Sea Sponges Facts

When you look at a sponge, the word "animal" might not be the first that comes to mind, but sea sponges are animals. There are over 6,000 species of sponges; most live in the marine environment, although there are also freshwater sponges. Natural sponges have been used by humans to clean and bathe with for at least 3,000 years.

Sponges are classified in the phylum Porifera. The word 'Porifera' comes from the Latin words 'porus' (pore) and 'ferre' (bear), meaning 'pore-bearer.' This is a reference to the numerous pores or holes on a sponge's surface. It is through these pores that the sponge draws in water from which it feeds.

Description

Sponges come in a wide variety of colors, shapes, and sizes. Some, like the liver sponge, look like a low-lying crust on a rock, while others can be taller than humans. Some sponges are in the form of encrustations or masses, some are branched, and some look like tall vases.

Sponges are relatively simple multi-celled animals. They do not have tissues or organs like some animals do; rather, they have specialized cells to perform necessary functions. These cells each have a job. Some are in charge of digestion, some reproduction, some bringing in water so the sponge can filter feed, and some are used for getting rid of wastes.

The skeleton of a sponge is formed from spicules which are made of silica (a glass-like material) or calcareous (calcium or calcium carbonate) materials, and spongin, a protein that supports the spicules. Sponge species may be most readily identified by examining their spicules under a microscope. Sponges do not have a nervous system, so they don't move when touched.

Species

There are an enormous number of species in the phylum Porifera, broken into five classes:

• Calcarea (Calcareous sponges)
• Demospongiae (Horny sponges)
• Hexactinellida (Glass sponges)
• Homoscleromorpha (Includes about 100 species of encrusting sponges)
• Porifera incertae sedis (Sponges whose classification has not yet been defined)

There are over 6,000 formally described sponge species, measuring from under a half inch to 11 feet. The largest sponge discovered to date was found in Hawaii in 2015, and has not yet been named.

Habitat and Distribution

Sponges are found on the ocean floor or attached to substrates such as rocks, coral, shells, and marine organisms. Sponges range in habitat from shallow intertidal areas and coral reefs to the deep sea. They are found in oceans and freshwater lakes throughout the world.

Diet and Behavior

Most sponges feed on bacteria and organic matter by drawing water in through pores called ostia (singular: ostium), which are openings through which water enters the body. Lining the channels in these pores are collar cells. The collars of these cells surround a hair-like structure called a flagellum. The flagella beat to create water currents.

Most sponges also feed on small organisms that come in with the water. There are also a few species of carnivorous sponges that feed by using their spicules to capture prey such as small crustaceans. Water and wastes are circulated out of the body by pores called oscula (singular: osculum).

Reproduction and Offspring

Sponges reproduce both sexually and asexually. Sexual reproduction occurs through the production of egg and sperm. In some species, these gametes are from the same individual; in others, separate individuals produce eggs and sperm. Fertilization occurs when the gametes are brought into the sponge by currents of water. A larva is formed, and it settles on a substrate where it becomes attached to the rest of its life.

Asexual reproduction occurs by budding, which happens when a part of a sponge is broken off, or one of its branch tips is constricted, and then this small piece grows into a new sponge. They may also reproduce asexually by producing packets of cells called gemmules.

Threats

In general, sponges aren't very tasty to most other marine animals. They can contain toxins, and their spicule structure probably doesn't make them very comfortable to digest. Two organisms that eat sponges though are hawksbill sea turtles and nudibranchs. Some nudibranchs will even absorb a sponge's toxin while it eats it and then uses the toxin in its own defense. Most of the sponges have been evaluated by the IUCN, as Least Concern.

Sponges and Humans

The modern plastic sponge in our kitchens and bathrooms are named after "natural" sponges, living animals which were harvested and widely used as long ago as the 8th century BCE as implements for bathing and cleaning, as well as in medical practices such as assisting in healing and to cool or warm or comfort a body part. Ancient Greek writers such as Aristotle (384–332 BCE) suggested the best sponge for such tasks was one that is compressible and squeezable but not sticky, and holds great quantities of water in its canals and expels it out when compressed. 

You can still buy natural sponges in health food stores or on the Internet. Artificial sponges were not invented until the 1940s, and long before that, commercial sponge harvesting industries developed in many areas, including Tarpon Springs and Key West, Florida.

Sources

• Brusca Richard C. and Gary J. Brusca. "Phylum Porifera: the sponges." Invertebrates. Cambridge, MA: Sinauer Press, 2003. 181–210.
• Castro, Fernando, et al. "Agalychnis" The IUCN Red List of Threatened Species: e.T55843A11379402, 2004.  
• Coulombe, Deborah A. The Seaside Naturalist. New York: Simon & Schuster, 1984.
• Denoble, Peter. The Story of Sponge Divers. Alert Diver Online, 2011.
• Hendrikse, Sandra and André Merks, A. Sponge Fishing in Key West and Tarpon Springs, American Sponge Diver, 2003
• Martinez, Andrew J. "Marine Life of the North Atlantic." New York: Aqua Quest Publications, Inc., 2003.
• UCMP. Porifera: Life History and Ecology. University of California Museum of Paleontology.
• Wagner, Daniel, and Christopher D. Kelley. "The Largest Sponge in the World?" Marine Biodiversity 47.2 (2017): 367–68. 
• Voultsiadou, Eleni. "Sponges: An Historical Survey of Their Knowledge in Greek Antiquity." Journal of the Marine Biological Association of the United Kingdom 87.6 (2007): 1757–63. Print.