Larval development and prevalence of Sacculina plana and its host Grapsus albolineatu

本研究針對台灣之扁蟹奴 (Sacculina plana) 在野外對白紋方蟹 (Grapsus albolineatus) 的寄生情形，以及在實驗室內的幼蟲發育及幼蟲選擇寄主偏好進行探討。扁蟹奴歸類於蔓足綱，對寄主白紋方蟹所造成的影響類似於其他蟹奴科種類對其寄主的影響。被寄生的白紋方蟹通常有多外體的現象，通常帶有一至三個外體的個體，最多則有七個外體，同一寄主身上的外體有處在不同發育狀態。白紋方蟹遭扁蟹奴寄生後的蛻殼成長率（10.5％）和未被寄生個體的蛻殼成長率（11.8％）沒有顯著差異。風吹沙測站寄生比率比香蕉灣測站高，但是香蕉灣測站處女外體的比率卻高於風吹沙測站，顯示寄生比率較低的地區，可能由於雄性幼蟲數少於高寄生比率地區，因此處女外體的比率會高於寄生比率較高的地區。根據幼蟲附著的位置及附著的偏好推斷，感染可能發生在白紋方蟹在潮池中蛻殼時。海浪強度可能是造成兩測站寄生比率不同的原因之一。 扁蟹奴幼蟲於25℃飼養環境下，無節幼蟲（nauplius）發育成腺介幼蟲（cyprids）需50-57小時，經歷四個齡期。無節幼蟲頭部前端具有一個無節幼蟲眼、兩側各有一未分節的角(frontal horns)，尾部則具有尾叉（furcal rami），另有三對附肢分別為第一觸角（antennule）、第二觸角（antenna）及大顎(mandible)。無節幼蟲及腺介幼蟲階段，雄性體型皆大於雌性；無節幼蟲階段，雌雄之間除了體長差異之外，雌性腹部亦較雄性腹部細。除此之外，雄性第四齡幼蟲的第一剛毛形態也異於雌性幼蟲，且第一剛毛附近有數個小孔存在。第一附肢的形態會隨著齡期產生變化，其餘的兩對附肢除了大小之外，形態上則沒有改變。幼蟲雌雄比例會隨著季節改變，春夏兩季以雌性幼蟲為主，冬天則是雄性幼蟲佔多數。 幼蟲附著時偏好軟殼蟹，軟殼蟹身上所佔的雌性幼蟲百分比顯著高於硬殼蟹。不論是軟殼蟹或硬殼蟹身上，皆未發現雌性幼蟲附著於剛毛基部、口器、關節薄膜處、爪、鰓、眼柄腹甲上，其餘地方則可見附著的雌性幼蟲。由於螃蟹殼厚度大於腺介幼體長度，因此硬殼蟹較不易被扁蟹奴感染。The study aims to investigate the relationship between Sacculina plana and its hosts, Grapsus albolineatus in Kenting area, southern Taiwan, and the larval development and preference at settlement of S. plana in laboratory. S. plana affected its host G. albolineatus as other sacculinids typically do affect their own hosts. Parasitized G. albolineatus usually carries one to three externae with a maximum of seven. Externae at different status might be found on one crab. The mean prevalence of S. plana at study area, Fengchueisha and Xiangjiaowan is 63.3％and 27.8%, respectively. The molt increment of parasitized crabs (10.5%) is not significantly different from that of unparasitized crabs (11.8%). Therefore, the mean carapace width of G. albolineatus at Fengchueisha that is smaller than that in Xiangjiaowan is not likely to result from the differences in growth ratio. Since the population densities of G. albolineatus at both sites are similar, it is suggested that the differences of environmental factors (mainly wave exposure) are likely to be the factors for the different prevalence of S. plana between the two sites. The larval development of S. plana includes four naupliar and one cypris stage. It takes about 50 to 57 h from hatching to cyprids at 25℃. The nauplius has a naupliar eye at the anterior end, a pair of frontal horns lateral and furcal rami at the posterior end. The nauplii also have three pairs of appendages, antennules, antennae, and mandibles. The body length of males is always larger than females in both nauplii and cypris stages. The abdomen of female nauplii is more slender than that of males and the morphology of seta 1 of both genders is also different. The morphology of antennule changes with different stages but morphology of antenna and mandible does not change, except for the size. There was a seasonal variation of sex ratio and higher percentage of females was found in summer and spring. The results of lab experiments showed that the percentage of female cyprids settling on recently-molted crabs was significantly higher than that those settling on intermolt crabs. Female cyprids never settle on the membrane of joints, the ventral surface of abdomen, eye stalks, claws, gills, the base of seta, or the mouthpart. It seems that larvae cannot penetrate hard shell crabs. Since G. albolineatus molts in tidal pools, it is suggested that the infection occurs in tidal pools (during molting).中文摘要……………………………………………………………………………...... i 英文摘要…………………………………………………………………...………....... ii Introduction…………………………………………………………………………….. 1 Taxonomy………………………………………………………………………..... 1 The adult stage of Rhizocephala – the interna...…….……………………………. 2 The adult stage of Rhizocephala – the externa......................................................... 3 Larval stage of Rhizocephala………………….…………………...………….….. 4 Effects of parasitism……………………………………………………................. 6 Sacculina plana and its hosts in Taiwan……………...…………………………… 7 Materials and methods……..…………………………………………………………… 9 Collection and maintenance of animals…………..……………………………….. 9 Field data collection……………………………………………………………….. 9 Observations on larval morphology……………………………………………….11 Settlement experiments……………………………………………………………13 Statistical analysis…………………………………………………………………14 Results………………………………………………………………………………… 15 Field observations………………………………………………………………... 15 Larval development……………………………………………………………… 19 Settlement experiments…………………………………………………………... 36 Discussion……………………………………………………………………………... 40 Field observations………………………………………………………………... 40 Larval development……………………………………………………………… 44 Settlement experiments…………………………………………………………... 47 Future works……………………………………………………………………... 48 Reference……………………………………………………………………………… 4