"Calcareous shells" refers to the hard outer coverings made of calcium carbonate that certain marine creatures, such as mollusks, crustaceans, and some types of algae, have. Full definition
Raised CO2 in aquatic systems can also lead to physiological stress, difficulty in building calcareous shells etc. (as will happen if atmospheric CO2 continues to build up beyond around 700ppm - the so called ocean acidification effect). (realclimate.org)
Pink Beach — locally called Pantai Merah — has a mixture of white and red sand formed from pieces of foraminifera, a marine protozoan with calcareous shells, and is one of only seven of its kind in the entire world. (blog.nezasa.com)
That additional acidity gained from carbon dioxide in sea water is affecting many species with calcareous shells and having the most significant effect on hard corals, which also use calcium carbonate to build their home (earthtimes.org)