Octopus coral

Small colonies of Galaxea fascicularis often form low domes but as they grow, the colonies become more irregular, massively hummocky or columnar and may eventually reach 5 metres (16 ft) across. The corals are the calcareous skeletons of polyps and the variability in shape is at least partly caused by the activities of horse mussels (Lithophaga spp.) which bore into the skeletons. The individual polyps are embedded in circular, tube-shaped corallites less than 1 centimetre (0.39 in) across, made of a limy material extruded by the polyps. Lining the corallites are a large number of ridge-like septa radiating from the centre. The polyps often feed in the daytime, and when their tentacles are extended the basic skeleton of the coral is hidden. The general colour of the coral ranges from green and grey to reddish brown. The tentacles are often a contrasting colour and are usually tipped with white. A few of the tentacles are modified into sweeper tentacles which can be extended to 30 centimetres (12 in) in length and serve to deter other organisms from settling close by. It has been found that the size and proximity of the corallites varies depending on the amount of light incident on the coral, even over different areas of the same colony. In bright positions, small, closely packed corallites maximise the photosynthetic potential of the zooxanthellae. In less well lit positions, there are larger corallites and polyps with longer tentacles with greater food capturing ability.