Hi-Tech Sponges
by Chuck Louch, PTMSC Docent
Despite their seemingly "primitive" organization, sponges have gone hi-tech. The fact that they are sessile and show little movement led early observers such as Aristotle and Pliny to consider them as plants and therefore primitive. Indeed, it was not until 1765, when internal water currents were first observed in sponges, that their animal nature was clearly established. The level of organization of sponges is, from our human perspective, very simple since they have no definite organs, no circulatory systems, and no definable nervous systems. Because of this seeming simplicity they are sometimes said to be at the "tissue level" of development, one step above single celled or colonial organisms, and yet they carry on all the functions of life very successfully. 
All sponges have internal skeletons. In the anatomically simplest class, the Calcarea, this is made up of small calcium carbonate bodies called spicules that come in a great variety of weird shapes as shown by the accompanying diagram. The largest and most complex of the sponges, the Demospongea, have skeletons made up of a fibrous, proteinaceous material called spongin. The once common bath sponge is really the skeleton of a member of this class. But the most fantastical skeletons are found in sponges of the Class Hexactinellida, the "glass sponges". As the name implies, their skeletons are made up of mostly six-parted spicules composed of silicon dioxide, the main component of glass. So far as I know, they are the only siliceous skeletons in the Animal Kingdom. Undoubtedly the most familiar member of this class is the Venus Flower Basket whose skeleton appears to be a vase made up of glass filaments woven together in an intricate pattern.
Most glass sponges are found in deep water and they are the dominant sponges in the Antarctic. There they extract silicon dioxide from seawater and use it to form the skeletal matrices of their bodies. But at least one species, Rossella racovitzae, goes one step further; it produces spicules that project out a finger-length or two from the body. Each of these is made up of a star-shaped cap at the distal end and a long, slender filament that penetrates into the interior of the sponge. And here's the interesting thing; the filaments are able to conduct light even when curved just like an optic fiber manufactured by an engineer.
The ability of a glass filament or fiber to conduct light depends on its refractive index relative to that of a surrounding transparent medium called a "cladding". The refractive index of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Commercial fiber optics are made up of a narrow glass core with a relatively high refractive index (light travels slowly) surrounded by a glass cladding with a lower index (light travels faster). In this system, light which enters the core, ricochets off the core-cladding interphace and so may be conducted by the core over long distances. The same system works in the sponge spicules where the cladding is water. Spicules have a refractive index of 1.49, which is very similar to that of the cores of commercial optic fibers, while the refractive index of seawater is 1.37.
The greater the difference in refractive index between a fiber's core and its cladding the larger the angle at which light can be captured and transmitted. Experiments show that the cut and polished ends of spicules can trap light at angles of up to 52 degrees from straight on so they are very efficient light gatherers. This light gathering ability is possibly enhanced by the four-pointed cap at the apex of each spicule. 
And now the big question: what can be the function of these light gathering and conducting structures? The most obvious idea is that they transmit light to symbiotic green algae for their use in photosynthesis and, indeed, this might be the case in Tethya seychellensis, a tropical sponge where green algae curl in strands around spicules, which radiate out from the interior to the surface. Although they found no green algae in R. racovitzae, investigators did find populations of diatoms that tended to collect along the length of the spicules. So, although more research needs to be done, this symbiotic relation between sponges and photosynthetic partners might be the answer to our question.
In any case, this is another example of how Nature has foreseen the engineering achievements of humans; birds fly, hummingbirds hover, fishes swim efficiently, and now the lowly sponge conducts light with a fiber optic system. What's next?
Illustrations by Chuck Louch.
References:
Peter Weiss: Soaking Up Rays. Science News, 160, 77, Aug. 4, 2001.
Robert D. Barnes: Invertebrate Zoology, 5th ed. Saunders 1974.