FOOD-INDUCED BODY PIGMENTATION QUESTIONS THE
TAXONOMIC VALUE OF COLOUR IN THE SELF-FERTILIZING
SLUG CARINARION SPP.
KURT JORDAENS1, PATRICK VAN RIEL1, SOFIE GEENEN1, RON VERHAGEN1
AND THIERRY BACKELJAU1,2
1
University of Antwerp (RUCA), Evolutionary Biology Group, Groenenborgerlaan 171, B-2020 Antwerp, Belgium. E-mail:
[email protected]
2
Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1000 Brussels, Belgium
(Received 5 May 2000; accepted 29 September 2000)
ABSTRACT
Body pigmentation is a popular taxonomic marker in slugs to discriminate closely related species. However, the genetic background of body pigmentation is known only for a few species, while in many others
body pigmentation is influenced by age, food and/or climate. In this study, we investigated the effects
of different food items on body pigmentation expression in two selfing pulmonate gastropods, Arion
(Carinarion) silvaticus and Arion (Carinarion) fasciatus. Both species mainly differ in the distribution of
yellow-orange granules on the body, which in A. fasciatus are concentrated in lateral bands, and in A. silvaticus are evenly scattered. Animals were raised individually under the same conditions, while they laid
eggs as a consequence of selfing. This F1 generation was afterwards divided into two groups, which were
fed with different food items. A diet of carrot, lettuce or paper had no effect on the distribution of the
yellow-orange granules in A. silvaticus, but provoked a loss of the yellow-orange lateral bands in
A. fasciatus so that externally these F1 specimens became similar to A. silvaticus. In both species, a diet of
nettle resulted in a strong yellow-orange pigmentation, which often formed yellow-orange lateral bands.
These results indicate that food can probably influence the ‘species-specific’ body pigmentation in Carinarion, and thus question the reliability of colour traits to distinguish A. silvaticus and A. fasciatus.
INTRODUCTION
The genetics of shell colour and banding patterns in
several stylommatophoran gastropods are relatively
well known and, for instance, the distribution of shell
morphs and changes of their frequencies in Cepaea
nemoralis is one of the classic examples of natural selection (e.g. Clarke, Arthur, Horsley & Parkin, 1978;
Cameron, 1992; Cook & Pettitt, 1998). In contrast, the
genetics of body pigmentation in slugs is known only
for a few species (e.g. Abeloos, 1945; Williamson, 1959;
Clarke et al., 1978; Evans, 1983; Reise, 1997). Yet,
colour traits are popular taxonomic markers because
colour variation in slugs appears to be common, is
sometimes very conspicuous and is often superficially
consistent (see references in Reise, 1997). However,
colour characters should only be used in taxonomy
when a sound knowledge of their phenotypic variation
and their genetic background is available, since body
pigmentation in several slug species may be influenced
by environmental factors such as climate and food
(Loens, 1890; Marenbach, 1939; Backeljau & De Bruyn,
1990; Backeljau, De Winter, Martin, Rodriguez & De
Bruyn, 1994), and individuals may change colour in the
Corresponding author: K. Jordaens
J. Moll. Stud. (2001), 67, 161–167
course of their lives due to both genetic and environmental factors (e.g. Simroth, 1885a,b; Gain, 1892;
Collinge, 1897; Boettger, 1949; Ant, 1957). These confusing factors underlying colour variation in slugs have
even led to the description of new species, which subsequently turned out to be colour morphs of one and
the same species, as determined by breeding experiments
and allozyme studies (e.g. Backeljau, De Brito, Tristão
Da Cunha, Frias Martins & De Bruyn, 1992; Backeljau
et al., 1994; Reise, 1997).
In the genus Arion Férussac, 1819, which comprises
approximately 40 species of terrestrial slugs divided
into four subgenera (e.g. Backeljau & De Bruyn, 1990;
Castillejo, 1997), colour characters are often used to
discriminate closely related species (e.g. Cain &
Williamson, 1958; Evans, 1983; Davies, 1987; Martín
& Gómez, 1988). A case in point are the three species of
the subgenus Carinarion Hesse, 1926, viz. Arion (Carinarion) fasciatus (Nilsson, 1823), Arion (Carinarion)
circumscriptus Johnston, 1828, and Arion (Carinarion)
silvaticus Lohmander, 1937, which, apart from some
subtle differences in the size and shape of the genitalia,
are mainly distinguished by colour traits (e.g. Wiktor,
1973; Kerney, Cameron & Jungbluth, 1983) (Table 1;
Fig. 1) and supposedly species-specific electrophoretic
protein profiles in Northwest European populations
© The Malacological Society of London 2001
KURT JORDAENS ET AL.
