Shell formation in two species of bivalves: the role of mantle cells and haemocytes

The shell of bivalves acts as structural support, defence against predators, stressors, and maintenance of homeostasis. Haemocytes and mantle epithelial cells participate in shell formation. Mantle epithelial cells produce the organic matrix of the shell and are associated with biomineralization because of their proximity to the shell surface and ability to maintain shell deposition ex vivo. Haemocytes play an immunological role and are involved in Ca2+ and CO32- intracellular transport implicated in shell mineralization. This study aims to evaluate the expression of molecules that play key roles in internal defence and the nervous system, in epithelial cells and haemocytes of the outer mantle margin of two species of bivalves, Polititapes aureus Gmelin, 1791 and Cerastoderma glaucum Bruguiere, 1789, to highlight their collaborative role in shell formation. Our data show for the first time the presence of serotonin/S100 protein, and TLR2/iNOS colocalization in the haemocytes and mantle epithelial cells of P. aureus and C. glaucum, suggesting that the mantle cells perform their canonical role of sensory and biomineralizing activity, while the haemocytes perform their typical immune function, and their collaboration in shell formation, providing new information on cellular interactions in shell formation and giving more details for taxonomical studies.