Intracellular calcium transients during nematocyst discharge in actinulae of the hydroid,Tubularia mesembryanthemum

The discharge of atrichous isorhiza (AI) from the aboral tentacle tips (ATT) is an integral process during settlement of actinula larvae of the marine hydroid Tubularia mesembryanthemum, since discharged adhesive tubules from AI anchor actinula to settlement substrata. Under experimental conditions, AI discharge was triggered either by contact with bacterial film-coated micropipettes or by exposure to artificial seawater (ASW) containing high concentrations of K+ ions (K+-ASW). Triggering of AI discharge required both contact by ATT with the substrata and the presence of Ca2+ions in the bathing solution. Changes in intracellular Ca2+ion concentration ([Ca2+]i) were monitored by measuring fluorescence images obtained from ATT of intact actinulae loaded with fura-2. An initial low [Ca2+]i(R340/380=1.0) value was rapidly elevated by K+-ASW treatment (R340/380=1.9) and simultaneously accompanied by AI discharge. These results suggest that Ca2+ions present in the bathing solution flow into nematocytes during the AI discharge process. J. Exp. Zool. 278:299–307, 1997.