Endogenous photoproteins, calcium channels and calcium transients during metamorphosis in hydrozoans

There are species of hydrozoans, Eutonina victoria, Mitrocomella polydiademata, and Phialidium gregarium whose eggs contain calcium-specific photoproteins. These cytoplasmic photoproteins are synthesized during oogenesis. During the cleavage stages of embryogenesis they are distributed to all of the cells of the developing planula larva. The amount of photoprotein slowly declines during the development of the planula larva, and markedly declines when the planula undergoes metamorphosis to become a polyp.Oocytes, unfertilized eggs, and fertilized eggs prior to the first cleavage do not produce light when treated with KCl. The ability to respond to KCl appears about the time of first cleavage, and is correlated with the appearance of active membrane responses. Both the KCl response and the action potentials will occur in sodium-free sea water, and both are inhibited by calcium channel blockers. These and other experiments suggest that voltage sensitive calcium channels first become active at about the time of first cleavage. These channels also appear on the same schedule in both unfertilized eggs and in enucleated egg fragments, which have been artificially activated with A23187.Developing planulae produce few or no spontaneous light responses before gastrulation. Later the frequency and magnitude of spontaneous light production increases presumably due to an increasing frequency and magnitude of calcium transients. Both the natural trigger of metamorphosis (bacteria) and an artificial trigger (CsCl) cause a conspicuous series of calcium transients. When these transients are inhibited by calcium channel blockers, metamorphosis is also inhibited.