Expression and activity of critical digestive enzymes during early larval development of the veined rapa whelk, Rapana venosa (Valenciennes, 1846)

Abstract

Metamorphosis is a vital developmental event in the life cycle of molluscs and involves extensive morphological and physiological changes. Remodeling of the digestive system is suggested to occur anticipatorily to enable the larva to shift its diet (from filter feeding on microalgae to feeding on small bivalves) after metamorphosis. Changes in the profiles and activities of digestive enzymes, the main executors of digestion, can reflect substantial remodeling of the digestive system. Artificial aquaculture of Rapana venosa, an important commercial shellfish in China, has been hampered because the transition of its food habit during metamorphosis makes determining the timing and dose for bait regulation difficult. In the present study, full-length cDNA sequences encoding cellulase and trypsin were characterized, and cellulase and trypsin mRNA expression levels were analyzed. Additionally, patterns in the activities of six digestive enzymes, including trypsin and cellulase, were investigated throughout the early developmental stage of R. venosa. In the present study, the full-length cDNA of the cellulase gene, comprising 2,086 bp, was found to contain a 1,719-bp open reading frame encoding 572 amino acids, and the full-length cDNA of the trypsin gene was found to be 1,587 bp in length and contained an 855-bp open reading frame encoding 284 amino acids. Quantitative real-time PCR showed that the cellulase levels in R. venosa increased beginning at the early intramembrane veliger stage, whereas cellulase activity was significantly increased in the one-spiral whorl stage. The mRNA expression and activity of trypsin were greatly increased in the juvenile stage (postlarva), whereas those of cellulase were decreased during this stage, which indicated functional changes in the digestive system during larval food habit transition. Our results showed that remodeling of the digestive system occurs prior to metamorphosis and suggest that animal bait should be provided as early as possible to R. venosa in the four-spiral whorl stage to meet its nutritional requirements for the development of its digestive system and to ensure successful metamorphosis of competent larvae.