The diverse and difficult to identify array of sponges that grow on tropical coral reefs...
Sponges have what is often considered the most basic body plan of all the multi-cellular animals. They are sedentary as adults, and resemble plants in their simplicity and adherence to the substrate. Because of their simplicity and stillness, they were originally classified as a plant, and were only recognized as an animal in 1765 when their internal water currents were noticed.
Sponges are covered in tiny holes that draw water in. This is accomplished by small beating hairs that bring the water in. As the water is pumped throughout the inside of the animal, it is filtered for food particles. The water is the pumped out, exiting the much larger holes.
This simple body plan and feeding technique of sponges has been very successful for a long time, for they have been around for half a billion years. During the Devonian period, well before the Dinosaurs, sponges were one of the most dominant forms of life on Earth.
In modern times, they are more obvious in areas of some water flow and nutrient load. Because of the generally clearer and nutrient poor waters of warmer seas, they are not always as noticeable around tropical coral reefs compared to their dominance along cooler water coastlines. However, there are many very attractively coloured species present around coral reefs. They vary in colour, size and shape, with some species growing simply as encrusting layers, but others look like balls, large cups, funnels, or into huge 'barrel' sponges.