Table 1. Morphological differences between the three Carinarion species.
Character
Body size
Yellow-orange lateral bands
Body side colour
Mantle spots
Epiphallus pigmentation
A. fasciatus
A. silvaticus
A. circumscriptus
Large
Present
Light
Absent
Absent
Small
Absent
Light
Absent
Absent
Small
Absent
Dark
Present
Present
(Backeljau, Ahmadyar, Selens, Van Rompaey & Verheyen, 1987; Backeljau, De Bruyn, De Wolf, Jordaens,
Van Dongen & Winnepenninckx, 1997a). Such profiles
were used to identify, for example, albino-like A. circumscriptus individuals (Backeljau, Jordaens, De Wolf,
Rodríguez & Winnepenninckx, 1997b). Albinism suggests that colour in Carinarion is probably partly under
genetic control, since in several other gastropods albinism shows a simple Mendelian inheritance with body
pigmentation being dominant to albinism (e.g. Luther,
1915; Ikeda, 1937; Richards, 1978; Dillon & Wethington, 1992, 1994).
Yet, the three Carinarion species self-fertilize over a
large part of their distribution (e.g. Foltz, Ochman,
Jones, Evangelisti & Selander, 1982; Backeljau et al.,
1997a) and differences in colour may therefore only
reflect a fixation of alternative alleles, rather than
suggesting taxonomic difference (Jordaens, De Wolf,
Verhagen & Backeljau, 1996; Backeljau et al., 1997a).
Moreover, there are strong indications that colour is also
environmentally/physiologically influenced (Simroth,
1885a; Ant, 1957). Because of this possible influence on
body pigmentation and the absence of species-specific
electrophoretic protein profiles in Central and Southeast European Carinarion populations (Jordaens et al.,
1996; Jordaens, 1999) the taxonomic status of the three
Carinarion species has been challenged (Jordaens et al.,
1996; Backeljau et al., 1997a). Nevertheless, apart from
species-specific esterase profiles (Jordaens, Van Riel,
Verhagen & Backeljau, 1999) and albumen gland proteins (Chichester, 1967; Backeljau et al., 1987) colour
seems the only way to discriminate the species.
Because the genetics of body colour pigmentation in
Carinarion are unknown, intermediate colour forms
have been described (Lohmander, 1937; Waldén, 1955;
Wiktor, 1973), and also a few anecdotal observations of
environmental effects on pigmentation in Carinarion
exist (Simroth, 1885a; Ant, 1957), colour in Carinarion appears to be an unreliable taxonomic marker. As
food was the major factor influencing body pigmentation in Arion empiricorum (Marenbach, 1939), we
studied the effects of different food items on body
pigmentation in Carinarion. We restricted our experi162
A
B
C
Figure 1. The three Carinarion species. A. Arion fasciatus; B. A.
circumscriptus; C. A. silvaticus. Scale bar 0.5 cm.
FOOD-INDUCED BODY PIGMENTATION IN CARINARION
ments to A. silvaticus and A. fasciatus because these two
species are morphologically most similar and sometimes difficult to distinguish in the field.
MATERIALS AND METHODS
Juvenile Carinarion were collected and individually raised in
plastic containers (diameter 8 cm, height 5 cm) on compost
and carrots. The containers were kept in a climate room
(20°C) under a 12 h light/12 h dark regime, with a relative
humidity of approximately 100%. Species identifications followed Lohmander (1937) and Kerney et al. (1983). When
fully grown, 26 specimens of A. fasciatus from Germany
(Görlitz) and Austria (Graz), and 19 specimens of A. silvaticus from several Belgian populations served as parents for a
breeding experiment (Table 2). Each individual parent was
kept in isolation and laid eggs as a consequence of selfing.
