Superficially, sponges and coral have a lot in common. They both just sit there underwater, filtering food particles, living in large colonies that provide habitats for other animals. Divers know that they come in many beautiful colors. But below the surface, these two organisms are completely different.
Sponges and coral come from different animal phyla, for one. Phyla is the most basic of all animal distinctions, and refers to animals with completely different body plans. Sponges are among the most simple of all animals, lacking true tissues, and deterring predators primarily by their lack of nutrition and glass-like shards found in their bodies called spicules. They are covered in little pores lined with cells equipped with flagella, which are both used to circulate water through the sponge and absorb food particles. Sponges are capable of living at any point in the ocean, from just off the shore to 8,500 m (29,000 ft) deep, or more. Sponges are members of phylum Porifera, and their alternate name is "poriferans."
Corals are cnidarians, related to jellyfish and anemones. More complex than sponges, they have differentiated tissue and a true gut. Corals look like single individuals, but they are in fact huge colonies composed of numerous genetically identical polyps just a few millimeters in diameter. These polyps have stinging tentacles, which is characteristic of cnidarians. Instead of depending on food particles for food, corals get most of their nutrition from symbiotic algae, which gives them their color. Corals cannot live as deep as sponges, most being found in the photic zone, where light can reach their algae, but some species are found at depths of 3000 m (9,842 ft).
Sponges and coral are both members of very early lineages that probably split off from other animals as early as 600 million years ago. For a very long time, it was thought that sponges were the most basal of sponges and coral, but recent genetic studies have indicated that the ancestors of coral, early cnidarians, actually split off from other animals first, and that sponges are likely from a lineage that was secondarily simplified.
