Dive Dry with Dr. Bill

#517: The Sulphur Sponge

I've never watched SpongeBob SquarePants and neither does Allison, my granddaughter. Heck, I don't even have TV reception at the house. I can't comment on the show's scientific accuracy or educational value. But I do remember the days when bath sponges were real sponges... the mass formed by the spongin fibers that made up the sponge's soft "skeleton." Nowadays they are mostly fake, made of cellulose fibers. That's a good thing because sponges are important in their natural habitat as you shall see.

Now sponges in southern California are not very obvious. Divers swim right past them without paying the least bit of attention. It's quite different in the tropics where sponges can be large enough to accommodate a diver's body within their barrel. Don't worry, they don't feed on humans. I'm looking forward to heading to the tropics, most likely the Philippines, this winter to film these giants to add to my stock footage from around the globe (and ensure I don't get hypothermia from diving here!).

But for now, I'm satisfied with our mostly diminutive species. As a marine ecologist, I try to understand the role of every critter in our kelp forest communities. That's the only way to try to put together the puzzle that constitutes a natural ecosystem... learn the individual "pieces" and how they interlink. In today's column I thought I'd write about one of our local members of the phylum Porifera, the sulphur sponge (Aplysina fistularis).

It is difficult to mistake this sponge for any other as long as you are diving reasonably shallow. The sulphur sponge's bright yellow color is usually a dead give-away. However sometimes I note algae growing on the exterior which may give them a greenish cast, or they may be brownish. The other yellow sponges I see in our waters are generally at deeper depths, often below 100 ft. This sponge's yellow color apparently comes from special fluorescent pigments.

When I first arrived on Catalina, I assumed the sulphur sponge was strictly a resident of our region. Back then it was known as Verongia aurea, and earlier as Verongia thione. Later, a biologist assigned it to the species name it currently is known by. The interesting thing is that Aplysina fistularis is a sponge found in other places around the globe such as the Caribbean and Gulf of Mexico!

In these other regions it has a different growth habit. It forms tall and cylindrical masses, and is known as the yellow tube sponge. Here it is essentially a low growing encrusting form. Some scientists feel these differences are not sufficiently genetic to make the two types different species. They feel they are simply different expressions of essentially the same genes due to environmental factors. High falutin' marine biologists like myself would refer to the two forms as ecophenotypes.

Sponges are really associations of different types of cells, each type specialized to carry out a specific function. They are among the simplest of animals, having no organs or true systems such as our nervous or digestive ones. Important functions including digestion, excretion and reproduction are carried out by the individual cells.

Some sponges use loosely arranged components to create a "skeleton" and make the organism rigid. Some use calcium carbon like that in clam shells or coral reefs and these are often encrusting forms that grow close to the substrate. Others have pointed structures of silicon dioxide (essentially the same as sand or glass) known as spicules that interlock to give them structure. The sulphur sponge is in a group that uses a fibrous material known as spongin to create its "backbone." This group is known as the demospongia.

Sponges are filter feeders. They draw water into their "bodies" using cells known as collar cells with flagella that whip the water into and through hollow canals. The individual cells filter out munchables from the water such as plankton, bacteria and organic matter. The waste water is then expelled through the larger opening in the sponge's mass. The sulphur sponge in turn provides food for an interesting relative of the nudibranchs and sea hares known as the yellow umbrella slug (Tylodina fungina). The yellow color of the body comes from the sponge it eats, and there is a brownish think shell on top that looks like a Chinese coolie's cap.

This sponge and many other species are of interest to medical researchers and pharmaceutical companies. During digestion of their food, these sponges produce metabolites, intermediate or end products of their cellular processes, which are used in their defense against predators, bacterial infections and to prevent other organisms from settling on them and growing over them. The sponges which use spongin for structure are softer and more palatable to predators than those which use calcium carbonate or silicon dioxide. Therefore they tend to produce more defensive chemicals, some of which could lead to medical breakthroughs for humans.

© 2012 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Local growth form of the sulphur sponge and that found in tropical areas (courtesy of Ryan Photographic);
microscopic view of spongin fibers that create its "skeleton" and Tylodina fungina that feeds on it.

This document maintained by Dr. Bill Bushing.
Material and images © 2012 Star Thrower Educational Multimedia