Abstract:Coral-inhabiting gall crabs are either obligate symbionts or parasitic associates with their host corals. They form a variety of galls/pits inside the skeleton of living corals. Nine genera of gall crabs on several scleractinian corals were used to test the hypothesis that galls vary among cryptochirid genera. Phylogenetic and morphometric observations were combined to analyze the possible evolutionary significance of gall construction. A high degree of conservation of gall shapes was observed in relation to the gall crabs' phylogeny. Gall/pit morphology and fidelity was studied in each of the different species of gall crabs. In addition, the correlation analysis results showed a significant linear relationship between crab size (carapace width) and its gall/pit size (opening diameter of gall/pit) (p < 0.001), demonstrating that crabs have the ability to create the gall/pit size which suits to their own size. Phylogenetically related crab species exhibit similar gall shapes; thus, the galls/pit morphology can be considered as an extention of the crabs' phenotypes.