Materials and Methods
Planulae collection
Planulae of S. pistillata were collected during two consecutive reproductive seasons (January±June 1998,
1999) in situ in front of the H. Steinitz Marine Biology Laboratory (MBL) at the northern Gulf of Eilat (Red Sea). During the reproductive season, mature colonies release planula larvae daily, about two hours after sunset (Rinkevich and Loya, 1979). Gravid colonies were enclosed before dark with plankton nets (45 um), each armed apically with a plastic flask. Nets with re- leased larvae were collected 4±6 h after sunset by SCUBA diving. Planulae of H. fuscescense are released from gravid colonies throughout the year (Benayahu,1991). They were collected by overnight ex situ maintenance of mature, field collected colonies in aerated aquaria at 24°C. All larvae were transferred to aerated aquaria at room temperature (24°C) and were used for experiments following the next 24 h.