Sponges filter oceans without organs—and may root the animal tree
Porifera are mostly marine sessile filter feeders among Earth's oldest macrobenthos, some building reefs, a few living in fresh water. Jellylike mesohyl between cell sheets channels seawater through pores; choanocytes drive the current that feeds roughly five to ten thousand known species.
A sponge is an animal built around moving water. Its hollow body is a jelly of collagen, the mesohyl, sandwiched between thin sheets of cells and pierced by pores. Inside, collar cells called choanocytes beat whip-like flagella to pull water in through small channels, often guarded by closable valve cells, strain out food and oxygen, and push waste out through a larger opening, the osculum. There are no true tissues, organs, or nervous, digestive or circulatory systems, yet researchers have counted 18 cell types, and most cells can crawl through the body and some can change type, so every living sponge can reshape itself.
Skeletons vary. Many sponges stiffen the jelly with spicules of calcium carbonate or silica set in spongin, a form of collagen, while soft encrusting kinds rely on the mesohyl alone. Demosponges make up more than 90 percent of known species and are the only ones in fresh water. Unlike other animals, sponges cannot secrete a protective mucus layer, so microbes coat their tissue and can account for 40 to 50 percent of their wet weight, which may be one reason they never evolved more complex bodies.
Most of the 5,000 to 10,000 known species eat bacteria and particles from the water, but some host photosynthetic cyanobacteria or algae that make more food than they use, and a few in food-poor places catch small crustaceans on hook-shaped spicules. Sponges breed sexually and asexually, can regrow from fragments that contain the right cells, and in cold seasons some freshwater species survive as tough gemmules.
Fossils such as Otavia may date back some 800 million years. Soft bath sponges were fished almost to collapse by the 1950s, and dolphins have been seen using sponges as foraging tools.
Source: Sponge
