Sponge Zoanthids

Are coral zoanthid polyps harmful to sponges?

Divers have long been told that sponge proliferation is a sign of  reef  decline.  Most of us enjoy seeing the beautiful colors & textures of sponges on the reef and revel in the critters that hide within them.  But what happens when sponges are on corals or vice versa?

While diving in the Caymans recently, many of us commented  on the beautiful and unusually vibrant colors of the sponges there. The sponges are prevalent and quite large in many dive sites.  They make great hiding places for sea creatures. And great diver portraits. (wink)...

While taking macro photos of a beautiful pink Vase Sponge, I noticed yellow polyps between the sponge matrix (see title photo).  This was something new for me, and you know how one thing leads to another? 
First, I needed to refresh my knowledge about these animals.

Corals vs Sponges

Sponges are totally separate from corals as animals.  Corals are in the Phylum Cnidaria, and Sponges are in Porifera.

Corals are actually individual animals (polyps) that resemble an anemone, living together in colonies. They are multi-cellular and fairly complex.  They have stinging nematocysts that can catch & eat fish and plankton.  But they also produce energy through specialized algae cells (zooxanthellae) via photosynthesis. As such they also need light to survive. When stressed, coral animals will eject their algae cells…and die.

Sponges do not have distinct digestive or circulatory organs.  They are filter feeders, their bodies are shaped to maximize water flow - they absorb nutrients as water passes through their jelly like central mesohyl cells. They are totally reliant on water circulation to deliver their nutrients and take away waste - and do an important job on the reef of cleansing nitrogens from the water.

So far, so good.  Now, how do they work together or do they?

Corals need sponges. But not too many.  

At our dive club meetings we’ve learned about sponges’ roles absorbing nitrogen on the reef (“Fish Pee in the Sea”). Sponges are a food source for angelfish. Another marine biologist presented his findings on how reefs with few angelfish are symptomatic of coral reef decline - eg. when angelfish are harvested, sponges proliferate, and displace corals.  Many of us have heard that too many sponges are bad news for coral.

But sponges also play a role in helping Corals - by underpinning coral structures which helps protect the coral colony (see photo left).

Can coral can grow on sponges?

Yes!  What I found on that Vase Sponge were polyps of a specific type of coral called Umimayanthus parasiticus.  (See macro below).

An ominous name, that implies parasitic behavior.  But is it?
This coral polyp is found in the Caribbean, Gulf of Mexico, Bermuda, Bahamas between 25 - 9o feet. It grows only on certain host sponges, such as the Vase Sponge, the Burrowing Sponge and Stinker Sponge (Spheciospongia), & Cliona sponges. The polyps attach themselves between the sponge matrixes and they benefit from the flow of water through the sponge.  

As they multiply (via spawning which happens generally in September on a full moon) - it seems reasonable that they might inhibit the flow of water that both the polyp and the sponge need.  But scientists recently are studying this animal in more depth and some are finding that both animals seem to thrive - implying some kind of mutualistic benefit.

So next time you see a Vase or Stinker Sponge in the Caribbean - slow down and check it out!  By diving slowly and being observant, we might learn something new or even find solutions to reef decline sitting right in front of us.