Egg clutches were collected and hatched juveniles (further
referred to as F1) were raised on compost and carrot until
they were 2 months old. At that time, half the number of F1 of
a single clutch was kept on compost and carrot, and the other
half was transferred to compost with either lettuce or nettle,
or to paper with carrot (Table 2). All food items are highly
preferred by both species (Rathcke, 1985; personal observation). Food and paper were changed twice a week, and compost every 2 months. When fully grown, body pigmentation
was scored in 507 F1 specimens of the 45 parents. Only a few
F1 specimens died before scoring. Body pigmentation was
scored by comparison with typical adult reference specimens
of the three species from natural populations.
RESULTS
There was no pigmentation variation among the F1
progeny of single parents when raised on the same diet.
However, there were conspicuous differences among
F1 progeny of single parents when raised on different
diets (Table 2; Fig. 2).
A diet of carrot, lettuce or paper provoked a loss of
the yellow-orange pigmentation in most A. fasciatus
(Fig. 2A and B). This was not so for nettle, which produced even more pronounced yellow-orange lateral
bands. The body mucus of many F1 specimens was
orange, whereas normally it is colourless in Carinarion
(e.g. Kerney et al., 1983).
Carrot, lettuce or paper had no apparent effect on
A. silvaticus. However, nettle produced a strong yelloworange pigmentation, which often formed yelloworange lateral bands (Table 2; Fig. 2C).
Especially with carrot and nettle, F1 of both A. silvaticus and A. fasciatus were sometimes also very dark
and lost the white colour of the body sides (Table 2;
compare Figs 1A, C and 2B with Fig. 2D), irrespective
of whether or not the yellow-orange pigmentation was
expressed (Table 2).
Table 2. Scoring of body pigmentation in 507 F1 progeny
of 45 Carinarion parents. C carrot; L lettuce; P paper;
N nettle. YB yellow-orange lateral bands present (),
weak () or absent (–); BS colour body sides with light
(L), intermediate (I) and dark (D).
Parent
Comparison
Numbers of F1
Pigmentation
YB
BS
A. fasciatus
11
12
13
14
15
16
17
18
19
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
C/L
C/L
C/L
C/L
C/L
C/L
C/P
C/P
C/P
C/P
C/P
C/P
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
C/N
6/6
1/2
9/9
6/7
8/7
7/6
5/6
8/8
6/6
5/5
7/6
3/3
3/4
6/7
2/2
7/3
8/6
5/5
10/6
6/5
12/15
6/7
4/4
7/5
1/2
7/8
–/
–/
–/
–/–
–/
–/–
–/–
–/–
–/–
–/–
–/–
/
–/
–/
–/
–/
–/
–/
–/
–/
–/
–/
–/
–/
–/
–/
L
L
L
L
L
L
L
I
L
L
L
L
L
D
D
D
D
L
D
L
L
L
L
L
D
L
A. silvaticus
11
12
13
14
15
16
17
18
19
10
11
12
13
14
15
16
17
18
19
C/L
C/L
C/L
C/L
C/L
C/L
C/L
C/L
C/P
C/P
C/P
C/P
C/P
C/N
C/N
C/N
C/N
C/N
C/N
3/4
3/3
6/8
7/8
3/4
4/5
5/3
8/8
5/4
6/6
10/9
2/3
2/2
3/4
6/7
2/2
7/3
8/6
5/5
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/–
–/
–/
–/
–/
–/
–/
L
D
I
L
D
L
L
L
D
L
L
I
I
D
L
L
I
D
I
163
KURT JORDAENS ET AL.
A
B
C
D
Figure 2. Environmental effects on colour expression in Carinarion. A. Arion fasciatus individual fed with nettle; B. A. fasciatus individual
from the same brood but fed with lettuce; C. two A. silvaticus individuals from the same brood but fed with lettuce (left) or nettle (right);
D. A. silvaticus individual fed with lettuce under laboratory conditions. Scale bar 0.5 cm.
DISCUSSION
Our results show that food may influence body pigmentation in Carinarion. In A. fasciatus, a diet of carrot,
paper or lettuce resulted in a loss of the yellow-orange
lateral bands and F1 externally became very similar to
A. silvaticus. Simroth (1885a), Ant (1957) and Backeljau & De Bruyn (1990; Fig. 3) also observed that A. fasciatus lost the yellow-orange lateral bands when the
animals were kept for several months in the laboratory;
however, they could not pinpoint the cause(s) of this.
