LARVAL SETTLEMENT OF A SYMBIOTIC HYDROID: SPECIFICITY AND NEMATOCYST RESPONSES IN PLANULAE OF PROBOSCIDACTYLA FLAVICIRRATA.

Planulae of the symbiotic hydroid Proboscidactyla flavicirrata settle on the rims of sabellid tubes by adhering sequentially and specifically to two different substrates. For both steps, nematocysts are the agents of adhesion. Although the cnidoblasts of these planulae require local excitation, they are subject to endogenous controls. Upon being swept into the plumes of tentacular cirri that sabellids extend from their tubes, planulae attach to individual pinnules (branches of the tentacular cirri) by discharging nematocysts. Within 2 min following attachment, the planulae cease discharging nematocysts in response to additional pinnule contacts. Planulae transfer to the rims of sabellid tubes by adhering with nematocysts when the sabellids retract their plumes. Cnidoblasts of planulae only become responsive to contact with sabellid tube after 4 min or more of continuous attachment to pinnules. Transfer initiates metamorphosis. Planulae are capable of settling and metamorphosing on Eudistylia vancouveri, a sabellid species that is sympatric with the normal hosts of Proboscidactyla, but will not support colonies. Apparently the deficiencies of Eudistylia as a host are manifested only after planula metamorphosis.