purpuratus can be found - the higher (HL) and lower (LL) limits of the intertidal distribution band and tide pools (TP) - influence energy allocation, oxidative balance and shell morphology in individuals of this species. Tide pools exhibited extreme fluctuations in salinity (from 28.6 down to 0 psμ) and pH (8.16 to 7.8), whereas intertidal rock wall habitats experienced recurrent aerial exposure. Clearance rates increased with salinity, with HL mussels showing higher clearance rates at 25-30 psμ, consistent with compensatory feeding after emersion. Oxygen consumption was broadly similar among microhabitats, although LL individuals showed higher consumption rates. Field-based condition index and biomass measurements revealed a clear energetic gradient (LL > TP > HL), indicating habitat-specific differences in energy acquisition and allocation. Oxidative balance varied over the tidal cycle: HL and LL mussels exhibited higher levels of lipid peroxidation and antioxidant activity during emersion-immersion transitions, whereas TP individuals maintained relatively stable oxidative profiles despite saline variability. Morphometric analyses revealed clear divergence in size and shell shape, with HL individuals displaying more compact shells and LL mussels attaining larger sizes. These results show that aerial exposure (HL, LL) and osmotic stress (TP) impose distinct but comparable energetic constraints, driving coordinated shifts in energy allocation, oxidative regulation and morphology.