In A. silvaticus, the regular appearance of orangeyellow lateral bands when fed on nettle meant that these
specimens externally resembled A. fasciatus. Finally, in
both species, F1 progeny of several parents had dark
body sides, as in A. circumscriptus, yet the dark mottling on the mantle and epiphallus in this species was
not induced by the diets tested. Lusis (1962) reported
that changes in the degree of pigmentation of the
hermaphrodite gland in Arion ater are correlated with
the sexual stages in the gland, but there are no indications that such an effect on pigmentation expression in
Carinarion exists (personal observation).
It is not clear which component(s) in the diet is (are)
responsible for the expression/suppression of the
lateral bands, the darkening of the body, and the colour
change of the mucus. Nevertheless, these observations
confirm and emphasize consistently that colour in
Carinarion is at least partly environmentally or physiologically influenced (Simroth, 1885a; Ant, 1957).
To what extent food-induced body pigmentation
occurs in natural populations needs further study, particularly since intermediate individuals have been
found (Lohmander, 1937; Waldén, 1955; Wiktor, 1973).
Moreover, there is great variation in the intensity of the
yellow-orange lateral bands in A. fasciatus, making
it not always easy to distinguish this species from A.
silvaticus (e.g. Lohmander, 1937; Waldén, 1955). The
nature and possible ‘adaptive’ significance of foodinduced changes in body pigmentation have also to be
explored. The few studies that have addressed this issue
in slugs suggested that darker animals better resist low
temperatures and/or high altitudes, and that pale
animals are better adapted to high temperatures and/or
low altitudes (for arionids: Albonico, 1948; Chevallier,
1977; for Deroceras rodnae: Reise, 1997). However, at
least for Deroceras juranum (violet coloured Deroceras
rodnae) this ‘adaptive’ interpretation has been ques-
164
FOOD-INDUCED BODY PIGMENTATION IN CARINARION
A
B
Figure 3. Loss of yellow pigmentation in A. fasciatus under laboratory conditions. A. Individual from wild population. B. The same individual
after three months of being kept under laboratory conditions and fed with lettuce. Scale bar 0.5 cm.
tioned (Jordaens, Backeljau, Reise, Van Riel &
Verhagen, 1998b). Simroth (1885a) and Albonico (1948)
showed that orange Arion empiricorum gradually became black when kept at low temperatures, and viceversa. We doubt whether this correlation also holds for
Carinarion. Many progeny became very dark in our
experiment despite the high temperature at which they
were raised. Moreover, animals may become very dark
at low, as well as at high temperatures (personal observation).
Our results suggest that pigmentation in Carinarion
seems to be determined both by genetic (i.e. albinism)
and environmental/physiological factors (i.e. pigmentation of body sides, colour of the mucus and yelloworange lateral bands). However we question the value
of colour traits to discriminate between A. silvaticus
and A. fasciatus for two reasons. First, for body pigmentation that has a genetic base, self-fertilization will
lead to the fixation of alternative alleles (i.e. different
morphotypes) and therefore does not necessarily represent taxonomic differences (Jordaens et al., 1996;
Backeljau et al., 1997a). Secondly, the pigmentation
changes introduced by dietary differences affect the
species specificity of the colour characters on which A.
silvaticus and A. fasciatus were originally defined.
Together with the strong population-genetic (Jordaens et al., 1996, Jordaens, 1999), morphological (Jordaens, 1999) and reproductive (Jordaens, Backeljau,
Van Dongen & Verhagen, 1998a) similarity between
A. silvaticus and A. fasciatus, these observations further
question the taxonomic status of these segregates.
ACKNOWLEDGEMENTS
We are very grateful to Olle Johnson, Sofie Thys and Bieke
Van Hooydonck for their help in collecting the material, and
Heike Reise for sending us animals and for commenting
on earlier drafts of the manuscript. P.V.R. and S.G. hold an
IWT scholarship.
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