Danièle Gaspard¹, Anthony Aldridge², Omar Boudouma³, Michel Fialin⁴, Nicolas Rividi^4
 
1 Museum national d’Histoire naturelle, CR2P, UMR 7207 CNRS-MNHN-UMPC Paris 06, Sorbonne Universités, 8 Rue Buffon CP 38, Paris 75005, France
² c/-4 Perry Way, Mapua 7005, New Zealand
³ ISTEP, Sorbonne Universités, UPMC, Service MEB, 4 Place Jussieu, 75252 Paris Cedex 05, France
⁴ CAMPARIS, OSU EcceTerra, 4 Place Jussieu, 75252 Paris Cedex 05, France
 
Brachiopods, benthic marine invertebrates archives, in their low-Mg shells, witness of modifications in the neighbouring environment via chemical proxies. Breaks in the shell morphology provide an age estimate in the course of ontogeny. Complementary analyses, highlighting growth marks, maxima and minima of deviations from underlying shell spiral growth (SSD), mapping (SEM-EDS) and Electron Microprobe (WDS) of certain elements (Ca, Mg, S, Mn, Fe) that compose the shell along with Scanning Electron Microscopy (SEM), all show indications and evidence of rhythms. These observations have been made on extant rhynchonelliforms species from Subantarctic waters: among them: Aerothyris kerguelenensis (Davidson, 1878) around Kerguelen and Crozet Islands (Southern Indian Ocean). Changes in growth rates were observed from early stages to recent ones (i.e. from the posterior shell region to the anterior region) and between valves (ventral and dorsal). Modifications are observed in the Mg and S concentrations, from the primary layer to the inner fibrous secondary layer, with fluctuations in accordance with the growth rhythm.  The external morphology and the anterior half-length in median longitudinal sections of shells emphasize the major growth rhythms and light secondary details, while the posterior region and the shell thickness highlight cumulative growth features.