This study talks about establishing that some corals can switch from depositing aragonite to calcite as the ratio of magnesium to calcium in sea water changes. The corals do experience a 65% slowing in rate of growth when they switch, though.
This study points out that corals have a reserve of plasticity that we didn’t know about before. That’s pretty cool. Now consider that even with this enhanced flexibility, corals all around the world are dying in the current situation as pollution and climate change alter their environment. In other words, these animals, with a lineage going back some 480 million years, have been able to cope with the changes that occurred over that time, but are finding it impossible to cope with what is going on now.
Some skeptics of climate change have offered the observation that change in temperature has happened in the past, and that the current change is nothing exceptional. I think that we can consider corals as indicator species in this regard: something that threatens their existence is also out of the ordinary.