The Ecology and Classification of Sponges

Investigating the Diversity and Taxonomy of Phylum Porifera

Sep 22, 2009

Sponges come in an incredible array of colors and an amazing array of body shapes. Their simple structure belies their wide spread distribution.

The variety and diversity of living sponge species is truly amazing. Some are round, others are flat and crusty, and some vase-like in shape. Some are gray or drab brown, but many are brilliant colors of red, orange, yellow, blue, violet, and even black.

Many species harbor symbiotic bacteria or unicellular photosynthetic protists that can color their construction.

Sponges range in size from a few centimeters (an inch or so) in diameter to giants more than 2 meters (7 feet) in height or diameter. Such gigantic sponges may be hundreds of years old.

The Ecology of Sponges

Sponges are benthic (bottom-dwelling) creatures and can be found living from the intertidal zone at the ocean’s edge to abyssal depths miles down. Sponges are worldwide in their distribution from the poles to the tropics and in some areas they literally blanket the sea floor.

Sponges prefer to inhabit quiet waters free of pore-clogging sediments that would make it difficult for them to feed and breathe. Most sponges anchor themselves to a firm substrate, such as rocks, where they grow as a thick film.

The greatest ecological role sponges play in their natural habitat is to provide a home for a wide variety of creatures both microscopic and macroscopic. The porous nature of sponges makes them ideally suited as biological boardinghouses for habitation by a host of smaller invertebrates and often by fishes such as gobies and blennies.

Ecologist A. S. Pearse once carefully picked apart a loggerhead sponge (Spheciospongia vesparium) the size of a kitchen sink with a total volume roughly equivalent to that of a 55-gallon drum and counted and classified all the creatures inside it. He recorded 17,128 total animals. Around 16,000 were alphaeid shrimp, but many other kinds of animals were found, including several fish up to 13 cm (5 inches) long.

Lobsters, crabs, shrimps, fish, and brittle stars retreat to into large sponges using their many openings and folds as a refuge when danger or daylight threaten. Zoanthids – small anemonelike animals resembling yellow daisies – often crowd together on the outsides of sponges as do some tubeworms in their thin and brittle parchment-like tubes.

Most residents use their host only for space and protection, but some rely on the sponge’s water current for a supply of suspended food particles. A classic example is the bond between a male-female pair of shrimp (Spongicola) and the hexactinellid sponge known as Venus’s flower basket (Euplectella).

The shrimp enter the sponge when young and small, only to become trapped in the glasslike case of their host as they grow too large to leave (prisoners of love?). Appropriately, this sponge with its mummified guests is a traditional wedding present in Japan – a symbol of the lifetime bond between two partners.

The Taxonomy (Systematics) of Phylum Porifera

The approximately 9,000 total species of extant (living) sponges are entirely marine save one family, Spongillidae, a group of fresh-water demosponges. The freshwater sponges number about 150 species, with approximately 27 species found in the lakes and streams of North America. Terrestrial sponges do not and cannot exist.

Recent molecular analysis of sponge phylogeny suggests that the traditional phylum designation of Porifera should be eliminated. In its place the class Calcarea should be delegated as a separate phylum with the other sponges placed in phylum Silicarea.

However, such taxonomic changes have yet to become widely accepted and prevalent in the literature (and textbooks), thus the more traditional classification of sponges is presented here.

Taxonomists carve two subkingdoms from the larger animal kingdom: Parazoa and Eumetazoa. The Parazoa (Gr. para, alongside + zoa, animals) are not bilaterally symmetrical and lack tissues and organs while the Eumetazoa (Gr. eu, true + meta, later + zoa, animals) are bilaterally symmetrical and do possess tissues and organs.

Sponges are the only living parazoans on the planet, all other animal types are eumetaozoans.

Although so simple in structure that they don’t even possess a true body, sponges may be the most widely distributed animals on the planet.

The copyright of the article The Ecology and Classification of Sponges in Zoology is owned by Dennis Holley. Permission to republish The Ecology and Classification of Sponges in print or online must be granted by the author in writing.