Coexistence of Different Shell Microstructure Types in a Single Shell Layer of the Veneridae (Bivalvia, Mollusca)

Abstract

Listen

Translate article

For the purpose of understanding the relationship of component taxa of the family Veneridae (Bivalvia), 65 extant species have been observed for their shell microstructure on thin sections and acetylcellulose replicas with the aid of optical and scanning electron microscopy. The observation revealed that the microstructure of the inner shell layer shows intraspecific variation. The inner shell layer of venerids generally consists of either homogeneous or complex crossed lamellar structure. However, these two microstructure types coexist in some species (e.g., Ruditapes philippinarum, R. variegata, etc.) and even within a single shell of the species. In contrast, intraspecific variations are not recognized in the microstructure of the middle, outer, or outer-most shell layer, but replacement or accretion of some microstructures occurs in the middle shell layer of some species; e.g., the crossed lamellar layer accretes to the homogeneous layer during the later stage of shell growth (Veremolpa mindanensis, etc.), or the crossed lamellar structure replaces temporarily the homogeneous structure at growth breaks (Mercenaria mercenaria, Protothaca (Notochione) jedoensis, etc.). These lines of evidence imply that these shell microstructures have been developed concurrently in venerids, and give us important clues for understanding the phylogenetic relationships of the Veneridae.