Water that is
too
warm is also bad forcorals. The upper temperature limit varies, but is usually around 30°to 35°C(86°to 95°F). The first outward sign of heat stress, or stress of other kinds, is
bleaching,
in which the coral expels itszooxanthellae (see Fig. 18.3). It is calledbleaching because the golden-brown orgreenish zooxanthellae give the coral mostof its color; without them, the coral is al-most white. Corals also slough off largeamounts of slimy mucus when stressed. If the warm conditions last too long or thetemperature gets too high the coral dies