Synchronous Mass Release of Mature Medusae from the Hydroid Halocordyle disticha (Hydrozoa, Halocordylidae) and Experimental Induction of Different Timing by Light Changes

Abstract

The timing mechanism for synchronous mass release of mature medusae of Halocordyle disticha was studied, using colonies from Shirahama, Wakayama, Japan, which were kept in a 450 I aquarium tank. In near natural conditions medusa release is correlated with sudden drop of light intensity such as occurs around sunset. Timing could be manipulated by controlling light intensity. Artificial sunset 2 hours earlier than normal caused mass release of medusae earlier than under natural conditions, whereas sunset artificially delayed 3 hours later than normal caused continuous release of medusa after the onset of darkness. The spawning of gametes of H. disticha is almost simultaneous with medusa release, and since the medusa has an ephemeral planktonic existence, synchrony of mass medusa release and also spawning of gametes may maximize fertilization success.