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Abstract
Phylum Mollusca is probably the third most
successful animal group after the arthropods and the
vertebrates. Slugs and snails belong to class Gastropoda,
generally characterized by having a distinct head with
tentacles and eyes, a broad flattened foot and a dorsal
visceral mass at least partly covered by the mantle.
Terrestrial slugs have proved to be one of the most
successful of all groups of mollusks. The reduction of the
shell has lessened the need for calcium salts with the result
that slug can live in a wider range of habitats than most
snails. Slugs are tolerant of water loss and the compressible
and worm–like body allows the animal to squeeze through
crevices in the soil in search of shelter.
In recent years, slugs have become increasingly
important, partly because several species are agricultural
and horticultural pests and partly because they have proved
to be useful experimental animals, particularly in the field of
neurophysiology. Also, slugs are used in drug industry as
slug syrup which is used to cure cough and bronchitis by
supporting the self-healing forces of the body. Moreover,
some slugs and land snails act as intermediate hosts for
helminth parasites causing serious diseases to human and
domestic animals.
To the best of the present author’s knowledge, the
knowledge about the Egyptian terrestrial slugs is still fragmentary, and partial information is available about their
distribution, their biology, and the ecotoxicological studies of
heavy metals on this group. Also, the majority of studies were
carried out in lower Egypt but Upper Egypt is completely
neglected. So, the present work is an attempt to fulfill this gap
and it aims to:
1- Make a survey of different terrestrial slug species inhabiting
different types of soil in Assiut governorate, Egypt.
2- Investigate the distribution and the seasonal fluctuations of
the common terrestrial slug populations and the effects of
some ecological factors which my have influence on terrestrial
slug species.
3- Study the life cycle of the common slug.
4- Investigate the seasonal changes of maturation in the
hermaphrodite gland of the common slug.
5- Investigate the difference in the accumulation behavior of
heavy metals in the terrestrial slug and an isopod species.
6- Study the impact of cadmium on digestive gland cells of
the common slug.
To achieve the above mentioned goals, random slug
samples were collected from different habitats located at
Assiut governorate, to make a survey for the different
terrestrial slug species. The samples collected included 39
samples, from Assiut governorate. The survey indicated the
occurrence of a total number of eight species. All the species
recorded belong to one order: Eupulmonata and two families;
namely: Limacidae and Agriolimacidae. The first family included two genera: Lehmannia and Limacus while the
second family included one genus: Deroceras. Genus
Lehmannia included three species namely: L. valentiana, L.
marginatus, and L. nyctelia. Genus Limacus included one
species: Limax flavus. Four slug species belonged to genus
Deroceras, namely: D. laeve, D. panormitanum, D.
reticulatum, and D. sturanyi.
The recorded species were divided into constancy
classes, and it was concluded that the constant slug species
was: Deroceras laeve. The accessory species was: Deroceras
panormitanum. The accidental slug species included 6 species,
namely: Deroceras reticulatum, D. sturanyi, Lehmannia
marginatus, L. valentiana, L. nyctelia, and Limax flavus.
For quantitative studies: Monthly samples were
taken from three main sites, at Al Malgaa, Assiut for a period
of one year; extending from April, 2005 till March, 2006.
Some ecological factors including air and soil temperatures,
pH, water content (Wt.C.), organic matter (Org.M.) were
monthly recorded during the period from April 2005 to March
2006, as well as their seasonal fluctuations at the studied sites.
Three slug species were found in these main investigated sites.
The three species recorded were: Deroceras laeve, Deroceras
panormitanum, and Deroceras reticulatum. The diagnostic
characters of the collected slug species are illustrated in the
investigation.
The total number of slug species collected from the
three studied sites during the period of investigation (from April 2005 till March 2006) was 192 individuals. from site I,
71 specimens were collected, constituting 36.98%
from the total catch. The collected specimens from site II were
78, constituting 40.63%. from site III, 43 specimens were
collected, constituting 22.40 %. Deroceras laeve was the most
dominant species recorded in the three sites.
The study indicated that the highest number of slugs
was obtained during spring in all sites. It was observed that, an
increase of density occurs from summer to autumn, from
autumn to winter and from winter to spring in all sites.
The effect of the studied environmental factors on the
abundance of the collected slugs was studied and it was
concluded that:
1. The densities of D. panormitanum (r= -0.376), (r= -
0.383), (r= -0.403), were negatively correlated with air
temperature, soil temperature, and soil pH respectively.
