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Diseases of Aquatic Organisms

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DAO 156:71-80 (2023)  -  DOI:

Novel tissue thickening around the notochord sheath found in deformed Japanese eel Anguilla japonica leptocephali

Akihiro Okamura1,*, Yoshiaki Yamada1, Naomi Mikawa1, Noriyuki Horie1, Katsumi Tsukamoto1,2

1IRAGO Institute Co., Ltd., Tahara, Aichi 441-3605, Japan
2The University of Tokyo, Tokyo 113-8657, Japan
*Corresponding author:

ABSTRACT: Even though reared leptocephalus larvae of the Japanese eel Anguilla japonica have a high incidence of notochord deformities (>60%), the cause is unknown. We performed histological examinations of the notochord and associated organs in reared larvae to better understand the process causing notochord deformation in eel larvae. In deformed larvae, unknown tissue thickening was discovered near the notochord sheath. Azan staining revealed that these tissue thickenings are most likely collagen fibers within fibrous connective tissue. This was almost identical to the connective tissue found in the primordium of the vertebral body around the notochord sheath in properly metamorphosing larvae. Furthermore, the amount of the thyroid hormone triiodothyronine (T3) was significantly higher in deformed larvae than in normal larvae, indicating that notochord deformity is probably linked to metamorphosis despite the immature stage of growth. We suggest that the aberrant growth of connective tissue surrounding the notochord sheath induced by incomplete metamorphosis causes deformities in eel larvae. The reason why deformed larvae have greater thyroid hormone levels is still unknown. It is important to assess how environmental and dietary factors affect the thyroid hormone levels of eel larvae raised in captivity.

KEY WORDS: Deformity · Notochord · Leptocephalus · Collagen · Metamorphosis · Thyroid hormone

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Cite this article as: Okamura A, Yamada Y, Mikawa N, Horie N, Tsukamoto K (2023) Novel tissue thickening around the notochord sheath found in deformed Japanese eel Anguilla japonica leptocephali. Dis Aquat Org 156:71-80.

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