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Sponges: The first Multicellular Organisms?

Increased co-ordination between colonial cells appeared with the evolution of the sponges (Porifera). Sponges are considered the oldest living animal phylum. The name Porifera means "pore bearer" in Latin. Sponges may be formless lumps on the sea floor reaching two metres in size. Their surfaces are covered with tiny pores through which water is drawn into the body by flagella and then expelled through larger vents. The sponges feed by filtering particles from this stream of water passing through its body. Some sponges produce a soft flexible silica-based substance which supports the whole organism, whereas other sponges secrete lime or silica to create a hard "skeleton" for support. Despite the elaborate skeletons that some sponges are able to produce they cannot be considered as an integrated multi-cellular animals since they have no nervous systems nor muscle fibres.

A sponge (unknown species)

Sponges are primitive, sessile, mostly marine, waterdwelling filter feeders that pump water through their matrix to filter out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs. In some ways they are closer to being cell- colonies than multicellular organisms. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m. Though the fossil record of sponges dates back to the Precambrian era, new species are still commonly discovered.

The structure of a sponge is simple: it is shaped like a tube, with one end stuck to a rock or other object and an open end, the osculum, open to the environment. The spongocoel, or interior of the sponge, is composed of walls perforated with microscopic pores that allow water to flow through the spongocoel.

Researchers now think that the most likely candidate for this "Animal Eve" is the sponges. Sponges are the only animals that if broken down to the level of their cells can miraculously reassemble and resurrect themselves. These seemingly inanimate creatures are also fantastic pumps, filtering tons of water to harvest just a few ounces of microscopic food.

How do we know sponges were our ancestors? It turns out that all organisms in their genes carry clues to their evolutionary history -- a unique set of acquired genetic changes passed on through countless generations.

Biologist Cristina Diaz from University of California in Santa Cruz has studied sponges in the waters off Sulawesi in Northern Indonesia, one of the most biologically rich environments on Earth. There, she searches for members of the group that started it all--the first animal group of Earth.

Diaz's explorations reveal sponges as a surprisingly colorful and diverse group of animals. Once considered plants, sponges showcase special traits of "animalness" -- like sporting multiple specialized cells that work together and communicate with each other in a uniquely animal way.

In her work off Sulawesi, Diaz brings to light the prodigious power of these seemingly quiet animals. Using luminous non-toxic dye, she demonstrates that simple sponges function as fantastically active pumps.

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