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Cuticle of Polyplacophora: structure, secretion, and homology with the periostracum of conchiferans

Checa, AntonioG., Vendrasco, MichaelJ., Salas, Carmen
Marine biology 2017 v.164 no.4 pp. 64
Bivalvia, Polyplacophora, biomineralization, calcification, epithelium, microscopy, secretion, California, Spain
The plesiomorphic state of the molluscan scleritome remains ambiguous. Chitons are significant because they show a mix of characters considered diagnostic in both aplacophorans and conchiferans, with both shell plates and small calcified girdle elements on an elongate body, and there is no consensus on the homology of structures involved in chiton plate calcification with those in conchiferan shells. Using light, scanning, and transmission microscopy, the present study examined the structure and formation of the cuticle and underlying epithelium in five chiton species collected over several years in southeast Spain and in California, USA. The cuticular matrix in chitons is similar to the translucent layer of the bivalve periostracum, although there are differences in the orientation of the densely spaced laminae with respect to the mantle surface. The cuticle arises in the accessory fold of the ventral girdle mantle. There is full continuity between the outermost layer of the cuticle and the thin outer layer of the tegmentum, suggesting that the latter is derived from the cuticular wedge permanently lying on the plate margins. A homology is proposed between the cuticle/associated girdle mantle epithelium of Polyplacophora and the periostracum/inner side of the outer mantle fold of the Bivalvia. Nevertheless, while the bivalve periostracum slides over the mantle epithelium during growth, the polyplacophoran cuticle remains stationary. The configuration of the cuticle, which overlaps the plate margins, allows it to efficiently close the plate biomineralization compartment, in a way comparable to that of the periostracum of conchiferans.