探討光潤金線蛭（Whitmania laevis）捕食有口蓋淡水螺類之偏好

Foraging Preference of the Leech Whitmania laevis on Two Operculated Freshwater Snails

光潤金線蛭（Whitmania laevis）是台灣平原緩流水域中以淡水螺類為主食之肉食性水蛭。本實驗旨在探討光潤金線蛭對共域的兩種具口蓋淡水螺類－－福壽螺（Pomacea canaliculata）與網蜷（Thiara tuberculata）－－的捕食能力及偏好，並且嘗試了解產生捕食偏好之原因。實驗結果顯示體長7㎝以上、體重介於0.5-0.7g之光潤金線蛭對均明顯偏好取食殼口長5㎜以下的網蜷及福壽螺個體。當同時提供光潤金線蛭最適合捕食尺寸-殼口長2.7至3.7㎜的網蜷與福壽螺時，則光潤金線蛭對網蜷有明顯的取食偏好。實驗發現螺類的味道並非造成偏好之因素，反之若將福壽螺殼口蓋破壞，則光潤金線蛭捕食福壽螺的偏好便顯著上升且高於共存的網蜷。因此推論殼口蓋密合度為造成光潤金線蛭捕食偏好之重要因子。根據實驗結果推論光潤金線蛭捕食偏好應可以最適捕食理論（Optimal foraging theory）來解釋，亦即不同螺類口蓋密合度之差異影響光潤金線蛭捕食所需付出能量，連帶的使捕食經濟效益改變而使光潤金線蛭產生對口蓋密合度較差的螺類之捕食偏好。 This study investigated feeding preference of the carnivorous leech Whitmania laevis on twooperculated freshwater snails, Pomacea canaliculata and Thiara tuberculata, inhabiting in the sameaquatic environment. Adult leeches longer than 7 cm with weights between 0.5 and 0.7g significantlypreferred to feed on both species of the snails with aperture length less than 5 mm. When the snails with apertures of 2.7 to 3.7 mm long were provided, the leeches preferred to feed on T. tuberculata than P.canaliculata. This is attributable to the fact that the body of the latter prey was better sealed withoperculum than that of the former prey. When the upper corner of P. canaliculata operculum wasartificially removed, the feeding preference distinctly shifted from T. tuberculata to P. canaliculata,suggesting that the snail operculum affected the feeding preference. The feeding preference wasindependent of the snail body odor. The foraging preference of W. laevis may be explainable by'optimal foraging theory,' i.e. the degree of opercular tightness of the snails affects the energy cost andforaging efficiency of the leech.