Chapter 3. Growth Pattern and Protandry of Achatina fulica
Introduction
In most cases of pulmonates that form reflected peristome lips, lip formation synchronizes with maturation of reproductive glands, thus lip reflection is commonly considered to indicate sexual maturation (Wolda, 1970). In the case of Achatina fulica, lip reflection does not occur, and shell continues to grow for a little while after full formation of reproductive system (Ghose, 1959; Pawson and Chase, 1984).
Pawson and Chase (1984) showed the growth pattern of A. fulica under laboratory condition, where the temperature was kept in the range from 20 - 24with light and dark cycle of 12:12. The snails showed rapid growth in the first month after hatching. In 4 to 6 months, they slowed down their growth and then sexually matured. Before 150-day old, sexual organs were found to be matured by dissection. In this age, pre-copulatory sexual behaviors have been observed (Pawson and Chace, 1984). Shell growth stopped about 3-6 months after the formation of reproductive system (Berry and Chan, 1968).
In A. fulica, the thickness of peristome lip is usually very thin while the shell was still growing. Fully matured snails stop shell growth, however, it is possible that calcium deposition would occur on the inner surface of shell thereafter. Thus, peristome would become thick even in fully matured snails. If it is the case, the peristome thickness would be an index useful for judging their sexual maturity. Unfortunately, there is no study dealt with the relationship between shell growth and peristome thickness in A. fulica. In this study, I investigated the reciprocal relationships between shell growth, maturation of reproductive system and peristome thickness.
Materials and Methods
1. Dissection of reproductive system
Many snails at different growth stages were collected from the study site in Chichi-jima in July, 1987. Collected specimens were boiled, and their soft bodies were pulled out. The soft bodies were preserved in 70% ethanol for dissection.
2. Growth pattern trace by mark-recapture experiment
To investigate the relationship between peristome thickness and shell growth in the field, a mark-recapture experiment was conducted from June, 1987, to November, 1988. All individuals found in a permanent quadrate (30m~ 30m), which was subdivided a 1m~1m grid with polyvinyl chloride stakes as marks, were marked and traced, since the grid enabled us to record the position of marked snails in the quadrate. To avoid the crushing of snails hidden under fallen leaves by feet at the time of survey, five hundred wooden steps, 25cm long~25 wide, were placed on the ground. The survey was made in daytime.
At the time of survey, the snails found were inspected on their individual mark numbers, then measured shell length for judging their age as defined just below, and then returned to the spots where respective individuals were found. The snails newly found were also marked and released there. Individual mark numbers were directly sculpted on the last whorl of the shell with a hand dental drill. It is ascertained that this marking technique gave no effect upon behavior and survival of the snail.
The survey was repeated 9 times as in the following schedule: 2 June, 20 July and 15 September, 1987, 4 February, 25 March, 6 May, 20 July, 8 August and 25 November, 1988.
In these cases, the snails found in the quadrate were classified into three categories according to peristome thickness; Subadult - possessing thinner peristome than 0.5mm, Intermediate (=middle-aged adult) - thickness of 0.5mm - 0.8mm, and Adult (=full grown adult) - thicker more than 0.8mm. All these snails were known to possess a developed genital system. 3. Comparison of genital system between adults and subadults.
To investigate the difference of genital systems between adults and subadults, typical snails belonging to the above mentioned 3 categories were chosen and dissected, but intermediate snails were not dissected. These samples were collected on 21 July, 1988, and the number of dissected animals were 104 in total, including 52 subadults and 52 adults. Before dissection, snails were boiled, and soft bodies were pulled out from shell for preservation in 70% ethanol solution.
Results
1. Morphology and function of genital system
2. Relationship between shell growth and peristome thickness
Figure 5 shows the histograms of shell length measured at the start of experiment on 2 June, 1987. The graph was shown separately for the following 3 groups; subadults, intermediates, adults. The graph for the total of these three groups was also added. According to the graph, the distribution of shell lengths shows a considerable difference between groups, and a mean was calculated to be 34.3mm in subadults, 52.5mm in intermediates, and 61.2mm in adults. The reciprocal differences between these means were significant (t-test, p<0.001). However, the graph for the total was shown by a continuous distribution with a mean of 45.9mm. In other words, if intermediates adults were neglected, subadults and adults are possible to separate, although all the individuals belonging to these three groups possess a developed reproductive system.
Figure 6 shows the same histograms obtained at the end of experiment. When we compare this figure to the above mentioned one, the histograms for subadults, intermediates and the total moved to right side as a whole. This means that there occurred shell growth during the period of experiment, except the case of adults.
Figure 7 shows the the histograms of shell growth rate in percent per month during the whole experimental period expressed by percent. According to this graph, the shell growth rate was most higher in subadults as compared with that in intermediates. Adults grown in a quite low rate. From these results, it is clear that the shell growth of this species gradually become slow with age. This fact was also clearly indicated by the graph showing the relationship between initial shell length and shell growth rate in Fig. 8.
Figure 9 shows the histograms of peristome thickness measured on subadults, middle-aged adults, and adults at the end of experiment, 25 November 1988, together with the graph for the total. The distribution pattern of frequency is quite resemble to that of shell length shown in Fig. 5. From this result, it can be said that peristome thickness have a close connection to the shell growth. Therefore both characteristics will become to be an useful index representing the reproductive status of each individuals. This fact also reconfimed by Fig.10 showing the relationship between shell length and peristome thickness.
3. Comparison of genital system between subadults and adults
The frequency distribution of shell length in subadults and adults was overlapped considerably, but a mean in the former (53.8mm) is significantly longer than that in the latter (61.8mm). On the other hand, the frequency distribution of protein gland weight in subadults and adults showed a great difference. In the case of subadults, there was no individual possessing a heavy protein glands more than 517mg, while in the case of adults we found the individual possessing the gland of 3110mg. In addition, the weight of protein gland among adults showed a great variation from 33mg to 3110mg, with a mean of 847.4mg (Fig. 11). The protein gland is generally primitive in subadults, but ready to develop in adults. However, as shown in Fig.12, there was no large difference in bursa copulatrix weight between subadults and adults, indicating the fact that they are ready to copulate. It is very interesting that we can judge whether or not a given individual already copulated several days before by the size of bursa copulatrix, because it became large after copulation. In subadults, those individuals which have small bursa copulatrix without gel like mucus in it are able to judge as virgin. Discussion
1. Self-Fertilization
In some early studies on the reproduction of Achatina fulica (Mohr, 1949; Ghose, 1959), it was thought that the snail is able to self-fertilize, since copulation was not observed between those snails which produced viable eggs. It has also been shown by Owiny (1974) in another achatinid snails (Limicolaria martensiana) that they can store allosperms received from mating partner and use them when it is becomes ready to breed, sometimes several months later. Kekauogha (1966) found that the individuals of A. fulica maintained in isolation never produced viable egg clutchs. Mead (1979) summarized all available evidences at that time and concluded that viable self-fertilization does not take place in A. fulica. From these reasons, it become to be considered that A. fulica has to copulate and get sperm form mating partners to fertilize their eggs.
2. Maturation of A. fulica.
In the present study, I showed that a growing individual have a very thin peristome. On the other hand, the individuals with thick peristomes show almost no shell growth. It is considered, therefore, that the peristome thickness becomes to be an useful index for determining whether or not a given individuals is in growing stage. Berry and Chan (1968) reported the reproductive condition of A. fulica, and concluded that, when protein glands were well developed, a significantly large number of eggs was produced, and when protein glands were
not developed, no or a few eggs were produced.
3. Protandry