In all their variety and abundance, coral reef sponges add an extra dimension of structural complexity and biodiversity to coral reef habitats.
The sponges (Phylum Porifera) are 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. The sponge body plan is simple, but elegantly designed for this purpose.
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.
Although sponges may not seem to be doing much, large specimens actually pump surprisingly quantities of water, often hundreds of gallons each day, as they feed. In fact, 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
Some coral reef sponges are known to use “chemical warfare” to bore their way into living coral heads, thereby creating their own living spaces.
Seagrasses are most often found in shallow, sheltered marine or estuarine waters. They are the only type of true plant to successfully colonize the sub-tidal sea.
The distribution of seagrasses within coral reef systems is restricted to the protected waters of the lagoon (back reef). Here, these plants often form dense and extensive seagrass meadows – a distinctive habitat-type within coral reef ecosystems.
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 reef face. 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, and 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.
In coral reef ecosystems, sponges are widely distributed throughout all three primary coral reef zones. They inhabit back reef seagrass meadows, encrust mangrove roots, and grow upon and between hard coral colonies.
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.
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