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Polyplacophoran molluscs show low morphological diversity compared to other marine invertebrate clades, yet chitons are ecologically important grazers that occupy a range of distinct ecological niches. We investigated a potential functional correlate of niche separation in three species of co-occurring mopallid chitons that have total ranges across differing environments (Mopalia muscosa, Mopalia lignosa, Katharina tunicata). We found that the force needed to fracture the protective valves varied significantly among species. K. tunicata, whose valves have a relatively reduced exposed dorsal surface, was significantly more resistant to fracture than the two Mopalia species (mean force: K. tunicata = 31.9 ± 4.5 N; M. lignosa = 12.5 ± 0.8 N; M. muscosa = 20.2 ± 0.8 N). In Mopalia spp., the terminal valves were significantly stronger than intermediate valves (i.e. higher force to fracture), whereas all valves in K. tunicata appeared to be functionally equivalent. To assess whether future chemical changes predicted under ocean acidification (OA) will affect these species differently, we measured the force to fracture of valves after 10 days of exposure to elevated pCO2 (control = 8.0 pH [407 ± 104, pCO2], elevated = 7.5 pH [1544 ± 249, pCO2]) for both live animals and dissected individual valves. Although previous experimental OA work found significant impacts of elevated pCO2 on adult mollusc shells over similar timescales, we saw no reduction in total strength related to treatment. Our data demonstrate that diversity in chiton valve morphology has functional implications and that physical changes in local topology and wave exposure may have stronger impacts on adult chitons than changes in ocean chemistry under future climate change.