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Résumé de la communication de Chiara LOMBARDI (ENEA, La Spezia, Italy) donnée à l’occasion du séminaire Les constructions en milieu marin / Constructions in marine environment du 14 au 19 mars 2016Many marine organisms, both plants and animals, build durable biogenic structures through the aggregation and accumulation of their calcareous skeletons. Bryozoans have a wide distribution in space - from tropical, to temperate and polar environments -, in depths - from intertidal environments down to abyssal depths - and through times - from Early Ordovician (480 My) to Recentand a great variability in shape and dimension. Among organisms making bioconstructions, bryozoans have been widely recognized as important habitatforming organisms. After their first appearance, the phylum expanded across freshwater, brackish and marine ecosystems.Bryo-constructors have different growth forms and habits. Proper 'constructors' are erect, large and well skeletonized mound or branched colonies, whereas 'binders' are encrusting species, which expand and unite the components of the framework and the settling sediment. Other erect, poorly skeletonized species, act as 'bufflers' reducing current velocity and enhancing sediment deposition, cavity filling within the framework [4]. The high plasticity of the taxon enable the species to perform more than one role in the success of building (e.g. Schizoporella errata).Bryozoan bioconstructions are composed of few to many thousands of conjoined, clonal modules (zooids), each ranging from 0.1 to a little over 1 mm in length. Although genetically identical, the zooids within a colony differ morphologically according to their developmental stage, the developmental stage of the colony when they were budded, and as result of functional differentiation (polymorphism).Each zooid is composed of a polypide and a cystid. The cystid comprises essentially the outer body walls of the zooid, including cuticle, mineralized skeleton and peritoneum and here, at level of the cystid, biomineralization processes are expected to take place.Looking at the evolution of biomineralization in bryozoans, the most phylogenetically basal class is Phylactolaemata- which includes entirely freshwater animals without biomineralized skeletons - followed by two other classes - Stenolaemata and Gymnolaemata - a sister grouping of predominantly marine species usually having calcareous skeletons. With a rich fossil record extending back to the Early Ordovician, Stenolaemata comprises four extinct and one extant order (Cyclostomata), all well calcified. Gymnolaemata consist of the paraphyletic order Ctenostomata, which is entirely soft-bodied, and Cheilostomata, today the dominant group of bryozoans, an order of species with calcareous skeletons which first appeared in the Late Jurassic (155My). Biomineralizational patterns and, especially, processes are poorly understood in bryozoans but are conventionally believed to be similar to those of the related lophotrochozoan phyla Brachiopoda and Mollusca. However, bryozoan skeletons are more intricate than those of these two phyla. Calcareous skeletons have been acquired independently in two bryozoan clades - Stenolaemata in the Ordovician and Cheilostomata in the Jurassic providing an evolutionary replicate.Bryozoan skeletal macrostructures comprise three different wall types (exterior, interior and compound) differing in the presence/absence and location of organic cuticular layers. Skeletal ultrastructures can be classified into wall-parallel (i.e. laminated) and wall-perpendicular (i.e. prismatic) fabrics, the latter apparently found in only one of the two biomineralizing clades (Cheilostomata), which is also the only clade to biomineralize aragonite. A plethora of ultrastructural fabrics can be recognized and most occur in combination with other fabrics to constitute a fabric suite. The proportion of aragonitic and bimineralic bryozoans, as well as the Mg content of bryozoan skeletons, show a latitudinal increase into the warmer waters of the tropics.Responses of bryozoan mineralogy and skeletal thickness to oscillations between calcite and aragonite seas through geological time are equivocal. Field and laboratory studies of living bryozoans have shown that predicted future changes in pH (ocean acidification) combined with global warming are likely to have detrimental effects on calcification, growth rate and production of polymorphic zooids for defense and reproduction, although some species exhibit reasonable levels of resilience.By building a framework, bioconstructors generate physical structures that modify the habitat and its structural complexity. Bryo-constructures are important determinant of the number, identity and abundance of species present in biological communities (e.g. biological habitat provision). The detrimental effect of climate changes on bryo-constructions represent a serious threat to both habitat and species diversity, and thus their knowledge, protection and conservation should be promoted under future scenariosSummary