非洲大蝸牛對東沙島皺紋陸寄居蟹族群結構的影響

Population Structure of Coenobita rugosus in Dongsha Island and Probable Role of the Invasive Species Achatina fulica

皺紋陸寄居蟹（Coenobita rugosus）是東沙島上數量龐大的優勢陸生物種，因島上缺乏大型海螺殼供應，許多個體背負著入侵種非洲大蝸牛（Achatina fulica）空殼。本研究旨在探討非洲大蝸牛對皺紋陸寄居蟹族群結構的影響，結果顯示皺紋陸寄居蟹體長分佈呈雙峰型，小個體利用海源螺種，大個體利用非洲大蝸牛殼，中間體型個體較少且佔據過大的蝸牛殼，善用非洲大蝸牛殼造成皺紋陸寄居蟹體型得以大幅提升。在此利用logistic regression model來描述與量化依賴非洲大蝸牛殼的程度，發現蝸牛殼使用率隨陸寄居蟹體長增加而驟增，兩樣區間有顯著不同的轉換殼種曲線，顯示釣魚區海岸蠑螺殼的供應比國碑一帶相對充足，可使陸寄居蟹緩慢漸次地改用非洲大蝸牛殼。本研究發現皺紋陸寄居蟹彈性適應有限殼源與海岸環境差異，或可借此監測台灣海岸的干擾與變化。

Coenobita rugosus is a dominant terrestrial species in Dongsha Island. Due to shortage of large shells, huge numbers of C. rugosus individuals are using the shell of Achatina fulica, a non-native species in this island. This study was aimed to investigate how A. fulica shell affects the population structure of C. rugosus. I find that the size frequency of C. rugosus demonstrated a bimodal distribution.The small-size population carried shell from marine snails while the large-size population carried A. fulica shell. Medium-size individuals were generally lacking in the statistics and they occupied A. fulica shell too big for their body size. These indicate that C. rugosus increased in average body size after A. fulica invaded Dongsha Island. The logistic regression model was used in this study to provide a quantitative description of shell preference in which C. rugosus individuals are increasingly carring A. fulica shell corresponding to the growing body size. The fitted curves derived from two sampling areas were significantly different indicating that more plentiful supply of turban shells on the fishing-allowed coast gave C. rugosus a wide-range of choices to gradually turn to using A. fulica as host shells. As found by this study the hermit crab is adaptive to host shells availability and environment changes, thus can be a useful resource for monitoring perturbation on Taiwan's coast.