Ocean acidity has increased by around a third since the 18th century. This has resulted in the weakening of California mussel shells as well as a decrease in their body mass.
Having weaker shells makes mussels more vulnerable to predators like crabs and carnivorous snails and the smaller body mass means they are more likely to dry out at low tide and less likely to withstand their metamorphic process.
Read more at The Journal of Experimental Biology