Abstract
Giant African snail, Achatina fulica (Ferussac), which was native in East Africa, was distributed by humans in Ryukyu, Amami, and Ogasawara Islands in Japan as well as other subtropical areas in the past. Observations of behavior of this species would be easy in the field because of its large body size. This species is also suitable for study of reproductive ecology. There are a tremendous number of studies on ecology of A. fulica. However, no one has investigated the reproductive ecology of this species in terms of evolution. In this study, I investigated the reproductive ecology of A. fulica, in relation to shell growth, maturation, copulatory behavior, and egg production in the field and laboratory, and the evolutionary significance of reproductive behavior of this species was discussed.
This species matured in two years after hatching and shell growth stopped at sexual maturity. The matured shell size was very variable. The growth rate of shell length was estimated from peristome thickness. There was a correlation between shell growth and sexual maturity. The reproductive gland was formed in one year after hatching, but shell growth continued in the following year. In most land snails, lip reflection is commonly considered to indicate sexual maturity. However, in A. fulica , lip reflection was not observed. Shell growth continued for a while even after reproductive system developed fully. The subadult produced only sperm. The adult produced both sperm and eggs, thus become complete hermaphrodite at this age. Clutch size varied from 3 to 200, and egg volume also from 35 to 85 mm^3. There were no seasonal and local differences in clutch size and mean egg volume. Clutch size and egg volume were positively correlated with shell length of parents. The snail age was classified into three categories by the following criteria: Juvenile - possessing immatured reproductive system, Subadult - possessing considerably developed reproductive system, but its shell growth continues, Adult - possessing full developed reproductive system and its shell growth stops.
From the trace of radio-tagged individuals, it was certain that the snail is nocturnal. Locomotive activity differed in its pattern and strength between adults and juveniles. Juveniles tended to move straitly in arbitrary directions, and the longest distance moved during 6 months was 500m. Both subadults and adults had their home ranges, but the ranges was wider in subadults than that in adults. Adults were supposed to have a distinct homing behavior. This species was not capable of self-fertilization. They copulated with other conspecifids and received allosperm from the mating partners to fertilize their eggs. Copulations were generally observed in nighttime, and the mean copulation time was 4.6 hours. Copulation time was positively correlated with amount of injected sperm. A simple pre-copulatory behavior proceeded copulation. Courtship behavior was completely different between initiator (upper-positioned snail) and acceptor (lower-positioned snail).
Success copulation rate after the spell of courtship was only 10.8%. Copulation was frequently rejected by the acceptors. The rejection of copulation often occurred between a small subadult and large adult. The adults preferred to copulate with the individuals large in body size. Subadults injected more larger amount of sperm than by adults. Subadults courted actively, and copulated more frequency than adults. Although subadults preferred to copulate with the adult large in body size, such a preference was week as compared with that of adults. A large difference in body size between subadult and adult brought about a failure of copulation.
It was concluded that A. fulica is protandraous in its reproduction. Since subadults were able to produce only sperm, it was thought that they would be desirable to copulate with adults with many eggs. In fact, subadults moved around a wide area for searching their mating partners and courted actively. On the other hand, adults produced both sperm and eggs. They staid within a narrow areas and showed passive pre-copulatory behavior. Both subadult and adult sometimes rejected transient partner when that partner was undesirable. The differences in copulatory behaviors between subadults and adults had a close relation to the deferences of reproductive conditions between them, especially the developmental status of reproductive organs.
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