Thyroid hormone mediates otolith growth and development during flatfish metamorphosis

Area of Science:

• Developmental biology
• Endocrinology
• Ichthyology

Background:

• Flatfish larvae undergo metamorphosis, transitioning from bilateral symmetry to asymmetrical forms.
• Metamorphosis in flatfish is regulated by thyroid hormone (TH).
• The role of TH in teleost inner ear development, specifically otoliths, is not fully understood.

Purpose of the Study:

• To investigate the role of TH in the development of the three otolith end-organs (sacculus, utricle, and lagena) during southern flounder (Paralichthys lethostigma) metamorphosis.
• To determine how TH influences the growth and mineralization of these otoliths.

Main Methods:

• Southern flounder larvae were treated with methimazol (TH synthesis inhibitor) and exogenous TH.
• Otolith growth and mineralization rates were measured and compared between treatment groups and controls.
• Developmental stages of the sacculus, utricle, and lagena were analyzed.

Main Results:

• TH significantly increased the growth rates of sacculus and utricle otoliths during metamorphosis, uncoupled from somatic growth.
• Methimazol treatment inhibited sacculus and utricle otolith growth, while TH treatment accelerated it.
• TH induced mineralization of the lagena otolith, but not its growth, whereas methimazol inhibited mineralization without affecting growth.

Conclusions:

• Thyroid hormone plays a critical, differential role in the development of southern flounder otoliths during metamorphosis.
• TH differentially regulates the growth of the sacculus and utricle, and the mineralization of the lagena.
• These findings provide insights into the hormonal control of vestibular system development in teleosts.