大里溪產霍甫水絲蚓Limnodrilus hoffmeisteri（Claparede）之生活史與族群分布

Life History and Population Distribution of the Tubificid Worm Limnodrilus hoffmeisteri (Claparède) in the Dali River

本研究經由飼養實驗，探討霍甫水絲蚓（Limnodrilus hoffmeisteri）之生活史中各生長階段所需時間和形態改變，並依形態特徵將其成長階段區分為卵繭期（cocoon stage）、幼年期（larvalstage）、青年期（juvenile stage）及成年期（adult stage）四大階段，並建立其體長、體節數之生長曲線。實驗發現養殖之個體最大體長可達3 cm，體節達143節。孵化後初期生長迅速，但性成熟後（約孵化後37日）因其可能將大部分能量用於產生卵繭，其生長速率即變緩。1999年12月至2001年2月間，於台中縣大里溪之中、下游區段分別調查霍甫水絲蚓生物量與其族群組成，探討其族群變化與環境因子間之關係，發現攜卵成蚓和幼年個體數量在冬、春季達高峰，推論大里溪中霍甫水絲蚓主要的繁殖季應在冬季至春季。其喜好生長在土壤有機物含量高及土壤生物量多的環境中，因此都市廢水污染嚴重之中游段族群量明顯高於下游。值得注意的是1999年秋九二一地震造成之擾動使冬季土壤有機物含量達高峰後，水絲蚓族群量亦由多變少，顯示霍甫水絲蚓的族群量有隨底質中有機物質減少而降低之情形。 Life cycle of the tubificid worm Limnodrilus hoffmeisteri (Claparède) may be divided into fourstages: cocoon, larval, juvenile, and adult. We briefly described morphological characters of the stages,and established daily growth curves of body length and segment numbers. The worms reached themaximum of 3 cm in length, and 143 in segment number in the laboratory culture. They grew rapidly at the larval and juvenile stages, but the growth decreased at the adult stage about 37 days after hatching. Asthe Dali River flows through Taichung City, its midstream section is seriously polluted with domestic andindustrial wastes. In order to study impacts of seasonal environmental changes on the worm distribution,we sampled the worm to estimate its abundance, biomass and life stage distribution in the middle anddownstream sections of the river from December 1999 to February 2001. The populations of oogeneticadults and larvae fluctuated seasonally, and the abundance of larvae and juveniles were at a peak inwinter and spring, suggesting the period as the reproduction season. The results of principal componentanalysis between the worm populations and environmental features, indicated that the worm preferredsoil substrate containing high concentrations of organic matters and high biomass. Therefore, it was moreabundant at the midstream section polluted with sewage discharges than that at the downstream sectionwhere the polluted water was diluted by inflow of clean water from tributaries. The 921 Earthquake of1999 caused mud-slides in the mountains upstream, increasing sedimentation in the Dali River.Subsequently, organic matters in the sediment decreased, with it decreased in the worm population. Theresults of this study suggested that the tubificid worm population in the Dali River fluctuated withchanges in organic matter contents in sediment.