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Not So Simple Sponges

What is 600,000,000 years old, has no nervous system, no body organs, no digestive system, no circulatory system, and lacks the ability to move about yet is an important, vibrant, colorful, living marine creature?  It’s the 10,000 or so known species of sponges!  Invertebrates, living in water from a few inches to over 5.5 miles in depth, most species of sponges are found in saltwater with a few, by comparison, living in freshwater. 

They can be found thriving under arctic ice and in tropical waters as well with the tropical species, generally, much more colorful.  Fossil records indicate sponges comprised some 50-60% of pre-historic reef life.  They are members of the kingdom Animalia, phylum Porifera from the Latin ‘porus’ meaning pores and ‘ferre’ meaning ‘to bear’…bearing pores.  Anyone who looks at a natural sponge knows it certainly bears pores, tens of thousands of ‘pores’ even in a very small piece of sponge.

Most sponges are filter feeders, taking water in, filtering out the bacteria on which they feed, and expelling the waste water.  Some species act as hosts to photosynthesizing organisms which produce more than enough food and oxygen for themselves and their sponge hosts.  And, there are a few species of sponges living in unfavorable environments that have become carnivores, feeding off small crustaceans that venture unaware on to the sponges’ surface.

There is current debate as to whether sponges have ‘tissue’ as we know it.  The outermost layer of a single sponge cell is known as the pinocoderm which is the equivalent of skin in mammalian species.  The next layer is known as the mesohyl, a jelly-like substances that contains amebocytes (free moving cells similar to white blood cells), fibrils (tiny fibers), and various skeletal elements depending upon the species of sponge.  It is the skeletal element component that gave taxonomists a means of classifying sponges.

The three main classifications of sponges are Calcarea, Demospongiae, and Hexactinellida.  Found within the mesohyl of the Calcarea sponges are spicules, supporting elements, composed of calcium carbonate silica.  The most common class of sponge, the Demospongiae, has mesohyl containing supporting spicules of both silica and spongin, a form of collagen.  The Hexactinellida sponges’ mesohyl contains spicules of silica and single cell like synctium filled with a cytoplasmic substance containing many nuclei.

The spicules supporting the Hexactinellida sponges, aka glass sponges, have a unique characteristic of great interest to scientists studying fiber optics.  These sponges can produce glass.  Whereas manmade glass requires high temperatures and the end product is brittle, glass sponge glass is created in cold water and it resists stress.  The glass sponges make glass by extracting silica acid from sea water and converting it into silica which then becomes glass skeletons.  The minute spicules in these glass skeletons not only display very intricate patterns but they are also less brittle than manmade glass, they are stronger, and they develop their own support systems compared to manmade glass which must have artificial support.  In addition, the ends of the spicules gather and focus light better than commercial fiber optic materials.  It is theorized that this light gathering/focusing ability evolved to provide light to symbiotic alga living within the glass sponges.

Most glass sponges live at great depths so few of us rarely have the opportunity to see them.  The Venus’ Flower Basket glass sponge holds a place in Asian culture where the gift of this sponge signifies a long and happy marriage.  Similar thoughts prevailed in Victorian England where a Venus’ Flower Basket could bring 500 pounds.  The belief attached to the Flower Basket arises from an unusual symbiotic relationship inside the sponges way beneath the surface.

Two tiny bioluminescent cleaner shrimp, one male, one female, live their entire lives inside the Venus’ Flower Basket sponge.  They are taken into the sponge as larva and once ‘hatched’ cannot escape.  The sponge provides them shelter and food, and the shrimp keep the sponge clean.  As the shrimp mature and propagate, their larva float out of the sponge and the cycle repeats itself. 

With little tissue binding individual sponge cells to one another, sponge colonies can be separated and, over time, will rejoin.  A species of freshwater sponge’s ability to repair was tested when scientists destroyed a small colony by forcing it through a fine-mesh sieve.  The individual cells eventually reunited and reformed their colony.

The sponges’ ability to survive is remarkable and reminds one of Star Wars and other sci-fi tales.  If water conditions become unfavorable to survival, some species of sponges produce ‘survival pods’ called gemmules.  The gemmules are composed of dormant, unspecialized cells that do not become active again until conditions improve.  Once revived, the cells in the survival pods either repopulate the skeletons of their parent colony or float away to begin a new colony.

The largest class of sponge, the Demospongiae, is the most commonly seen; the barrel sponges are a good example.  This class of sponge is currently under study for its propensity of harboring all manner of bacteria and micro-organisms as well as its ability to ‘signal’ colonization.  Scientists believe identifying the signaling mechanisms could have great benefit to developing new treatments in wound and surgical recovery.  Additionally, scientists have discovered bioactive compounds in sponges which show great promise in the treatments for AIDS and some cancers.

NOTE:  The photo accompanying this article is a section of a Venus’ Flower Basket showing the glass skeleton.  The sponge was torn from its deep hold along with thousands of other living ‘by catch’ creatures by a bottom trawler several years ago.  Although damaged, it was rescued from the pile of trawler refuse and given to me for use in educational programs.

After 30 years as a wild and domestic animal rescuer, rehabber, and educator in the states, Becky Bauer became a scuba instructor and award-winning journalist covering the marine environment in the Caribbean.  She is a contributing photographer to NOAA.

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