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Abstract
Cheilostome Bryozoa Anoteropora latirostris, a colonial marine invertebrate, constructs its skeleton from calcite and aragonite. This study presents firstly correlated multi-scale electron microscopy, micro-computed tomography, electron backscatter diffraction and NanoSIMS mapping. We show that all primary, coarse-grained platy calcitic lateral walls are covered by fine-grained fibrous aragonite. Vertical lateral walls separating autozooid chambers have aragonite only on their distal side. This type of asymmetric mineralization of lateral walls results from the vertical arrangement of the zooids at the growth margins of the colony and represents a type of biomineralization previously unknown in cheilostome bryozoans. NanoSIMS mapping across the aragonite-calcite interface indicates an organic layer between both mineral phases, likely representing an organic template for biomineralization of aragonite on the calcite layer. Analysis of crystallographic orientations show a moderately strong crystallographic preferred orientation (CPO) for calcite (7.4 times random orientation) and an overall weaker CPO for aragonite (2.4 times random orientation) with a high degree of twinning (45%) of the aragonite grains. The calculated Young’s modulus for the CPO map shows a weak mechanical direction perpendicular to the colony’s upper surface facilitating this organism’s strategy of clonal reproduction by fragmentation along the vertical zooid walls.
Highlights
Bryozoans represent a diverse phylum of mostly marine, colonial suspension feeding animals, present in nearly 7,000 recent and about 20,000 fossil species in all types of marine habitats from the intertidal to the deep sea, and the polar regions towards the tropics
We present a detailed characterization of the internal structure and biomineral architecture of colonies of recent Anoteropora latirostris, a bryozoan that is free-living and does not encrust with their basal colonial surface
Micro-CT images allow accurate imaging of individual zooidal chambers and study of the zooid chamber and interconnection between zooids
Summary
Bryozoans represent a diverse phylum of mostly marine, colonial suspension feeding animals, present in nearly 7,000 recent and about 20,000 fossil species in all types of marine habitats from the intertidal to the deep sea, and the polar regions towards the tropics. Bryozoan skeletal hard parts, consisting of calcite and/or aragonite[4,5] are abundant throughout the geological record since the Ordovician[6], which makes them interesting as potential paleo-environmental and -climatic proxy archives[7,8,9,10], as well as ‘sentinels’ of ocean acidification[11,12,13] They are an ideal group for the study of evolution, since they are abundant and morphologically complex. We present a detailed characterization of the internal structure and biomineral architecture of colonies of recent Anoteropora latirostris, a bryozoan that is free-living and does not encrust with their basal colonial surface. Brood chambers (ovicells) (Fig. 1c) and avicularia (Fig. 1d) are present[19,20]
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