扁蟹奴 (Sacculina plana) 幼蟲發育及白紋方蟹 (Grapsus albolineatus) 遭寄生情形

Abstract

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).