to the algae's photosynthetic activity the internal pH of the coral can be raised; this behavior indicates that the zooxanthellae are responsible to some extent for the metabolism of their host corals. Stony Coral Tissue Loss Disease has been associated with the breakdown of host-zooxanthellae physiology. Moreover, Vibrio bacterium are known to have virulence traits used for host coral tissue damage and photoinhibition of algal symbionts.Therefore, both coral and their symbiotic microorganisms could have evolved to harbour traits resistant to disease and transmission. Reproduction Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas. Reproduction is coordinated by chemical communication. Sexual Corals predominantly reproduce sexually. About 25% of hermatypic corals (reef building stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic. It is estimated more than 67% of coral are simultaneous hermaphrodites. Broadcasters About 75% of all hermatypic corals "broadcast spawn" by releasing gametes—eggs and sperm—into the water to spread offspring. A genetic ability to detect blue light allows these sightless organisms to detect a full moon, triggering the simultaneous release. The gametes fertilize at the water's surface to form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony forms several thousand larvae per year to overcome the odds against formation of a new colony. Synchronous spawning is very typical on the coral reef, and often, even when multiple species are present, all corals spawn on the same night. This synchrony is essential so male and female gametes can meet. Corals rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve temperature change, lunar cycle, day length, and possibly chemical signalling. Synchronous spawning may form hybrids and is perhaps involved in coral speciation. The immediate cue is most often sunset, which cues the release. The spawning event can be visually dramatic, clouding the usually clear water with gametes. Brooders Brooding species are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, sinking onto the waiting egg carriers that harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occur even with these species. After fertilization, the corals release planula that are ready to settle. Planulae The time from spawning to larval