In dark zones of submarine caves, photosynthesis-related production and water movement can be negligible or absent. This situation induces sessile animals like Porifera and Bryozoa to reduce their presence, shifting from massive to tiny encrusting morphologies. Notwithstanding this general rule, true engineer organisms can develop forming three-dimensional structures in particular situations. Near the entrance of Mediterranean submarine caves, where light and hydrodynamism are not limited, bio-concretions are similar to those found in the coralligenous. Coralline algae with their convolute calcareous thalli are the primary engineers, while Porifera, Cnidaria, and erect Bryozoa compete for more sheltered and dark positions. Epi- and endolithic Bivalvia are present as well, together with the boring Porifera Clionaidae, representing the demolition community which establishes a dynamic equilibrium with the building one. Due to light decrease, until complete darkness, bioconstructions of innermost cave sectors lack algae and consist of a simplified association if compared with that at the entrance. They are dominated by a few selected taxa of skeletonized invertebrates, essentially Polychaeta Serpulidae, Porifera, and Bryozoa, to which carbonate-producing bacterial communities add. These peculiar frames known as biostalactites are conical to cylindrical in shape and typically restricted to semi-dark and dark cave portions, reaching considerable sizes and protruding from the ceiling and/or walls. The gravity force shapes in a more tenacious arrangement the structure. The presence of boring organisms is strongly reduced. The complete set of growth/structural development has to be investigated yet. Due to the recent discovery and description of these biostalactites and their apparent rarity, data are scant, especially those related with the associated vagile faunas and the biodiversity.
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