日本鰻銀化階段鱗片發育及皮膚結構之改變

Abstract

本研究首次利用組織學的方式，除了確認鱗片重疊程度與鰻魚銀化體色呈現之關聯性，並獲得日本鰻皮膚中purine結晶的清楚結構，藉此判定鰻魚的銀色體色呈現是由iridophore所造成。 對降海洄游的鰻魚而言，銀化是其必經歷程，在銀化過程中，鰻魚在型態、生理及行為上皆發生改變，以適應即將面對的海水環境，是以銀化過程被視為變態。體側皮膚金屬光澤的出現是判斷銀化的一項重要特徵，亦是其最原始的定義，此色澤為一種結構色，會受到鱗片重疊及嘌呤結晶累積的影響。在鮭魚的研究，甲狀腺素在銀化過程中扮演著重要角色，雖然對於鮭魚及鰻魚的銀化目的相同，然而目前對於鰻魚銀化的判斷標準，則多與性成熟相關，性腺發育被當做判斷銀化標準的主要指標。本實驗聚焦於淡水養殖日本鰻鱗片排列及組織結構對於外部體色的關連。本實驗對淡水養殖之日本鰻皮膚進行採樣，藉由光學及穿透式電子顯微鏡觀察其結構與鱗片堆疊情況。鰻魚之鱗片包覆於真皮上層內，鱗片覆蓋之面積增加及鱗片間重疊情況已可在淡水銀鰻中觀察到，並伴隨著金屬光澤的出現。本研究首次對日本鰻皮膚中purine結晶的型態進行描述，更進一步辨認出下在鰻魚中purine結晶是以iridophore之形式存在。本實驗亦藉由外源性藥物的處理，觀察甲狀腺素及雄性素對日本鰻銀化過程所造成的影響，並發現以thiourea (TU)處理之魚隻腹部以及胸鰭有黑色素累積的情況出現。此結果顯示日本鰻的銀化，可能在降海前便近乎完成，與性成熟間的關聯並不明顯，且鱗片重疊伴隨著皮膚內嘌呤結晶的累積。甲狀腺素可能參與銀化的早期階段，但銀化的完成以及進入生殖洄游階段，必需在甲狀腺活性下降後才會發生。

These results for the first time provide the silvering color of Japanese eel is asso-ciation with scale arrangement increase and determine the existence of iridophores in eels by the ultrastructure of the purine crystals deposition in the skin of the Japanese eel. Silvering is a required process in catadromous eels’ downstream migration. Dur-ing silvering, the morphology, physiology and behavior in eels are changed for adap-tation to the upcoming ocean life stage. The ventral skin exhibited metallic silvering is the original definition and important characteristic of silvering. Silvery body color of fish is a structural color, which is caused by overlapping of scales and purine accu-mulation in skin. In salmonids, thyroid hormones play an important role in smoltification. Although the salmonid smoltification the same as eel silvering, the sil-vering of eel are generally regarded association with sexual maturation, the silvering stage are determine by the status of gonad development. This study focus on association between skin-scale complex and skin color of cultured freshwater Japanese eels. The skin was excised from flank region of Japanese eels which cultured in fresh water, and observed by light and transmission electron microscope. Iridophores under the outer chromatophore layer can be found. Scales were embedded in the upper of the dermis. In cultured freshwater Japanese eels, scales develop intercrossly and are overlapped in appearance accompany with metallic hue. This is the first time iridophores have been found in skin of eels. The effect of thyroid hormone and androgen on silvering process was preliminary studied by ex-ogenous drugs. In thiourea (TU) treated eels, melanization can be found in ventral skin and pectoral fins. The results suggest that the scale development in Japanese eel has been started or even finished in the fresh water, not directly induced by gonad maturation. Scale arrangement increase may be accompany with deposition of purine crystals during silvering process. Thyroid hormone may stimulate the onset of silvering, but a complete of silvering or spawning migration behavior express may start after the activity of thyroid gland was decrease.