蚯蚓照射紫外線A或紫外線B (UV-A or UV-B)後對爬行行為、呼吸及體表氧化壓力之研究

The study of crawling behavior, oxygen consumption and oxidative stress of skin in earthworms after UV-A or UV-B exposure

莊淑君(Shu-Chun Chuang)

指導教授 ： 陳俊宏

國立臺灣大學/生命科學院/動物學研究研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.01728

摘要

本文主要探討兩個主題：蚯蚓為何在大雨過後的夜晚到清晨爬出地表的原因及日出後為何不爬回土壤而死於地表。以往的學者常將”為何爬出地表”及”爬出地表後會死亡”視為一件事，但在本文中發現爬出地表的原因與耗氧有關，而紫外線的照射是蚯蚓致死的重要原因。
從呼吸生理的角度，綜合實驗結果及野外觀察，有一類蚯蚓包含了A. gracilis，會在下過大雨後的夜晚爬出地表，其耗氧量在夜晚(從晚上到清晨)較白天高，因此在大雨後，土壤飽含雨水，會將孔隙封住，使得土壤中氧氣降低，由於這類蚯蚓清晨時的活動力及耗氧量皆最高，因此當蚯蚓將土壤及雨水中氧氣用盡時，就會被迫此時必須爬出地表呼吸；但另一類包括P. corethrurus的蚯蚓，耗氧量較低，對無氧環境的耐受力高，因此不需在下過大雨後的清晨爬出地表，以求獲得更多的氧氣。
蚯蚓在爬出地表後會死亡的原因與紫外線有很大的關係，以UV-A及UV-B進行實驗發現，一般而言，UV-A所產生的傷害遠低於UV-B，不同蚯蚓對紫外線的耐受性也不同，P. corethrurus耐受性最佳，M. posthuma次之，而A. gracilis則最差。根據本文的研究發現這些耐受性與多種生理反應有關，如：呼吸調節，行為收縮，體表組織的氧化壓力等等有關。UV-B會影響Amynthas gracilis會的爬行能力並造成死亡率的增加。由照射時立即的行為反應及之後發生的體表病變，可分為急性及慢性反應，急性為蚯蚓照射時會立即出現肌肉強烈收縮，有不正常S型爬行行為及跳動；慢性反應則會造成表皮和肌肉細胞的破損及個體的死亡。在測量蚯蚓照射UV-B後耗氧會降低，但泡過水再加以UVB照射，其耗氧會明顯降的更低，推測是由於蚯蚓氧氣的取得是藉由濕潤的體表讓氧氣滲透，再經由肌肉的收縮使血液中的氧氣帶至身體各處，因此當紫外線照射後，體表的傷害會阻擋部分的氧氣進入血管中，且紫外線使肌肉無法正常運動，造成蚯蚓血液循環變緩，最後造成缺氧（耗氧降低）而死。在缺氧情況下，容易造成細胞的氧化壓力增加，再者，紫外線也可能直接影響體表組織的氧化壓力，在測量與氧化壓力有關的物質及酵素發現，蚯蚓體表組織在照射UV-B後，脂質過氧化現象會增加，抗氧化酵素(catalase及GPx)的活性會降低，並且直接測量其中的H2O2發現含量會增加，顯示氧化壓力亦是造成蚯蚓體表傷害及死亡的原因之一。
因此UV-B對A. gracilis的影響，可能開始於神經傳導的異常，造成肌肉的收縮不正常，爬行能力降低，並且出現體表的病變，這些變化導致蚯蚓的耗氧降低，所需氧氣無法獲得，加上體表細胞的氧化壓力增加，大量細胞死亡，最後造成蚯蚓的死亡。而P. corethrurus或M. posthuma沒有如此嚴重的行為及死亡現象，可能與在耗氧上的適應，體表氧化壓力的耐受性較佳有關，甚至可能身體具有一些物質可以有效保護他們免於紫外線的傷害，但此部分有待進一步實驗證實。

關鍵字

爬行行為 ； 蚯蚓 ； 紫外線 ； 氧化壓力 ； 呼吸

並列摘要

The main purpose of this thesis is to investigate two topics: 1. why do earthworms crawl out of soil at night after a heavy rain. 2. why earthworm do not crawl back into soil after sunrise and die on the ground? In the past, researchers considered them as the same. In this study, I found oxygen consumption was one of the major reasons for earthworms crawled out the soil and ultraviolet exposure is the lethal event for earthworms.
Two types of earthworms were investigated. Amynthas gracilis, may crawl out of the soil after a rain. However, the secondary types of earthworms like Pontoscolex corethrurus, never disperses after a rain, which suggest that they could stay in oxygen-deprived water for a couple days. A. gracilis has a diurnal rhythm of oxygen consumption which consumes more oxygen at night. This could explain why A. gracilis disperses on the soil surface at night after a heavy rain. When a rain is heavy enough to fill up the soil, the water in the soil content less oxygen for earthworm respiration. A. gracilis showed higher activity and consumed more oxygen at night, they were forced to surface at night after a heavy raining day to consume more oxygen.
After the earthworms surface the soil, they might die in few hours after sunrise. According to the results for this study, ultraviolet exposure is the cause of death. In general, UV-B has stronger damage effect than UV-A. Different species of earthworms have different tolerance to UV exposure. In this study, P. corethrurus showed the highest tolerance and A. gracilis is most sensitive to UV. The damage of ultraviolet radiation on earthworms can induce two responses: acute and chronic response. The acute response is charactering by the appearance of abnormally strong muscle contraction generate S-shape movement and jumping behavior. The phenomenon might be caused by bad coordination between circular muscle and longitudinal muscle in the earthworms. The chronic response showed damage of the skin and muscle cells and generate high mortality rate. The oxygen consumption of A. gracilis was significantly decreased after UV-B exposure. And the effect was more significant in the drown earthworms. Since the circulation of earthworms is mediated by muscle contraction, and the oxygen has to diffuse through their moisture skin, it is reasonable too explain abnormal muscle contraction and damaged epithelium could cause the stifle of earthworm. In addition, UV also increased oxidative stress on earthworm tissue. It was found the lipid peroxidation and the activity of antioxidant enzymes (such aaaaas catalase, GPx or SOD) were increased in the UV-B exposure earthworms. Therefore, I suggest that oxidative stress is the other reason that caused earthworms dead after UV-B exposure.
The adaptation of low oxygen consumption and high toleration of oxidative stress may explain why P. corethrurus and M. posthuma could alive under such ultraviolet exposure. They maybe have other specialized mechanisms to prevent the damage of ultraviolet exposure. However, it need more study to prove that in the future.

並列關鍵字

crawling behavior ； oxygen consumption ； oxidative stress ； earthworm ； ultraviolet

參考文獻

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