Phylogenetic constraint and phenotypic plasticity in the shell microstructure of vent and seep pectinodontid limpets

Pectinodontid limpets of the genus Bathyacmaea are endemic to hot vents and cold seeps and exhibit greatly variable shell and radular macro-morphologies, rendering reliable species-level identification challenging. Here, we analyzed shell microstructures of western Pacific vent/seep Bathyacmaea limpets using scanning electron microscopy and Raman spectrophotometry to test its usefulness in providing phylogenetic signals. Bathyacmaea shells comprised of two forms of calcitic microstructure including irregular spherulitic prismatic type-A (ISP type-A) and semi-foliated (SF), as well as the aragonitic crossed lamellar (CL) microstructure. Despite marked differences in macroscopic shell morphologies once leading them to be classified into different species or even genera, six morphotypes of Bathyacmaea nipponica from different chemosynthetic localities and substrates shared an outermost ISP-A layer and alternating layers of SF and CL structures in their outer and inner shell layers. A genetically divergent lineage recovered from the South Chamorro Seamount, however, differed in having a simple three-layered shell composition consisting of ISP-A, SF, and CL structures, in that order, from the outside, and an unusually thin inner shell layer consisting of only CL structure. Moreover, the ratio of aragonite and calcite varied with habitat conditions, with calcite dominating in vents and aragonite dominating in seeps. These results suggest that the shell microstructure of pectinodontids is under phylogenetic constraints and provides useful taxonomic signals, while the mineral polymorphism in aragonite/calcite ratio varies according to environmental factors. Furthermore, microstructures of two ‘species’ from Cretaceous seeps confirmed the same patterns in fossil lineages.