November, 2004:
The sponge is the earliest, most primitive multicelled animal, Sogin says. Some scientists believe the ability to grow different cell types started animals on the evolutionary road to becoming humans. With just a few kinds of cells, only loosely connected, the sponge manages to produce a variety of asymmetrical shapes, from cups and fans to tubes and piecrust shapes. Sponges survive handsomely on their own and can even shelter other sea creatures: Scientists found a large sponge in the Gulf of Mexico hosting 16,000 snapping shrimp and 1,000 other aquatic animals. The sponge’s cells, its calcium carbonate or glasslike silica spicules, and the mass of collagen that forms its visible body all create a network of tunnels and chambers, with little flailing hairs called cilia on the walls that wave the water through and filter out plankton and waste. No matter how large the sponge, it can eat only what its individual cells can absorb.
Sponges are also the earliest sexual reproducers; most are hermaphroditic, producing both eggs and sperm, which they release into the water. The sperm drift along until they find their way into the tunnels and caves of another sponge. But the sponge has other reproductive options. If you push one through a sieve, breaking free its individual cells, these cells will drift until they find each other, then stick together and create an exact genetic duplicate of the parent. If wounded, a sponge doesn’t need to grow new tissue; it simply moves old cells into the wound to close it. These techniques have helped sponges survive at least 500 million years. A few have remarkable capabilities. One, living in a Mediterranean underwater cave, traps small crustaceans with the sharp, glassy spikes jutting from its body, then surrounds them with its cells and digests them.
Biologist Calhoun Bond, then at the University of North Carolina at Chapel Hill, found in 1986 that sponges don’t just sit still—many actually move. Using time-lapse microscopy, he filmed freshwater sponges slowly crawling across the bottom of their containers. He found that larger, saltwater sponges do the same by extruding flat paddlelike extensions of their bodies and pulling themselves along, often climbing the sides of their glass tanks in labs. One sponge, a lavender beauty called Heliclona loosanoffi, moved four millimeters a day.
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