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Most sponges living in Recent seas have a skeleton composed of collagen or spongin
fibres, with or without an inorganic component of silica or calcium carbonate (spicules). Only about
15 species of Recent sponges are known to build solid secondary calcareous skeletons of aragonite or
calcite, in addition to, or instead of their primary spicular skeletons. The secondary basal skeleton
is superficially similar to that of scleractinian corals, and this group of sponges is called 'coralline
sponges' or 'sclerosponges'. They appear to be living representatives of several groups of sponges from
the late Paleozoic and Mesozoic, previously grouped together under the concept of 'Sclerospongiae',
which is now demonstrated to be clearly polyphyletic.
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Sponges with a secondary basal skeleton had a major functional role in past reef systems, being the main
reef building organisms (e.g. 'Stromatoporoids', 'Chaetetids', and 'Sphinctozoans') during long periods
of the Earth's history. They were only replaced by hermatypic corals in their reef building function
since the late Jurassic, but some of those ultraconservative taxa are still dwelling in the cryptic
habitats of Recent coral reefs. Here, they still play an important functional role for the stabilization
of the internal reef framework in shallow water, due to their rigid calcareous basal skeleton,
especially if they occur in high densities (e.g. Saipan, Marianna Islands, 9-16m: 672 individuals m-2,
65% cover). Studying different aspects of these 'living fossils', not only can we learn about the
ecology, community structure and phylogeographic relationships of
'relict faunas', but also about basic metazoan biomineralisation
processes.
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