Different stages of spicule development were examined in the transcriptome of Onchidoris muricata, a gastropods nudibranch mollusk belonging to suborder Doridina. These mollusks possess unique subepidermal spicules that form during the early postlarval development. During ontogenesis, the spicules undergo a biomineralization lead to the formation calcareous spicules. The internal structure of the spicules is directly influenced by calcium deposition. Based on morphological data, three size groups of mollusks have been identified. In the first group (mollusks ranging in size from 100 um to 1 mm), the spicules are in the initial stages of formation and are loose. In the second group (individuals measuring 1 mm to 4 mm), the spicules undergo a maturation process, becoming harder and exhibiting a layered or concentric internal structure. The spicules of the third group (specimens larger than 4 mm) are fully mineralized and have a monolithic internal structure. The molecular mechanisms underlying spicule formation, mineralization, and regulatory processes are still unknown. In this study, we obtained transcriptomic sequences from three size groups of mollusks and performed a differential analysis of gene expression. The objective was to identify genes that might play a role in the mineralization of spicules in O. muricata. The visceral organ complex was removed in all molluscs except group #1. The sample for each group contained several molluscs. Each group contained 3 biological replicates. Total RNA was isolated using ExtractRNA reagent (BC032, Evrogen). RNA was enriched for polyA fraction, and a library with TruSeq2 adapters was prepared. Libraries were sequenced on Illumina NovaSeq6000 (150 PE).
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