Mussids are
hermatypic or
reef-building corals and can be either solitary or colonial. They are generally massive corals with robust, dense skeletons. The
corallites (stony cups secreted by the
polyps in which they sit) are large, with the
septa (stony ridges) decorated by long teeth. The polyps are large and fleshy, and in certain species, the body cavity becomes inflated with water during the day, partially revealing the underlying skeleton. All species are
zooxanthellate, that is, they contain
symbiotic, single-celled
photosynthetic dinoflagellates that live in the tissues and provide the coral with nutrients produced by photosynthesis during the day. At night, the tentacles of the polyps expand and capture
zooplankton. Budding in mussids is always intracalicular, that is to say occurring inside the oral disc of the polyp, within the whorl of tentacles. The
corallites are either separate, or arranged in series, and when the
coenosteum is present, it extends beyond the wall of the septa ("costate"). The septal teeth are pointed and even, either arranged transversely to the plane of the septa or in random directions. With the exception of
Scolymia lacera, the teeth are the same size in each cycle of septa. The
columella is "trabecular", in that it lacks discrete individual corallites, because the budding of new polyps within the tentacles on the oral disc results in a series of mouths surrounded by a continuous whorl of tentacles; such intramural budding results in "trabecular" mouths, which are not individually encircled by tentacles. The resulting corallites that merge form the meandering valleys between costate septa typical of brain corals. ==In aquaria==