瀕危四斑細蟌稚蟲的糞便DNA條碼食性分析

Abstract

四斑細蟌(Mortonagrion hirosei)是棲息在半淡鹹水蘆葦草澤中的小型豆娘，此獨特棲地使其分布侷限值得保育關注。臺灣目前已知的四斑細蟌棲地僅有五股濕地、社子島濕地和磺港溪濕地。過去研究發現四斑細蟌的出現與蘆葦密度、鹽度及水深有關。不同棲地所能提供的食物資源可能不完全相同。研究物種在不同棲地的食性可以了解該物種的食性範圍，進而分析食性與特定棲地的相關性，做為保育工作中棲地維護之依據。常見的食性研究為直接觀察、分析消化道內含物及排遺。其中分析排遺花費成本低且為非侵入性的取樣，對於族群量小的物種較無影響。本研究使用排遺分析探討不同棲地四斑細蟌稚蟲的食性。測試的假說為社子島濕地的稚蟲，其排遺內的食物種類較磺港溪濕地的稚蟲豐富。假說預期不同棲地稚蟲的食性有明顯差異。從2020年8月至2022年7月間，每月於社子島之2樣點及磺港溪之4樣點中各調查1次，每個樣點調查時間為30分鐘，以細目篩網(網目≦0.7mm)進行稚蟲與棲地中底棲生物的採集。將稚蟲帶回實驗室並測量記錄其體長、頭寬、尾鰓長。在48小時內收集稚蟲第一次的排遺，然後使用95％酒精保存，以孑孓、紅蟲飼養稚蟲至下次採集時再野放回原棲地。調查期間兩樣區共採集232隻稚蟲，蒐集257個排遺進行DNA條碼分析鑑定稚蟲的食物種類，初步結果顯示，社子島四斑細蟌稚蟲獵物的種類較磺港溪多，其中社子島四斑細蟌稚蟲同類相食的比例較磺港溪低。

The four-spot midget (Mortonagrion hirosei) is a small damselfly that inhabits brackish reed wetlands. Due to its highly specialized habitat requirements, its distribution is extremely limited and thus of conservation concern. In Taiwan, this species is currently known only from three sites: Wugu Wetland, Shezidao Wetland, and Huanggang River Wetland. Previous studies have shown that the presence of M. hirosei is associated with reed density, salinity, and water depth. Since the food resources available may differ across habitats, investigating the dietary composition of this species in different locations can help assess its dietary breadth and the relationship between diet and habitat characteristics, thereby providing insights for habitat management in conservation efforts. Common methods for studying insect diets include direct observation, gut content analysis, and fecal analysis. Among these, fecal analysis is low-cost, non-invasive, and minimally disruptive—an important consideration for species with small populations. This study employed fecal DNA barcoding to investigate the diet of M. hirosei larvae across different habitats. We hypothesized that larvae from Shezidao Wetland would exhibit a more diverse diet compared to those from Huanggang River Wetland, reflecting differences in prey availability between habitats. From August 2020 to July 2022, monthly surveys were conducted at two sampling sites in Shezidao and four in Huanggang River. At each site, larvae and benthic organisms were collected using a fine-mesh sieve (mesh size ≤ 0.7 mm) during 30-minute sampling periods. Collected larvae were brought back to the laboratory, where body length, head width, and tail gill length were measured and recorded. Feces were collected within 48 hours and preserved in 95% ethanol. Larvae were fed with mosquito larvae and bloodworms until the next field survey, at which point they were released back to their original habitats. Over the course of the study, 232 larvae were collected, and 257 fecal samples were obtained for dietary analysis using DNA barcoding. Preliminary results showed that larvae from Shezidao Wetland had a greater diversity of prey species compared to those from Huanggang River Wetland. Furthermore, the occurrence of conspecific DNA—indicative of cannibalism—was lower in Shezidao samples than in those from Huanggang River.