Bubble coral

 
During the night, the bubble coral (bub-ble-cor-ull) sometimes withdraws all its tentacles, as well as its bubbles, revealing its hard skeleton.  Bubble corals are characterized by having large water filled bubbles (vesicles) covering large sharp sepia.  The vesicles are expanded during the day and look like cream colored grapes.  During the night the vesicles are deflated and tentacles become apparent.  In the Pacific Ocean and parts of the Red Sea, bubble coral can be found in varying species, colors and forms.  They maintain their egg-like appearance during the sunlit hours (maybe an egg-like disguise?), then deflate at dark, manifesting finger-tentacles that feed on plankton, etc.
 
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