Star Thrower Educational Multimedia (S.T.E.M.) Dive Dry Column

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Dive Dry with Dr. Bill

124: Feathers That Sting, not Tickle

It's time I switched from writing about great white sharks to a "dangerous" marine critter that most divers and snorkelers actually might encounter in Catalina's waters. One of the hidden "hazards" of diving here are very small relatives of the corals and jellyfish (oops, sea jellies) known as the white feather hydroid and the ostrich plume hydroid. Like their other "kin," these small colonies consist of numerous individuals each possessing many stinging cells known as nematocysts. Even though some have called me very thick skinned, their tiny "arrows" seem to penetrate me quite easily. If I land on one while positioning to get a picture, I usually end up on top of dozens or even hundreds of these colonies each with numerous individuals! Ouch! Although our water temperatures down to depths of 100 feet or more have been a balmy 59-61 degrees this winter, one of the reasons I wear a full length wet suit is to avoid their stings.

I thought this week's subject would be an easy column to research. Boy was I wrong. I found almost nothing in my biological reference books or on the Internet on the white feather hydroid! Very surprising for a species that is so common here. These hydroid colonies do look just like feathers. In fact, I once mistook a seagull feather for one. Fortunately there was much more on their larger relatives, the ostrich plume hydroids.

These hydroid colonies are usually found in clusters. Each colony has a central stalk with lateral branches extending on either side of it. There may be a dozen or more small feeding individuals on each side branch. They are protected by a cup-like exoskeleton into which they can withdraw the tiny tentacles. The arrangement of hundreds of individual feeding polyps makes them quite efficient at filtering food, including plankton and organic matter, from the water.

Politically, these hydroids are neither Democrat nor Republican, but lower case communists. All the feeding individuals are connected through a common digestive system. That way even the laziest polyp benefits from food caught by the rest. In the case of hydroids, this is important. Associated with the feeding individuals are smaller, mouthless (and therefore non-feeding) polyps armed with the stinging cells or nematocysts. Since they are the defenders, they are fed by the rest of the colony. As for the truly "lazy" polyps, they are tolerated (and fed) if such exist.

If one looks closely at these feathery "plumes," you may see larger structures known as corbulae which contain the sexually reproducing medusae. This phase of the hydroid would look like tiny jellyfish if they were free swimming as in some hydroids. Each corbula contains only one sex. The male medusae release sperm into the water, while eggs from the females are held within the corbulae. Here they develop into tiny worm like larvae which crawl along the bottom for a while before producing a new colony. The fact that these larvae have a brief free crawling stage means that new colonies are often established near the "parent" colony.

The power of the nematocysts on these tiny colonies is astounding. However, while this impressive defense may be enough to keep "the doctor" (me) away, it doesn't seem to bother some of the predators that feed on the tender little polyps. Such predators include sea spiders and skeleton shrimp, which can often be seen moving and feeding among the colonies branches. Some nudibranchs also find these hydroids quite tasty. They may incorporate the untriggered stinging cells into their own bodies which helps them defend themselves.

Top two rows show white feather hydroid and colony structure (colony length under 2");
bottom row shows reproductive corbulae on ostrich plume hydroid.