The sponges (Phylum Porifera) are among the most primitive all animals, lacking muscular, nervous, circulatory, or digestive systems of even the most rudimentary kind.
Sponges feed by filtering the surrounding seawater of extremely tiny organisms like bacteria and the smallest of plankton.
Although they may not seem to be doing much, large specimens actually pump surprisingly quantities of water, often hundreds of gallons each day, as they feed. It has been estimated that together, all of the sponges in the Caribbean Sea filter an amount of water about equal to the volume of the entire sea itself every single day
The Sponge Body Plan
The sponge body plan is simple, but elegantly designed for its way of life. The external surface of the sponge body is covered with tiny pores called ostia into which water is forced inward (but never outward).
Water drawn in through the pores eventually reaches one or more internal cavities lined with another type of specialized cells called collar cells.
These are the driving force behind the pumping mechanism. The water is then forced out through the large body cavity opening called the osculum.
In the process, the collar cells trap tiny bacteria and other small organisms and pass them to other specialized cells for digestion. The water flow created by the collar cells also brings fresh oxygen-rich water to the working body cells and removes carbon dioxide.
Some coral reef sponges are known to use "chemical warfare" to bore their way into living coral heads, thereby creating their own living spaces.
Distribution and Diversity
Within the coral reef biome, sponges occupy all three major coral reef zones, including seagrass meadows and mangroves of the lagoon as well as upon and between the massive hard coral colonies of the reef crest and fore reef.
Around 2,000 sponge species are known from Indo-Pacific coral reefs alone. Many species are highly variable in both shape and color, depending upon the environmental conditions to which they are exposed. Thus, identification of these animals or determining their true diversity within a given area is a daunting task even to seasoned professionals.
Common Types of Coral Reef Sponges
Coral reef sponges are commonly described in terms of large-scale body form, although these forms do not represent formal taxonomic categories. Three types are common: vase sponges, tube sponges, and encrusting sponges.
Vase sponges rise from the reef substrate as spheroid structures.
The central cavities of larger species form a habitat in themselves, often housing a variety of coral reef animals including reef fishes as well as invertebrate animals such as shrimps, crabs, and others.
The largest vase sponges are usually found in deep water on the lower fore reef.
Some of the largest types of vase sponges have voluminous central cavities - big enough to easily contain a fully-equipped scuba diver.
Tube sponges exhibit a branched body form that grows from the reef as a closely aligned series of vertical columns.
Some are quite large, while others are fairly small. Tube sponges are common in both shallow and deeper portions of coral reef ecosystems.
Tube sponges are among the most colorful of all coral reef invertebrate animals. Many species display brilliant yellow, orange, or reddish hues.
As with many coral reef creatures, it is only with underwater lights or flash photography that one can truly appreciate the actual coloration of these animals.
As the name implies, encrusting sponges form a comparatively thin but often expansive layer atop reef substrates.
Like tube sponges, these forms are often brightly colored, displaying a variety of hues in the yellow to red range. However, the vividness of these colors is often masked by the rapid attenuation of this portion of the color spectrum with increasing water depth.
Encrusting sponges are common throughout the entire depth range of hard coral growth, from shallow lagoon patch reefs to the deepest portion of the reef face. They often cover reef surfaces in crevices between coral colonies.