so in other words, when crs are stressed, they constrict their chromatophores to become pale, and calcium ions are responsible for this action?
Well, the study concludes that calcium ions are needed for the transport, but they don't know which direction it's used in: the expansion, or contraction, of the area of pigment. But the process whereby the pigment moves needs calcium ions present. In many cases in physiology, however, the calcium ion is used to mediate the active process (i.e., as long as calcium is present, the engine is running, and then is pumped back across a membrane afterward as the process goes quiescent and passive).
Based on my personal observations, I'd guess that the calcium is used in the dispersion of pigment - that the act of EXPANDING the pigment area is the active process, and the act of retracting the pigment area is the passive process. It could be that there are different mediators involved in the different directions, too. Usually, in physiological systems, there is a sort of baseline level of activity that is moderated up or down - it's not just as simple as an on-off switch.
My guess is that calcium ions are used toward the active
process, and that the active process is that of expanding
pigment, and that inhibition results in passive retraction of the pigment.
Why I think this is that if you keep crystals relatively happy, they are fairly pigmented (so, a moderate level of active process going on, all the time, as baseline). But if you make them really happy, their pigment intensifies, such as when you finally feed them after a few days of not feeding. Cuttlefish increase their pigment into big displays in response to stress, positive or negative stress. This suggests that increasing the pigment area is the active process.
It's likely a lot more complicated that this, too. Usually, in physiological systems there is a reservoir of the thing like calcium that is under pump action that in the resting state the pumps are pumping the calcium into the reservoir and keeping a level of calcium in the reservoir. Then, when a stimulus comes for activity, the reservoir membranes are altered such that they leak the calcium out of the reservoir, and into a general area, where something else has calcium receptors that, when filled with calcium, make something else happen. So, the calcium, when loose, randomly finds a receptor to park in, causes a reaction, then eventually gets removed from the receptor via a recycling enzyme, and then pumped back into the reservoir, whose membranes have now recovered and are no longer leaky.
Biology is coo-el.