Considered the most primitive of multicellular animals, sponges are really loose colonies of cells. Most sponges live in saltwater, with only a few freshwater species. Adult sponges do not move from place to place (they are sessile) but come in a dazzling variety of shapes and colours. Sponges range from slimy encrusting forms to beautiful 3D structures with lacy skeletons made of silicon. They are found from shallow waters down to the depths of the ocean. Some grow to a metre or more in height and they are a prominent and colourful feature of all marine environments. Sponges have been evolving for many millions of years and have developed an amazing array of chemical toxins to ward off animals that might eat them and plants that might grow over them. The pharmacology industry is only now beginning to discover potential uses for the wide range of chemicals found in sponges.

• A body that is a loose collection of cells without true tissues or organs.
• Cells that can adapt to any function the organism needs (totipotent cells).
• Special cells that that drive water currents through the body called choanocytes or collar cells.These flagellated cells line the inside of the sponge
• A larval stage that drifts in water currents , changes into a sessile adult stage and feeds by filtering tiny particles from the water.
• A skeleton made of calcium carbonate or silicon dioxide in the form of spicules and /or collagen fibres, called spongin. Or have no skeleton at all!

Sponges, bath sponges, Venus's flower basket, Calcarea, Demospongiae, Hexactinellids

Many sponges have an amorphous shape: they look like a slimy, spongy or prickly layer on the rock. Other types are asymmetrical or radially symmetrical and have openings in the outer surface called oscula which lead to the canals of the water system. Shapes of upright sponges can vary from globes to vases and may have many branches. They come in every colour imaginable including lavenders, blues, yellows, bright reds, orange and white. The beautiful skeleton of Euplectella aspergillum is known as 'Venus's Flower Basket'. The delicate 3D structure is possible because the 6-rayed spicules made of silicon fit together in a variety of ways. This group of sponges called Hexactinellids: The "hex" in their name refers to their 6-rayed spicules. Sponges used chimneys long before humans existed. They adapt their shape to take advantage of the different water speeds. Water flow near the bottom is usually slower than that higher in the water column. The faster water flow over the chimney's exit hole actually sucks water through the sponge's pores increasing the efficiency of its collar cells whose job it is to create a water flow through the sponge, allowing the sponge to feed. Sponges are colonial animals which are made up of many individual cells which specialise to fulfil the animal's many needs. This means some cells specialise in reproduction, others in feeding and so on. The acid test as to whether an animal is truly multicellular or made up of many colonial individuals is to put them through the blender! If you do this with a sponge the resulting soup will reassemble and recreate itself, given the right conditions. If you did this with a mouse you would soon realises that it is a truly multicellular animal which will never squeak again!

Some of the first reefs made by animals hundreds of millions of years ago were made of sponges. They were found in many parts of the world until they were gradually outcompeted by the corals. Sponges are found in all seas living mainly in shallow waters but some occur at great depths. Unpolluted shallow water habitats support especially rich sponge faunas. Where currents are strongest they form flattened or rounded shapes, but in still waters can grow into tall and often tree-like structures which greatly increase the surface area. The majority attach themselves to any suitable surface such as rocks, hard-shelled animals or seaweeds. Some bore into rocks, shells or coral and are very important in the process of bio-erosion which over time helps to build reefs . Some crabs (Family Dromiidae or decorator crabs) disguise themselves by attaching pieces of sponge to their shells. Sponges are dominant animals in many benthic marine environments. They even occur in large numbers and huge sizes in Antarctica.

Sponges were considered to be plants by the Ancient Greeks and Romans. Observations of water currents generated by cells pointed to their animal nature and filter feeding mode of life. Sponges feed by filtering small food particles (some even eat bacteria) out of water pumped through the system by flagellated collar cells. Huge volumes of water are processed: a 10 cm. individual of Leucandra pumps 22.5 litres of water a day!! Water passes directly over cells responsible for food gathering and gas exchange and at the same time removes wastes and reproductive products. Particulate matter is taken directly into the collar cells which digest them and food is then passed into the body by specialised amoeba-like cells.

Because of their impressive chemical toxins sponges have few predators. One group of animals which has beaten their chemical defenses are the beautiful nudibranchs. These animals add insult to injury by not only being immune to the toxins, but by being able to actually reuse them in their own defence! The toxins of sponges are effective against many bacteria and viruses but they are not completely effective. A fungal disease attacked a crop of commercial bath sponges in the Bahamas in the 1930's and almost wiped them out.

Sponges have no males and females nor specialised reproductive organs. Individual cells become sex cells and produce either eggs or sperm. Sperm are released into the water where they pass to a neighbouring sponge, are captured by a collar cell which then changes shape and function to become an amoeboid cell and transports the sperm to the oocyte or egg producing cell. Egg and sperm combine to form a zygote which develops into a motile (flagellated) larva. These usually have a short life in the plankton before settling and growing into a new sponge. All sponges have great powers of repair and regeneration. In a laboratory situation new sponges can be grown from cells taken from an adult organism. Growth rates vary widely between species and can depend on water temperature and the amount of food present in the water.

Sponges live attached to any suitable surface including other living organisms. One species Siphonodictyon coralliphagum infests Montipora corals and exudes a toxic chemical which kills the coral polyps nearby and so avoids overcrowding by other sponges. Large sponges have a huge number of organisms living on and in them. Many sponges on coral reefs have blue green algae living within them which pay rent to their sponge landlords by producing sugars in much the same way as dinoflagellates do with corals. Many organisms live on sponges, including various crabs, feather stars, worms and sea cucumbers. Small sea cucumbers are one of the most common commensal animals seen on sponges. There may be hundreds of them feeding on the large organic debris which accumulates on large barrel sponges. Small prawns sometimes enter sponges as larvae and develop to adults inside the 'cage' from which they grow too large to escape. Sponges with a pair of prawns were seen as a symbol of great fidelity in Japan. This species lives at depths of more than 1200m. off Japan and the Philippines but may be endangered due to over collecting.

Some sponges can cause dermatitis partly due to silica spicules and partly to a toxin. Other sponges have antibacterial properties, but the potential of this group have not yet been fully explored. Medical science is researching the chemical compounds of sponges with great interest for their pharmacological properties. In the Mediterranean and the Bahamas sponges are harvested for the bath sponge industry. Most of these are in the genus Spongia and have a skeleton made of spongin fibres only with no spicules. When dried and cleaned they form soft, dense sponges which are used for bathing or applying makeup. At one time they were planted and grown as a crop but have now been largely replaced by synthetic sponges. Sponges are often abundant at the entrance of harbours and river mouths because they thrive on the large amounts of bacteria and organic debris usually present in those habitats. But large amounts of sediment in the water can be a problem, as it can clog their pores and smother the sponges. Trawling can also be a problem for some sponges as they are often dragged up in nets with the target species and later discarded as by-catch.