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Abstract
Mollusca is the second largest phylum of the animal
kingdom, forming a major part of the world fauna. The
Gastropoda is the only class of molluscs which have
successfully invaded land. They are one of the most diverse
groups of animals, both in shape and habit. Among
gastropods, land snails (subclass: Pulmonata) are one of the
most numerous with almost 35,000 described species of the
world.
On reviewing literature conducted on land snails of
Egypt, as far as can be ascertained, most studies were focused
on Lower Egypt but Upper Egypt was neglected. So, the
present investigation was designed to fulfill this gap and to
promote and enhance the studies of land snails, especially
those which have economic importance.
The study aims to:
1- Make a survey of different terrestrial snail species
inhabiting different types of soil in Assiut governorate, Egypt.
2- Study the horizontal distribution of common terrestrial snail
populations in Assiut governorate to make the distribution
map.
3- Investigate the seasonal fluctuations of the terrestrial snail
populations and effects of some ecological factors (which my
have influence on terrestrial snail species) in Assiut city.
4- Study the life cycles of common snails.5- Study the effects of some substrates on certain traits of a
common snail.
To achieve the above mentioned goals, the following
procedures were taken:
For qualitative study, random land snail samples were
collected from 95 different locations in Assiut governorate,
during a period; from April 2007 till October 2008, to make a
survey for different terrestrial snail species present.
For quantitative studies, four sites from the above mentioned
locations were chosen. The first two locations were at Al-
Malga, Assiut, these are: Mangoes orchard and Mandarin
orange. The third location was a vegetated land at El-Wasta.
The fourth location was an ornamental farm of Assiut
University. For quantitative studies, monthly samples were
taken from the main four studied sites mentioned above, for a
period of one year; extending from October 2007 till
September 2008.
Some ecological factors including air and soil temperatures,
relative humidity, percentage of soil water content (%Wt. C.)
and percentage of soil organic matter content (%Org. M.),
light and soil pH were monthly recorded during the period of
study, as well as their seasonal fluctuations at the studied sites.
A part of the present study was focused on common snails
Monacha obstructa and Oxyloma elegans since they were the
most dominant terrestrial snail species recorded in the four
sites. The life cycles of these species were studied in laboratory, which will help in controlling these species since
they are considered as agricultural pests.
Effects of different substrates on certain traits of Oxyloma
elegans were studied.
The results can be summarized as follows:
The survey indicated the occurrence of a total number of
nine species. Four species were recorded for the first time in
Egypt namely: Pupoides coenopictus, Oxyloma elegans,
Vitrea pygmaeum and Cecilioides acicula. All the species
recorded belong to order: Pulmonata and include eight
families namely: Pupillidae, Vallonidae, Succineidae,
Zonitidae, Helicidae, Hygromiidae, Helicodiscidae and
Ferussaciidae.
The recorded species were divided into constancy classes,
and it was concluded that there was no constant snail species.
The accessory snails included Oxyloma elegans, Monacha
obstructa, and Eobania vermiculata, while the accidental
species included Vallonia pulchella, Theba pisana,
Helicodiscus singleyanus inermis, Vitrea pygmaea, Pupoides
coenopictus and Cecilioides acicula.
Four snail species were found in the four main investigated
sites. The four species recorded were: Monacha obstructa,
Oxyloma elegans, Eobania vermiculata and Theba pisana.
The total number of snail species collected from the four
studied sites during the period of investigation was 3117
individuals. from site I, 63 specimens were collected, constituting 2.02% from the total catch. The collected
specimens from site II were 82, constituting 2.63%. from site
III, 1017 specimens were collected, constituting 32.63%.
from site IV, 1955 specimens were collected, constituting
62.72%.
The study indicated that the highest number of snails
(Monacha obstructa, Oxyloma elegans and Theba pisana) was
obtained during winter and spring in all sites. In case of
Eobania vermiculata the highest number was obtained during
summer.
The effect of the studied environmental factors on the
abundance of the collected snails was studied and it was
concluded that:
1. The abundance of Monacha obstructa was positively
correlated with percentage of soil water content, percentage of
organic matter and soil pH.
2. The abundance of Oxyloma elegans was negatively
correlated with air temperature and positively with relative
humidity, percentage of soil waster content and pH.
3. The percentage of soil water content was positively
correlated with the abundance of Eobania vermiculata and
Theba pisana.
4. The total abundance of the four snail species is affected
positively by the percentage of soil water content.
Studies of the population dynamics of Monacha obstructa
and Oxyloma elegans indicated that:In case of O. elegans, the percentage of young snails
during different seasons was comparatively low, since it
ranged between zero in autumn and 20% in spring. In case of
M. obstructa, the percentage of young was comparatively
higher in different seasons than that of O. elegans. In M.
obstructa, it could be noticed that young flourish in spring
(30%) but they reached minimal percentages in summer (7%)
and winter (20%). In case of adults of both Monacha
obstructa and Oxyloma elegans, it was observed that they
were generally present with high percentages during all
seasons. The percentages of adults in summer were markedly
increased inspite of decreasing the population in general. In
case of the percentages of olds of the two studied species, it
could be noticed that in Monacha obstructa, this percentage
was markedly lower than Oxyloma elegans.
Studies of the life cycle of Monacha obstructa and
Oxyloma elegans:
The study indicated that there were several differences
between the two studied species. The lifespan in M. obstructa
took about 12 months while in O. elegans it took about 8
months. It was noticed that in Monacha obstructa, laying eggs
occurred during two periods. The first period took about six
months (from mid-autumn till mid-spring) while the second
period took about two weeks at the beginning of autumn. In
case of Oxyloma elegans, laying eggs took about 6.5 months,
starting from mid-October until the end of May. In the present
study, it was recorded either in laboratory or in the field that in Monacha obstructa and Oxyloma elegans eggs are absent
during summer months. Also, many traits (14 characters) of
Monacha obstructa and Oxyloma elegans were recorded
during rearing and concluded that the two species differ in the
studied traits.
Study the effects of cross fertilization and self fertilization
on certain traits of reared Monacha obstructa and Oxyloma
elegans:
For both investigated species, there were significant
differences in the three traits between single and paired
individuals, these traits are:
1: No. of clutches laid/snail: there was a marked increase
in the paired snails than in single one. This increases the
fecundity.
2, 3: Mean egg weight and diameter of egg: there was a
marked increase in single individuals than in paired
individuals.
In case of the trait: mean hatchling size; there was a
marked increase in paired snails in Monacha obstructa and the
opposite was observed in Oxyloma elegans.
Study the effects of substrate on certain traits of Oxyloma
elegans:
The effects of substrate on certain traits of Oxyloma
elegans could be concluded that there are highly significant
differences in all the studied traits except in the two traits
namely: length of adult and weight of adult. It could be
concluded also, that the soil as a substrate, is the most suitable habitat for the snail, followed by leaves. The least suitable
substrate was the twigs, since none of eggs hatched in this
case and the majority of traits were markedly reduced
including length of adult when die, lifespan, no. of clutches
laid/snail, mean no. eggs/clutch, no. of eggs/snail, mean egg
weight and diameter of egg.