Unusual biomineralization processes in marine invertebrates: the example of cirratulid polychaetes from Peru

Carbonate biomineralization in marine organisms normally occurs as three distinct types, known as: “controlled”, “induced” and “influenced”. Among invertebrates, reef-building polychaetes of the family Cirratulidae appear to provide the first example of “dual-phased” biomineralization as they simultaneously combine “controlled” and “induced” types in the formation processes of their aggregated tubes. Recently, Kočí et al. (2020) described the general morphology and internal framework of some bioconstructions of a Miocene cirratulid species and stressed the need of further comparative studies to better understand the nature of the calcareous matrix, tube ultrastructure and biomineralization processes. The main purpose of this work is to study the mineral structure of some Miocene (Diplochaetetes) and Recent (Dodecaceria) cirratulid bioconstructions from southern Peru through optical microscopy, epifluorescence, SEM-EDS analyses and morphometry. Results have also been statistically processed in order to highlight the possible relationships between size and chemical composition of the skeletal component with the morphology of the bioconstructions themselves, which in turn hints to environmental parameters. Three linear correlations, considering as variables morphological parameters (i.e., tube diameters) and the weight concentration of the main elements (Ca and Sr), were tested. Both fossils and recent specimens show similar correlations among composition and morphometric parameters, suggesting that the growth/evolution of these bioconstructions is presumably unaffected by environmental chemo-physical conditions. The primary ultrastructure of the tube walls points to controlled biomineralization, while the “intertube” areas of the bioconstruction, filled by a calcareous matrix frequently agglutinating extraneous sand grains, would be the product of an induced biomineralization. Agglutinated sand grains have various compositions and many of them are igneous, their origin being attributable to nearby coastal batholiths. Until now the means by which the “dual-phased” biomineralization processes take place in cirratulids have not been described in detail, probably due to several systematic misinterpretations occurred in the past prior to the description of Fischer et al. (1989), where fossil cirratulid specimens were frequently reported as sponges or corals instead. We also highlighted that the unique dual-phased biomineralization system of cirratulids seems not to have evolved since the Eocene, when they appeared.