88
of E. serratus more often at the surface of ponds than
females.
The present study aimed at assessing phototaxis in
adults of the Sudanese fairy shrimp Streptocephalus
proboscideus (Frauenf.) under laboratory conditions.
Effects of spectral composition were also examined.
An extrapolation is made to migratory behaviour in the
field based on a series of suggestive field observations
on Streptocephalus torvicornis.
Field observations on Streptocephalus
torvicornis
(Waga)
In May 1987, while collecting live S. torvicornis, a
series of deep rock pools ('gueltas') were visited along
the south rim of the Fadnoun plateau, Tassili-n-Ajjer,
Algeria. Several of them are circular, steep-walled
excavations of the floor of deep Wadi Canyons (e.g.
at Wadi Amais, and at guelta Adessei). When filled up,
depth may reach 6-10 m. Our attention was initially
caught by the fact that in one such guelta we collected
only (pinkcoloured) males. Because of the transparent
water of the guelta, females (notable by their bright
red ovisac) could be seen swimming, but in deeper
strata only. Few ever came closer than 2.5 m to the
surface. We confirmed this observation at five more
gueltas subsequently.
Additionally, in gueltas situated at the foot of a cliff,
we found that populations concentrated in the shaded
part of the pool during daytime (e.g. in guelta Dider).
In one 7 m deep guelta in northern Mauritania (Gueltet
Zli), where we performed a study on the diurnal vertical migration of the zooplankton on 7-8 Feb 1976, a
sizeable population of S. torvicornis (all pale animals)
was present as well. Although our sampling technique
(a two liter plankton trap) was totally ineffective in
capturing the Anostraca, we observed few specimens
at the surface (except, again, in the shade provided by a
rocky shore) during the day. During the night, by contrast, swarms of Streptocephalus appeared and stayed
near the surface.
Materials and methods
Test specimens of S. proboscideus stemmed from mass
laboratory cultures initiated from surface mud with
resting eggs from a temporary pool in Al Gedid (Sudan,
Africa). Only adults were used during experimentation.
Experiments were conducted in a perspex column
(H: 2 m; diam.: 20 cm) filled with dechlorinated tap
water and illuminated from above with a 2000 W light
source. The distance between lamp and water surface
was 35 cm. The column was externally divided in
10 equal compartments. Black pebbles on the bottom
minimized light reflection (see De Meester & Dumont,
1988). Prior to testing, groups of 10 adults of a given sex were given 3 hours adaptation to dechlorinated tap water (21±1 DC) and darkness. From the start
of each experiment (t=O), the number of animals in
each 20-cm section was recorded at 2-min intervals
for 40 minutes. The vertical distribution of the animals
was reconstructed by averaging all observations from
t = 10 min onwards. The observations of the first 10 min
were discarded because they were strongly influenced
by the initial flight reaction ofthe animals (see also De
Meester & Dumont, 1988). All testing was completed
between 14.00 and 20.30 h. Sexes were tested separately to exclude interference from mating behaviour. No
discrimination was made according to female reproductive state. Phototaxis was considered positive if a
significant movement occurred in the direction of the
light, and negative if the animals moved away from
the light source.
Coloured cellophane filters were used to study the
effect of spectral composition on male and female
photoresponses in comparison with their behaviour in
'white' light (no filter). Transmission spectra of the
cellophane sheets are presented in Fig. 1. Light transmission of colour filters in percent of white light is:
yellow=77.3%, red=40.2%, and blue=22.4%. After
dark adaptation, animals were first exposed for 40 min
to white light, after which their response to the positioning of a colour filter was tested. Filters were used
in the following order: yellow, blue, red. Each of the
colours was applied for 40 min., and there was a 10 min
period of white light between the different treatments.
The vertical distribution of the animals in coloured
light was reconstructed as described for white light.
The complete experimental series, including dark
adaptation, took 6 h 10', and was run 3 times with
males and 3 times with females, each time with different test animals. Even upon transformation, the data
did not conform to assumptions for parametric testing
(see Sokal & Rohlf, 1981). Frequency distributions of
animals over compartments were compared between
colours and between sexes by means of the G-test
statistic. In addition, the uniformity of the distributions
of E. serratus more often at the surface of ponds than
females.
