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Résumé de la communication de Manuel MALDONADO (CEAB-CSIC, Spain) donnée à l’occasion du séminaire Les constructions en milieu marin / Constructions in marine environment du 14 au 19 mars 2016From the Jurassic oceans where they once abounded about 140 million years ago, a small number of "rock-like" animals, technically known as lithistid sponges, has survived to our days.The appellative of "rock" comes from these sponges being hard as stones because they are characterized by a massive internal skeleton, being the living part of the animal reduced to not more than a thin layer of cells that grow around the skeleton. The lithistids make a rare and vestigial fauna, which in the Mediterranean Sea represents only about 2% of all sponges. One of these sponge species, Leiodermatium pfeifferae, has recently been discovered at a depth of 800 m in the Balearic Sea, forming a massive aggregation around the top of a seamount. These sponges are rigid, convoluted plates, as large as 80 cm in height, and can grow one on top of each other, building a sort of accretive, reef-like formation.Unlike modern coral reefs constructed with calcium carbonate, the uncovered sponge reef can be said to be built by glass. The massive skeleton of these sponges is made of silica, a material with the same chemical formulation than the glass used by humans to make, for instance, windows and wine bottles. Even more surprising is the fact that the accumulation of skeletal glass caused over the sponge generations results from the amazing architectural intertwining of millions of irregular microscopical pieces, called desma spicules, that are elaborated one by one within the sponge body. The cellular and molecular controls driving the delicate assembly of the silica pieces to elaborate the massive and rigid mega-skeletons that in turn make possible the development of the glass reef remain largely unknown. Neither are the reasons known for the unexpected proliferation of these relict sponges in the modern Mediterranean. A unique and extraordinary reef-like formation has been grown in the dark and cold environment of a bathyal seamount, an astonishing analog to the sponge reefs that populated the warm and well illuminated continental shelves in the Jurassic ocean.Summary