Figure 7.12 Calcium Ions as Second Messengers The concentration of Ca²⁺ can be measured using a dye that fluoresces when it binds the ion. Here, fertilization in a sea star egg causes a rush of Ca²⁺ from the environment into the cytoplasm. Areas of high Ca²⁺ concentration are indicated by the red color, and the events are photographed at 5-second intervals. Calcium signaling occurs in virtually all animal groups and triggers cell division in fertilized eggs, initiating the development of new individuals.