GAs of corals typically consist of circumscribed areas of tissue and skeleton that have grown at a faster rate than the surrounding tissue and skeleton. Affected areas have morphological features visible underwater that differ from normal. This includes changes in the shape and size of skeletal elements and polyps, discolored (lighter) tissue, and complete loss of polyps.
GAs are composed of coral tissue and supporting skeleton, but they differ markedly in morphology from the surrounding tissue and skeleton. Some characteristic features include aberrant corallite shape and coenosteal structures, presence and development of abnormally large polyps and skeletal elements, discolored tissue, and often more rapid growth than that of the surrounding polyps.
Acroporids often exhibit whitened protuberant masses of gastrovascular canals characterized by reduced to absent calices and polyps and loss of normal polyp structure. These growths have been termed hyperplasia;
In branching corals, GAs can appear anywhere on the colony, while plating corals typically have GAs only on the upper surfaces.
GAs in Acropora from the Indo-Pacific have been divided into 7 distinct morphological types: nodular, crateriform, exophytic, fimbriate, vermiform, annular, and bosselated.
In massive Porites, two types of GAs are described. Type 1 is the widespread, commonly seen type of massive GA with a raised appearance, usually distinctly demarcated with paler tissues than surrounding normal tissues. Type 2 GAs are typically very pale or white with occasional patches of pink pigmentation, a rough surface, and fewer polyps.
In imperforate species, such as Platygyra, three distinct morphological types of GAs have been reported:
Type I have a chaotic formation of corallites with thin septa and presence or absence of columella;
Type II are characterized by raised distinct mounds of normal corallites with absence of pigmentation;
Type III have corallites with thickened primary septae and very thin secondary septae.