Coral Bleaching in a Temperate Sea: From Colony Physiology to Population Ecology Maoz Fine and Yossi Loya 6.1 Introduction While shifts in coral reef species composition following bleaching events have been reported (Glynn 1988; Brown and Suharsono 1990; Gleason 1993; Guzman and Cortes 2001; Loya et al. 2001) and widely discussed (Glynn 1993; Brown 1997; Done 1999; Hoegh-Guldberg 1999; Wilkinson 2000), little is known about the effect of bleaching on coral populations in temperate zones or nonreef environments. A major factor influencing shifts in coral community structure is differential susceptibility of species to bleaching and their recovery capabilities following a bleaching event. Reef-building corals are not all equally susceptible to the in- fluence of increased temperature. Some coral species have been shown to sur- vive severe bleaching events (Glynn 1993; Brown 1997; Hoegh-Guldberg 1999; Wilkinson 2000; Loya et al. 2001), leading to major structural shifts in coral communities (Glynn 1993; Brown 1997; Ostrander et al. 2000; Loya et al. 2001). Once these communities have shifted, it would probably require a lengthy pe- riod of time to return to their original states (Hoegh-Guldberg 1999). The population of Oculina patagonica, an invading species to the Mediterra- nean Sea from the southwestern Atlantic Ocean, has been studied extensively during the last decade (Fine et al. 2001). O. patagonica was first recorded in the Mediterranean Sea in 1966 near Savona harbor (Gulf of Genova; Zibrowius 1974). In 1973, it was first noticed in southeastern Spain (Zibrowius and Ramos 1983; Zibrowius 1992). At present, it is known to inhabit many natural sites and harbors along hundreds of kilometers of coastline in southeastern Spain. Along the Mediterranean coast of Israel, O. patagonica was first re- corded in 1993 (Fine et al. 2001). The intrinsic and extrinsic factors shaping the current population of O. pata- gonica were studied, including the way that bleaching events affect its popula- tion dynamics through reduction of gametogenesis, regeneration, and com- petitive abilities. Bleaching in O. patagonica was first recorded in 1993 and explained by a bacterial infection, to which high water temperature was a con- tributing factor (Kushmaro et al. 1996, 1998). Since then, 70–90% of coral colo- nies in the study population have exhibited bleaching during the summer months, when water temperature reaches a maximum of 29–30 °C, and recov- ered in the following winter (Figs. 6.1, 6.2; Kushmaro et al. 1998; Fine et al. 2001). The studies investigating this phenomenon revealed that the causative 6