Physiological Mechanisms for Degeneration of Blind-Side Hypermelanosis in Pond-Cultured Japanese Flounder (Paralichthys olivaceus)

Abstract:

Blind-side hypermelanosis is a serious problem among intensively cultured Paralichthys olivaceus, resulting in significant economic losses. To date, no treatment is available to prevent this problem due to lack of knowledge. However, the degeneration of blind-side hypermelanosis was observed among pond-cultured P. olivaceus recently. To investigate the possible mechanisms, three types of P. olivaceus were identified based on the pigmentation on the blind side: normal blind-side type (NBS), blind-side hypermelanosis type (BSH), and blind-side degeneration type (BSD), and used for the comparison of morphological and associated endocrine changes at hormonal and mRNA levels. For BSD type, the number of melanocytes in the hypermelanosis-degenerated parts of blind side was reduced significantly comparing to those in the eye-side parts and in the blind-side parts in the BSD type. Moreover, the scales in the hypermelanosis-degenerated parts experienced the transformation from ctenoid to weak ctenoid, and cycloid shape. Meanwhile, the number of ctenoid scale spines on the blind- side decreased in hypermelanosis-degenerated parts among BSD fish. The plasma melanocyte-stimulating hormone (MSH) and melanin-concentrating hormone (MCH) levels were determined and compared between the three types of P. olivaceus. Our results showed that the BSD plasma MCH level was significantly higher than that of NBS and BSH, whereas the BSH had the highest MSH level among all three types of fish. Gene expression analysis showed that the BSD pituitary MCH mRNA level was remarkably higher than that of BSH, but for POMC mRNA level, the BSD was significantly low. Results from the present study significantly improved the understanding of the hyperpigmentation on the blind- side of P. olivaceus and help to develop the protocol to control this problem in aquaculture production.