Sea Sponges & Cnidarians
Among Earth's oldest and simplest animals, sea sponges have existed for over 500 million years. Uniquely, they completely lack brains, muscles, nervous systems, and internal organs.
As sessile organisms, they anchor permanently to surfaces like rocks or reefs, relying entirely on water currents. They function as efficient filter feeders by pumping water through microscopic pores called ostia. Inside, specialized cells called choanocytes capture food particles like bacteria and plankton before the filtered water exits through a large top opening known as the osculum.
Black encrusting octopus sponge - San Salvador Bahamas
Giant barrel sponge - Roathan Honduras
Purple rope sponge - San Salvador Bahamas
Branching sponge - Roatan Honduras
Branching vase sponge - Florida Keys USA
High-veined encrusting sponge - Roatan Honduras
Blue vase sponge - Roatan Honduras
Orange encrusting sponge - Florida Keys USA
Giant barrel sponge spawning - San Salvador Bahamas
Most sponges are hermaphrodites, meaning a single sponge can produce both eggs and sperm. Since they can't swim to find a mate, they rely on the ocean currents in a process called broadcast spawning. A "male" sponge releases massive clouds of sperm out through its osculum into the surrounding water. The ocean current carries the sperm to neighboring sponges. When a "female" sponge pulls in this water to feed, her specialized collar cells catch the sperm just like they would catch food. The fertilized egg develops into a tiny, free-swimming larva. It is released into the water, swims around for a few days, and eventually glues itself to a hard surface to grow into a permanent, stationary adult.
A sea sponge's lifespan varies widely depending on species, growth rate, and environment. Shallow-water sponges, living in warm, dynamic reefs, typically survive a few years to a few decades. Deep-sea sponges, in cold, stable depths with few predators, grow slowly and live exceptionally long.
Sea sponges are vital to marine ecosystems, especially coral reefs, by filtering thousands of liters of water daily, removing bacteria, organic matter, and pollutants. They capture inaccessible nutrients and convert them into forms usable by other organisms. They provide habitat in their hollow tubes and intricate structures offering ideal shelter, breeding grounds, and homes for diverse small marine species.
Sea Sponges
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.
Cnidarians
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.
Stony Corals
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