Hydroids, close cousins of the soft corals, are widespread reef inhabitants. Like hard corals, they build stony skeletons and live in colonies. Hydroids also feed very much like corals, depending on their zooxanthellae as their primary sources of nutrition. In addition, hydroids are suspension feeders as well as carnivores that capture their prey with stinging tentacles.
Several characteristics distinguish hydroids from hard corals. Unlike corals, hydroids experience both a polyp and a medusa stage of life. Plus, individuals in hydrozoan colonies specialize and divide the labor needed to support them, with some individuals taking charge of reproduction while others are involved in feeding, defense, and other functions.
Hydroids form three kinds of colonies: plantlike, coral-like, and jellyfish-like. The plant- and coral-like species can often be found on reefs. The plantlike colonies resemble slender stalks topped with multiple branches. One example, Stylaster elegans, grows to about 9 inches (22.75 cm) in length and is commonly found under coral overhangs.
The coral-like species produce colonies that are covered with skeletons of calcium carbonate. Their skeletons are so similar to hard corals in appearance that they have earned the nickname “false corals”; however, the two types are significantly different. Instead of growing in cup shapes, as true corals do, false coral form smooth skeletons that are dotted with tiny holes where the polyps are housed. Their ability to form skeletons makes coral-like hydroids important reef builders. In many systems they contribute a significant portion of limestone to the reef structure.
One group of false corals is well known to scuba drivers. The fire corals (Millipora) can deliver a painful sting to a fish or any other animal that touches them. Skeletons of Millipora may be purple, yellow, brown, or green, and they have white edges or tips on them. As in all false corals, the pores in the skeletons of Millipora house two types of polyps, feeding polyps and stinging polyps. Feeding polyps remain safely retracted within the skeleton until a meal is near; however, stinging polyps, which look like fine hairs waving in the water, are extended much of the time. These nematocyst-laden tentacles defend the colony, and are always poised and ready.