For the Reefnet readers that are not familiar with the coral animal describe it and explain what happens when corals are touched, bumped or walked on. The coral animal has two thin layers of living tissue, the ectoderm and endoderm. They are simple organisms with few organs, primarily tentacles surrounding a mouth that leads into a blind sac or coelenteron. This colenteron is partitioned by membranes that increase surface area for digestion. The ectoderm layer secretes a calcium carbonate skeleton.
Corals deposit calcium carbonate underneath the layers of tissue in species-specific patterns.
Because coral tissue is only a few millimeters thick, any contact can abrade this tissue from the surface. A certain amount of abrasion is not catastrophic for corals because they are very plastic and can regenerate quickly. In the case of corals, bacteria and fungus can invade the abraded areas. In fact, I believe that diseases are often initiated by injury and that tumors can be triggered by an injury in which the healing process goes awry. If the injury is chronic, bacteria, fungi and algae may move in and begin to invade and weaken the coral.