Shell microstructures of Recent solemyids from Japan (Mollusca: Bivalvia)

Solemyidae (Mollusca: Bivalvia) belongs to the Protobranchia which is the ancestral and possibly paraphyletic group of the Bivalvia (Zardus, 2002) and they are probably the oldest of the chemosymbiotic bivalves (Taylor et al., 2008). All extant members possess the symbiotic bacteria and the overall similarity in shell morphology between fossil and extant solemyids suggest that symbiosis is an ancestral condition that became established prior to diversification in this group (e.g. Oliver and Taylor, 2012). But the simplicity of their external shell morphology inhibits our understanding of their evolutional history. In order to examine more morphological characters, we focused on shell microstructures. Molluscan shells are composed of structural units of calcium carbonate called shell microstructures and they are similar in phylogenetically close taxa of molluscs (e.g. Fuchigami and Sasaki, 2005). Thus, investigations of shell microstructures can provide clues for systematic and phylogenetic analysis of molluscs, including fossil taxa. The purpose of this study is to reveal the relationship between the shell microstructures and molecular phylogeny of solemyid bivalves.

Shell microstructures of five solemyid species from Japan were characterized by SEM. All five species examined had outer and inner shell layers, and were categorized into four groups according to the shell microstructure composition. Group 1 consist of Solemya (Petrasma) pervernicosa and Solemya (Solemya) tagiri. The outer layer of their shells is characterized by radially elongate simple prismatic structure (RESP) type A, and the inner layer by irregular prismatic structure. Group 2 is represented by Solemya (Solemya) pusilla. The outer layer of its shell has RESP type B structure, and the inner layer structure is homogeneous. Acharax japonica is in Group 3. Its outer shell layer has RESP type C, and the inner layer is characterized by laminar, homogeneous, and irregular complex cr¬ossed lamellar structures. Group 4 is represented by Acharax johnsoni. Its outer shell layer has a reticulate structure and the inner layer has a cone complex crossed lamellar structure. The organic content of both shell layers of A. johnsoni is highly contained as sheets. A. johnsoni is deep sea species that is typically found in chemosynthetic communities. Thus, organic sheets in their shells probably protect the shells against corrosion in deep sea. Four groups are categorized based on shell microstructure composition, but the groups are not consistent with previous solemyid systematics at the generic (Solemya and Acharax) and subgeneric (Petrasma and Solemya) level.

The phylogenetic analysis of mitochondrial 16S rRNA gene and nuclear 18S rRNA and 28S rRNA genes were performed in the sequences of five solemyid species from Japan with additional sequences of other solemyid and protobranch bivalves. Phylogenetic trees were reconstructed using the maximum likelihood (ML) method. The results of the molecular phylogenetic analysis and the observation of the shell microstructure were well agreed with each other. These results indicate solemyid shell microstructures may provide crucial characters for their phylogenetic grouping.

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First Name: 
Kei
Last Name: 
Sato
Telephone: 
819017883796
Affiliation: 
The University of Tokyo
First Name: 
Rei
Last Name: 
Nakashima
Affiliation: 
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology
First Name: 
Ryuichi
Last Name: 
Majima
Affiliation: 
Yokohama National University
First Name: 
Hiromi
Last Name: 
Watanabe
Affiliation: 
Japan Agency for Marine-Earth Science and Technology
First Name: 
Takenori
Last Name: 
Sasaki
Affiliation: 
The University Museum, The University of Tokyo
Choose keywords that are most applicable to your abstract. (Three maximum.): 
Evolutionary history (fossil and molecular data)
Abstract ID: 
CBE5-164