Soft coral – Most corals secrete limestone which forms a hard skeleton, but "soft" corals like this one do not. Although they don't help to build a permanent coral reef, soft corals provide food and shelter for lots of organisms.
Basket star – By day, basket stars coil their long arms and hide in small nooks and crannies on the reef. At night they feed, unfurling their arms and capturing small particles with their "branchlets."
Crown-of-thorns sea star – This large sea star, named for its sharp, spiky skin, feeds on living coral tissue and can cause severe damage to coral reefs. The triton shell -- a very large sea snail -- is one of its few natural predators. But triton shell populations have been badly depleted in many parts of the world because they are collected for use as food and ornaments.
Cuttlefish – Despite its name, the cuttlefish is not a fish -- it's a cephalopod, and closely related to octopuses and squids. Considered one of the most intelligent non-vertebrate animals, cuttlefish hunt prey on the reef, mostly by night. Masters of camouflage, they can change color patterns almost instantly. They use color changes to catch prey, avoid being eaten by predators, and communicate -- both with other cuttlefish and other species.
Sponge – Despite their outward appearance, sponges are animals. They draw water in through small pores (visible in this close-up photo) to capture floating food particles. Sponges tend to live in mutually beneficial relationships with other species -- crabs, shrimps, barnacles and brittle stars spend their entire lives on or inside the host sponge. Many sponges also harbor a vast diversity of microbial life -- in some, the biomass of microbes can outweigh the biomass of the sponge itself.
Sponge on a shell – The exterior of this shell has been covered with a red encrusting sponge, while the interior has been filled with small anemones or "mushroom" corals. Vacant space is hard to come by on a coral reef and competition is intense -- every spare surface is colonized quickly.