Metamorphosis in the Brachiopod Terebratalia: Evidence for a Role of Calcium Channel Function and the Dissociation of Shell Formation from Settlement.

Larvae of Terebratalia will not undergo metamorphosis when maintained in a sterile environment unless they are 9-10 days old; under these conditions the frequency of normal metamorphosis is low. Four-day larvae are normally induced to metamorphose when they contact a suitable substrate. They will also undergo metamorphosis when they are treated with high K+ seawater in the presence of Ca2+. Additional experiments indicate that both substrate-induced and high K+ seawater-induced metamorphosis may involve the function of voltage-dependent calcium channels. Metamorphosis involves settlement of the larva followed by formation of the protegulum, the initial shell. In larvae that have been aged in a sterile environment and in larvae treated with high K+ in seawater with low Ca2+, partial metamorphosis takes place. Under these conditions the larva does not settle, however a protegulum forms. Substrate-induced metamorphosis does not occur in the absence of the distal end of the pedicle lobe of the larva which normally makes contact with the substrate, however, treatment with high K+ seawater containing Ca2+ induces partial metamorphosis in these larvae. These experiments suggest that there are at least two centers in the larva that control metamorphosis.