Euendolithic Conchocelis stage (Bangiales, Rhodophyta) in the skeletons of live stylasterid reef corals

Stylasterids are azooxanthellate lace corals, which may show an unusual colouration in coral reef ecosystems, ranging from red to purple on the light-exposed side of their colonies. In the present study, it was discovered that the calcareous skeletons of such corals are actually invaded and eroded by cryptic carbonate boring algae that represent Conchocelis phases in the life cycle of bangialean rhodophytes. For the first time, the shape, organisation and distribution of Conchocelis filaments within the skeletons of two different genera of lace corals are reported and documented by light and electron microscopy. Such description and characterisation of Conchocelis can help to distinguish between morphologically and taxonomically different endolithic microorganisms penetrating coral skeletons, including their recognition in fossil borings as indicators of depositional depth, i.e. of the extent of the photic zone and light conditions in ancient marine environments. The results are discussed in the light of a possible symbiotic or parasitic relationship between Conchocelis phases and their stylasterid host corals