Glass sponges may be the longest-lived animals on Earth
Deep in the ocean, sponges built from glassy silica may live for millennia. Some marine biologists tentatively put their maximum age at up to 15,000 years. These strange animals have no ordinary skin cells, cannot contract, and yet send electrical signals through their bodies to stop feeding when the water turns murky.
Hexactinellids, or glass sponges, have skeletons of silica spicules with four or six points, fused into a rigid lattice. Most classifications put them in Porifera alongside ordinary sponges, though a minority view gives them a phylum of their own, Symplasma. In the genus Euplectella a protein called glassin speeds up the conversion of silicic acid drawn from seawater into silica.
They are fairly rare and mostly live 450 to 900 metres down, though Oopsacas minuta turns up in shallow water. They occur in every ocean and are especially common around Antarctica and in the North Pacific. Typically cup-shaped, pale white to orange and 10 to 30 centimetres tall, they have a large central cavity that often opens through a sieve formed by the skeleton, and some species fuse together into reefs. The average maximum length is around 32 centimetres, but some reach a metre, and those are thought to be roughly 200 years old.
Inside, they are unusual. Much of the body is a syncytium, a continuous mass of cytoplasm with many nuclei, hung like a cobweb on the silica scaffold. Remaining cells connect to it through cytoplasmic channels that ferry nuclei and organelles. Instead of the collar cells other sponges use to pump water, they have bell-shaped chambers lined with collar bodies, each with a flagellum but no nucleus, whose beating draws water through the network for feeding.
Their conduction system lets impulses race across the body. Experiments on Rhabdocalyptus dawsoni showed that when sediment is detected, a signal shuts down feeding, and a related species has its own tolerance threshold before it stops. The Venus' flower basket anchors itself with a crown-like tuft of fibres at its base, each 50 to 175 millimetres long and about as thick as a human hair.
Source: Hexactinellid
