Expression of calcification-related ion transporters during blue mussel larval development.

The physiological processes driving the rapid rates of calcification in larval bivalves are poorly understood. Here, we use a calcification substrate-limited approach (low dissolved inorganic carbon, ) and mRNA sequencing to identify proteins involved in bicarbonate acquisition during shell formation. As a secondary approach, we examined expression of ion transport and shell matrix proteins (SMPs) over the course of larval development and shell formation. We reared four families of under ambient (ca. 1865 µmol/kg) and low (ca. 941 µmol/kg) conditions and compared expression patterns at six developmental time points. Larvae reared under low exhibited a developmental delay, and a small subset of contigs was differentially regulated between ambient and low conditions. Of particular note was the identification of one contig encoding an anion transporter (SLC26) which was strongly upregulated (2.3-2.9 fold) under low conditions. By analyzing gene expression profiles over the course of larval development, we are able to isolate sequences encoding ion transport and SMPs to enhance our understanding of cellular pathways underlying larval calcification processes. In particular, we observe the differential expression of contigs encoding SLC4 family members (sodium bicarbonate cotransporters, anion exchangers), calcium-transporting ATPases, sodium/calcium exchangers, and SMPs such as nacrein, tyrosinase, and transcripts related to chitin production. With a range of candidate genes, this work identifies ion transport pathways in bivalve larvae and by applying comparative genomics to investigate temporal expression patterns, provides a foundation for further studies to functionally characterize the proteins involved in larval calcification.