The present study aimed at assessing phototaxis in
adults of the Sudanese fairy shrimp Streptocephalus
proboscideus (Frauenf.) under laboratory conditions.
Effects of spectral composition were also examined.
An extrapolation is made to migratory behaviour in the
field based on a series of suggestive field observations
on Streptocephalus torvicornis.
Field observations on Streptocephalus
torvicornis
(Waga)
In May 1987, while collecting live S. torvicornis, a
series of deep rock pools ('gueltas') were visited along
the south rim of the Fadnoun plateau, Tassili-n-Ajjer,
Algeria. Several of them are circular, steep-walled
excavations of the floor of deep Wadi Canyons (e.g.
at Wadi Amais, and at guelta Adessei). When filled up,
depth may reach 6-10 m. Our attention was initially
caught by the fact that in one such guelta we collected
only (pinkcoloured) males. Because of the transparent
water of the guelta, females (notable by their bright
red ovisac) could be seen swimming, but in deeper
strata only. Few ever came closer than 2.5 m to the
surface. We confirmed this observation at five more
gueltas subsequently.
Additionally, in gueltas situated at the foot of a cliff,
we found that populations concentrated in the shaded
part of the pool during daytime (e.g. in guelta Dider).
In one 7 m deep guelta in northern Mauritania (Gueltet
Zli), where we performed a study on the diurnal vertical migration of the zooplankton on 7-8 Feb 1976, a
sizeable population of S. torvicornis (all pale animals)
was present as well. Although our sampling technique
(a two liter plankton trap) was totally ineffective in
capturing the Anostraca, we observed few specimens
at the surface (except, again, in the shade provided by a
rocky shore) during the day. During the night, by contrast, swarms of Streptocephalus appeared and stayed
near the surface.
Materials and methods
Test specimens of S. proboscideus stemmed from mass
laboratory cultures initiated from surface mud with
resting eggs from a temporary pool in Al Gedid (Sudan,
Africa). Only adults were used during experimentation.
Experiments were conducted in a perspex column
(H: 2 m; diam.: 20 cm) filled with dechlorinated tap
water and illuminated from above with a 2000 W light
source. The distance between lamp and water surface
was 35 cm. The column was externally divided in
10 equal compartments. Black pebbles on the bottom
minimized light reflection (see De Meester & Dumont,
1988). Prior to testing, groups of 10 adults of a given sex were given 3 hours adaptation to dechlorinated tap water (21±1 DC) and darkness. From the start
of each experiment (t=O), the number of animals in
each 20-cm section was recorded at 2-min intervals
for 40 minutes. The vertical distribution of the animals
was reconstructed by averaging all observations from
t = 10 min onwards. The observations of the first 10 min
were discarded because they were strongly influenced
by the initial flight reaction ofthe animals (see also De
Meester & Dumont, 1988). All testing was completed
between 14.00 and 20.30 h. Sexes were tested separately to exclude interference from mating behaviour. No
discrimination was made according to female reproductive state. Phototaxis was considered positive if a
significant movement occurred in the direction of the
light, and negative if the animals moved away from
the light source.
Coloured cellophane filters were used to study the
effect of spectral composition on male and female
photoresponses in comparison with their behaviour in
'white' light (no filter). Transmission spectra of the
cellophane sheets are presented in Fig. 1. Light transmission of colour filters in percent of white light is:
yellow=77.3%, red=40.2%, and blue=22.4%. After
dark adaptation, animals were first exposed for 40 min
to white light, after which their response to the positioning of a colour filter was tested. Filters were used
in the following order: yellow, blue, red. Each of the
colours was applied for 40 min., and there was a 10 min
period of white light between the different treatments.
The vertical distribution of the animals in coloured
light was reconstructed as described for white light.
The complete experimental series, including dark
adaptation, took 6 h 10', and was run 3 times with
males and 3 times with females, each time with different test animals. Even upon transformation, the data
did not conform to assumptions for parametric testing
(see Sokal & Rohlf, 1981). Frequency distributions of
animals over compartments were compared between
colours and between sexes by means of the G-test
statistic. In addition, the uniformity of the distributions