2. The data obtained clearly indicate that there is no
noticeable significant correlation between the
abundance of D. laeve and D. reticulatum with all
studied factors.
3. The interaction of all factors (multiple regression) has
highly significant effect on the abundance of D.
panormitanum (R= 0.575, P < 0.01). This regression for
the other species was not significant.
Statistical multivariate analysis of variance
(MANOVA) indicated that no significant differences of the abundance of the studied three species at the three investigated
sites during the period of investigation were observed.
A part of the present study was focused on the common
slug species Deroceras laeve since it was the most dominant
terrestrial slug species recorded in the three sites. 121
specimens were collected, constituted 63.02% of the total
number. The life cycle of this species was studied in
laboratory, which will help in controlling this species since it is
considered as an agricultural pest. It is considered as an aphally
polymorphic species. So, several (mostly two or three) sexual
morphs co-occur. It was concluded that, the length of adult
specimens ranged between 1.5-3cm. The age of adults laying
eggs ranged between 35-83 days. The number of eggs per
specimen ranged between 9-57 egg. It was observed that laying
eggs occur in clusters during a period of 7-22 days. The age of
death recorded ranged between 41-90 days.
In the present study, the hermaphrodite gland of the
species D. laeve was studied during different seasons of the
year to record its state of maturation. It was noticed that, it
differs in its structure and developmental stages during
different seasons of the year. It reached its maximum maturity
in winter and spring and its least maturity state was in summer
and autumn.
The impact of cadmium on the survival ratios of
Deroceras laeve and the isopod Porcellio laevis were studied.
It was concluded that, slugs are highly affected by cadmium
than isopods. The effect of cadmium on slugs started clearly at 5o μg/g for soil and food treated groups. In case of both
treated groups, the effect started at 100 μg/g. The effect of
cadmium on isopod of the experimental groups started at 200
μg/g, 400 μg/g and 600.
Also, Cadmium concentration and patterns of
accumulation in both slug and isopod species were studied in
laboratory for three weeks: after 3 weeks of exposure to
cadmium through food and/or soil, the Cd concentrations in
Deroceras laeve (slug) and Porcellio laevis (isopod) as well as
food and soil were measured.
Regarding cadmium patterns of accumulation in slug and
isopod, the bioaccumulation factors: (BAF) (indicating the
ratio of metal concentration in animal to food metal
concentration) and bioconcentration factors (BCF) (indicating
the ratio of metal concentration in animal to soil metal
concentration) were calculated for every treated groups.
Overall, cadmium concentrations, BAF and BCF were highly
significant in isopod than in slugs.
Generally, slugs and isopods showed increased cadmium
concentrations in case of metal addition in soil and/or food.
Also, the histological structure of the digestive gland of
Deroceras laeve using Transmission Electron Microscope was
performed to record the role of this gland in the elimination of
Cd as a heavy metal. It consists of two unequal lobes,
compound tubular and brown in color. Each lope consists of
large number of digestive acini bound together by loose connective tissue and are characterized by their thin basal
lamina. Moreover, numerous haemocoelic vessels and
sinusoids are found between the acini. Digestive gland-acini
are lined with simple epithelial cells arranged around a narrow
irregular lumen. The lumen of acini usually contains large
number of bodies containing vacuoles and food debris. The
simple epithelial cells of the digestive gland can be
differentiated into two main cell types: digestive cells and
calcium cells. A third type of cells, excretory cells thought to
be a step in life cycle of digestive cells and another type of
thin cells are distributed, apparently at random between the
different cells. In an experiment on the slug D. Laeve to test
the effect of different Cd concentrations in food and in soil on
the cells of digestive gland and to see the role of this gland in
the elimination of heavy metals, seven groups were prepared
as follows: Group 1: control (without treatment).
Group2: treated with 50μg/g cadmium in food.
Group3: treated with 50μg/g cadmium in soil.
Group4: treated with 50μg/g cadmium in both food
and soil.
Group5: treated with 400μg/g cadmium in food.
Group 6: treated with 400μg/g cadmium in soil.
Group7: treated with 400μg/g cadmium in both food and soil.
It was concluded that the digestive gland plays an important
role in the elimination of heavy metals entering the slug.