The Beneficial Role of the Thyroid Hormone Receptor Beta 2 (thrb2) in Facilitating the First Feeding and Subsequent Growth in Medaka as Fish Larval Model

During the early growth stages of fish larvae, there are significant challenges to their viability, so improving their visual environment is essential to promoting their growth and survival. Following the successful knockout of thyroid hormone receptor beta 2 (<i>thrb2</i>) using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 technology, there was an increase in the expression of UV opsin (short-wave-sensitive 1, <i>sws1</i>), while the expression of other cone opsins was significantly decreased. Further analysis of the retinal structure demonstrated that the <i>thrb2</i> knockout resulted in an increased lens thickness and a decreased thickness of the ganglion cell layer (GCL), outer plexiform layer (OPL), and outer nuclear layer (ONL) in the retina. The slowing down of swimming speed under light conditions in <i>thrb2</i>^-/- may be related to the decreased expression of phototransduction-related genes such as G protein-coupled receptor kinase 7a (<i>grk7a</i>), G protein-coupled receptor kinase 7b (<i>grk7b</i>), and phosphodiesterase 6c (<i>pde6c</i>). Notably, <i>thrb2</i>^-/- larvae exhibited a significant increase in the amount and proportion of first feeding, and their growth rate significantly exceeded that of wild-type controls during the week after feeding. This observation suggests that although the development of the retina may be somewhat affected, <i>thrb2</i>^-/- larvae show positive changes in feeding behaviour and growth rate, which may be related to their enhanced ability to adapt to their environment. These results provide novel insights into the function of the <i>thrb2</i> gene in the visual system and behaviour and may have implications in areas such as fish farming and genetic improvement.