Chapter 4. Oviposition and egg clutches in Achatina fulica
Introduction
Land snails generally show a wide variation in number of egg production from species to species. For example, A. fulica produces 1000 or more eggs in one brood (Ghose, 1959), while Boninena spp., an endemic species of the Ogasawara Island, does so only one or two eggs (Kurozumi, 1988). If the egg size have a close connection to the clutch size, the production of large size eggs may result in a reduction of clutch size.
Many studies have shown that A. fulica often produces a great number of eggs at once, although the reported clutch size and egg number differed greatly from author to author (Mohr, 1949; Bequaert, 1950; Rees, 1951; Ghose, 1959; Kekauoha, 1966; Nisbet, 1974; Pawson and Chase, 1984). Pawson and Chace (1984) concluded that such a large difference in reported values may be due to measurement error or inadequate sampling. However, their conclusion has not yet been confirmed.
In this study, I examined the oviposition and egg hatching of A. fulica in the field and laboratory, and many adults were dissected for examining the clutch size and the volume of individual egg.
Materials and Methods
1. Observations on oviposition and egg hatching
2. Dissection of pregnant snails.
The sampled snails from three sites, Mikazuki-yama, Miyanohama-michi and Yoake-yama, during June, 1987 and April, 1989, were boiled and their soft bodies were pulled out for preserving in 70% ethanol. Each snail was dissected to check whether it carried eggs. In dissection, all the number of eggs found in an uterus were considered to belong one clutch, and the length and width of an individual egg were measured for calculating egg volume. Egg volume (V) was calculated by the following formula:
is a half of egg length.
These characters were compared between two ages; middle-aged adults (intermediates) and fully grown adult (adult), since there was no subadult which had eggs. The above classification into three age categories (subadult, intermediate, and adult) was already described in Chapter 3.
Results
1. Oviposition and egg hatching of A. fulica in laboratory.
2. Observation clutch size and egg volume.
A thousand sampled snails were dissected, and about two hundreds were found to be pregnant. Sixty nine individuals among them carried eggs in fully formed shell. No subadult carried eggs, indicating their incomplete ability of egg production.
Clutch sizes observed in three local populations from Mikazuki-yama, Yoake-yama, and Miyanohama-michi, showed no significant difference each other (ANOVA, p>0.05). No significant seasonal difference in clutch size was also detected among the samples collected in the following five seasons (April, May, June, August, and October ; ANOVA, p>0.05). In other words, there are no local and seasonal differences in clutch size. Figure 13 shows histograms of clutch size in full grown adults and intermediates. Difference of clutch size between full grown adults and intermediates is significant (Mann-Whitney's U-test; P<0.001). This result indicates that full grown adults (=adults) produce larger clutchs than that of intermediates.
There was no significant difference among mean egg volume in three local populations from Mikazuki-yama, Yoake-yama, and Miyanohama-michi (ANOVA, P>0.05). Reciprocal differences of mean egg volume among the following five seasons, April, May, June, August, and October, were also not significant (ANOVA, p>0.05). Figure 14 shows distribution of mean egg volume in different clutchs obtained from full grown adults and intermediates. According to this graph, the mean values of egg volume calculated for full grown adults and intermediates were significantly different (Mann- Whitney's U-test; p<0.001). This result indicates the fact that full grown adults (=adults) produce larger eggs than that of intermediates.
Although there were no local and seasonal differences in clutch size and egg volume, these two characters show a great variation even in a local population. For example, the clutch size of Yoake-yama population varied from 20 to 137. About the same things can also be said on the variation of egg volume in a single clutch. As shown in Fig.15, the frequency distribution of egg volumes examined on egg from a single clutch collected different places considerably differed each other. Although actual figures were not shown in the graph, the mean volume of eggs produced by No.1140 individual was twice as large as that produced by No.1362, in spite of the fact that they were collected from the same locality, Yoake-yama in July, 1988.
Figure 16 shows the relationship between shell length and clutch size produced by snails with different shell length. Although there is a great variation, clutch size tends to increase as going to snails with large shell length. The correlation coefficient was calculated to be 0.417. In Fig.17, the same relationship was drown between shell length and mean egg volume calculated for each clutch. There is also positive correlation between these two characteristics (r=0.246; p<0.05). From these results, it can be said that large snails tend to bear large volume eggs in a large clutch.
Discussion
1. Variation of clutch size and egg volume in A. fulica
Certain characteristics of the egg clutch of A. fulica and three species of Archachatina are listed in Table 2. The clutch size is strongly variable in A. fulica. The volume (size) of an individual egg also shows a wide variation. It is interesting that different authors reported different mean sizes eggs. Pawson and Chase (1984) examined the reported data on these characteristics as compared with their data obtained from the culture of A. fulica under laboratory conditions. They concluded that a possible source of the reported variability in egg size might be brought about by measurement error, inadequate sampling, and nutritional influence on production. However, the results of the present study revealed that A. fulica tend to produce very variable clutch size and egg volume from individual to individual. These variabilities exist even within the same local population and same season. Consequently, it is concluded that A. fulica has a great inherent variability in the reproductive performance.
2. Ecology of egg laying in pulmonate land snails