Photo copyright Tim Nicholson.
Tube Sponge, Acervochalina sp
Taken at Gota Baraka, Egyptian Red Sea.
Sponges are amongst the simplest members of the animal kingdom. They have no digestive, nervous or circulatory systems for example.
The lack of sophistication in sponges has been shown in a classic experiment. A sponge was pushed through fine silk to completely break it down. However, it soon succeeded in resassembling itself.
A sponge feeds by drawing water into its central cavity through its pores - the tiny holes covering its body. The filtered water leaves through the big outlet pores.
With their varied colours and intricate shapes, sponges add beauty to the reef or rock. Some sponges are greenish, violet or pinkish in colour because they harbour symbiotic algae. The more symbionts and light, the stronger the colour. When there is low light these sponges are a much paler colour - almost white.
The sponge uses the algae as a source of oxygen, a screen against sunlight and as a food source. It provides the algae with a secure habitat.
Our picture shows a Tube sponge, Acervochalina species. This sponge can produce a chemical which is highly toxic to Pocillopora and Acropora corals.