Gut structure and diet adaptability of yellowbelly flounder: A step towards integrating Rhombosolea leporina into New Zealand aquaculture

Abstract

Aquaculture is a rapidly expanding industry as wild fish stocks decline due to overfishing, climate change, and increased seafood consumption. Flatfish, being highly prized, have faced significant declines in the wild, but several species are now being cultivated in land-based recirculating aquaculture systems to meet market demand. Flatfish are low-trophic feeders and thrive in relatively simple farming setups, making them viable candidates for sustainable aquaculture. In New Zealand, the yellowbelly flounder (Rhombosolea leporina), or pātiki, is a sought-after species in commercial and recreational fisheries. Introducing this species into aquaculture could support the local seafood market and aid in replenishing wild stocks. However, limited research exists on yellowbelly flounder compared to other flatfish, particularly regarding its digestive system and dietary needs. Feed costs are a significant challenge in aquaculture, especially when striving to reduce reliance on unsustainable ingredients like fishmeal and oil while maintaining low feed conversion ratios. Understanding the digestive tract structure and function of the yellowbelly flounder is critical for developing sustainable, species-specific diets. This study examined the gut structure and function of wild-caught adult and juvenile yellowbelly flounder, as well as juveniles fed a mussel-based diet, a fishmeal-based pellet diet, or subjected to starvation. The histological analysis examined the muscularis externa, mucosal folds, and goblet cell distribution throughout the alimentary canal, while qPCR assessed the expression of the cholecystokinin hormone paralogs (ccka and cckb). Unexpectedly, findings suggested that yellowbelly flounder may be agastric, differing from other flatfish. Consistent gut tissue patterns, goblet cell presence, and ccka and cckb expression throughout the canal supported this conclusion. The pellet diet did cause higher ccka expression compared to the mussel diet, but overall, the results suggested the yellowbelly flounder is adaptable to varying feeds. While limited by a small-scale study, these results provided a basis for further exploration of yellowbelly flounder digestive capabilities in hopes of facilitating this species into the New Zealand aquaculture industry.