Dear Sammie, The largest clams in the world are members of the tropical genus Tridacna. The sea urchins themselves are not very dangerous. They have spines which can hurt you, but the major problem is with the bacteria that coat the spines. If you ever get a sea urchin spine puncture, be sure to use some type of antiseptic to clean out the wound. An interesting question.
Corals are the skeletons of living animals.
They are made by cnidarians that look like sea anemones, but they secrete a skeleton either externally or internally. Thus, all the coral reefs that you hear or see are the leftovers of animals. The reefs get larger and larger as animals die and new animals grow on top of the old layers. Most rocks and other large structures get to the bottom from the bottom. There are active geologic sites known as ridges where plates of the earth's crust are moving apart and new material is made.