Transcriptomic analysis of the clam Meretrix meretrix on different larval stages

The clam Meretrix meretrix (Mollusca: Bivalvia) is an important commercial species in China. The deficiency of genomic and transcriptomic data is becoming the bottleneck of further researches on its complex and unique developmental processes. To improve this situation, in this study, a large-scale RNA sequencing was conducted on M. meretrix on larval stages. In particular, mRNAs of trochophore, D-veliger, pediveliger, and postlarva were purified, reverse transcribed, and sequenced through 454 sequencing technology. A total of 704,671 reads were obtained and assembled into 124,737 unique sequences (35,205 contigs and 89,532 singletons). Further analysis showed that 118,075 (94.66%) of these sequences were low-expression-level transcripts. Fifteen thousand two hundred fifteen (12.20%) of the unique sequences were annotated by searching against Uniprot Protein Knowledgebase, while the others (109,522, 87.80%) were left as novel sequences. Gene ontology analysis of the annotated sequences showed that most of them were assigned to certain gene ontology terms. By analyzing the depth of each unique sequence, a preliminary quantification analysis was conducted. An amount of sequences that showed a dramatic transcript discrepancy among the four larval stages were screened, which were related to development, growth, shell formation, and immune responses etc. As the first attempt on large-scale RNA sequencing of marine bivalve larvae, this work would enrich the knowledge of larval development of marine bivalves and provide fundamental support for further researches.