Cnidarians
Stony corals make up a large part of the cnidarian phylum. Being stationary, they are easy to spot and observe. Most of the time, it is easy to see the tiny living polyps that build a shared, hard skeleton. Over centuries, it is this very skeleton that becomes the foundation of the reef. These colonies construct a diverse array of structures, ranging from tree-like branches, leaves, plates, and cups, to massive, brain-like boulders.
Branching type: Staghorn coral - San Salvador Bahamas
Brain type: Grooved brain coral - San Salvador Bahamas
Brain type: Butterprint rose coral - San Salvador Bahamas
Encrusting style: Lobed start coral - Roatan Honduras
Leaf style: Lettuce coral - San Salvador Bahamas
Fleshy style: Knobby cactus coral - San Salvador Bahamas
Bipinatte sea plume - San Salvador Bahamas
Sea rods - San Salvador Bahamas
Common sea fan - Florida Keys USA
Gorgonians are another prominent group within the cnidarian phylum. Similar to stony corals, they are stationary and easy to observe as they sway gracefully in the ocean current, though they can often be confused with plants. Despite their plant-like appearance, it is easy to see their eight-tentacled living polyps. However, the skeleton they build is much more flexible, which is why they are often referred to as soft corals. Rather than forming the hard foundation of the reef, they attach to it, creating lush, three-dimensional underwater forests. These colonies grow into a beautiful structures, ranging from intricate, branching fans and feathery plumes, rods, to long, wire-like whips.
Stony Corals
Gorgonians
Anemones
Giant anemone - San Salvador Bahamas
Golden zoanthid on sponge - San Salvador Bahamas
Clownfish in an anemone - Indonesia - (Credit: Noemi Marz / Ocean Image Bank)
Other Cnidarians
Unbranched Hydroid - Roatan Honduras
Christmas Tree Hydroid - San Salvador Bahamas
Upside Jellyfish - mangroves - (Credit: Ron Watkins / Ocean Image Bank)
Sea anemones form another iconic group within the cnidarian phylum. While typically stationary and easy to spot, they are not permanently fixed and can actually slide slowly across the reef. Often mistaken for brightly colored underwater flowers, an anemone is actually a single, large living polyp rather than a colony. Because they do not build any skeleton at all, their soft, fleshy bodies anchor directly to the reef, displaying a vibrant crown of tentacles that famously shelters other marine life.
Hydroids and jellyfish complete the picture of the incredibly diverse cnidarian phylum. Hydroids are stationary colonies of tiny polyps that attach to the reef, often resembling delicate, feathery ferns. Jellyfish, on the other hand, represent a completely different lifestyle. While closely related to a single polyp, a jellyfish is actually known as a medusa—essentially an upside-down, solitary animal that pulses and drifts freely through the open ocean rather than anchoring to the reef.
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