Fire corals and hydroids are a large group of solitary and colonial animals which have a complex life cycle which often involves an alternation between sessile (fixed in one place) polyp phase and a free-swimming medusa stage, similar to the True Jellyfish. Fire corals are a common sight when diving Koh Lanta.
In some groups of fire corals and hydroids, either the polyp or the medusa stage may be missing. Some families are able to produce calcium carbonate skeletons and form colonies, similar to the hard corals, however despite exhibiting very similar characteristics, these are not true corals.
They capture their prey and ward off enemies using stinging cells called nematocysts. Nematocysts are tiny cellular structures consisting of coiled, hollow and usually barbed, thread-like stingers, which usually contain poison.
The nematocysts are located throughout the tentacles that radiate either from the exterior in the sessile (stationary, fixed in one place) stage, or downward from the edge of the umbrella dome in the medusa (jellyfish) stage.
The reef building 'fire coral' family Milleporidae family can quickly grow large skeletons covered with many surface pores which contain stinging nematocysts.