Sponges

Animals that can reinvent themselves

Sea Sponge

Sponges are amongst the most primitive and basic animals alive. They don’t have a nervous, digestive or circulatory system, but feed by filtering the water for organic particles. They come in all shapes and sizes, and date back at least 580 million years.

Sponges have a sac-like body formed from millions of tiny interlocking silica needles. They draw water through the body into a central cavity, remove organic nutrients, then eject the water again through a larger hole usually at the top of the body.

Axinella
A sponge from the sea bed on Lyme Bay

Needles and nodules

Sometimes the overall shape of the sponge is preserved as a fossil. But at other times it’s just the textures, or the doughnut-shaped central cavity. This is because the silica needles are easily dissolved in the ground water that percolates through the rocks.

Fossil Nodules

The gel-like silica then hardens again to create flint in chalk, and chert in limestone or sandstone. Sometimes the silica collects around solid objects such as other sponges, then the sponge breaks down, resulting in hollow flint nodules.

Masters of reinvention

Sponges have remarkable powers of regeneration. If you put one through a blender, mash it to a pulp, then put it back in sea water, the sponge will remake itself!