The perks of being endolithic

Bioeroding sponges erode and live in calcium carbonate substrates such as coral. The most obvious reason for this lifestyle is shelter from grazers and predators, but this has never been conclusively shown. To investigate this we collected Siphonodictyon mucosum Bergquist, 1965 and Cliothosa aurivillii (Lindgren, 1898), 2 lesser-studied bioeroding sponges common in coral rock at Orpheus Island, central Great Barrier Reef, Australia. During sampling we broke up blocks of dead coral that contained the sponges and observed post-fracture healing processes. Even though both sponges produce large erosion chambers and their endolithic tissue is soft and easily ruptured, they initially displayed early signs of healing, showing some scarring after only 3 to 4 days. However, endolithic tissue of S. mucosum specimens exposed to direct sunlight did not continue to heal, but rotted away, while shaded tissue fully healed, toughened and turned brown-black like the external fistules. Likewise, endolithic tissue of C. aurivillii showed signs of healing by contracting, toughening and flattening, but after about 2 wk, some C. aurivillii showed signs of healing nevertheless began to rot and had patches of orange sediment underneath them. Their undersides were teeming with copepods. A closer investigation revealed that the copepods had orange stomach contents the colour of the sponge tissue and produced rust-coloured faeces collecting underneath the sponges. Hence, being endolithic has multiple sheltering functions beyond the obvious escape from grazers and predators, very likely including shading, shielding from parasites and disease, and saving on maintenance costs.