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First published online June 15, 2006
Journal of Experimental Biology 209, 2509-2514 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02300
Green land and blue sea: a coloured landscape in the orientation of the sandhopper Talitrus saltator (Montagu) (Amphipoda, Talitridae)
1 Dipartimento di Biologia Animale e Genetica, Università di Firenze,
Via Romana 17, 50125 Firenze, Italy
2 Istituto Nazionale di Ottica Applicata, Firenze, Italy
* Author for correspondence (e-mail: ugolini_alb{at}dbag.unifi.it)
Accepted 25 April 2006
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Summary |
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 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
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Key words: orientation, landscape, colour, vision, sandhopper, Talitrus saltator
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Ugolini, 2001a;
Ugolini, 2001b;
Ugolini et al., 2002;
Ugolini, 2003). Local cues are
also important: through learning, sandhoppers can modify the innate compass
reference direction to improve their orientation along the sea–land axis
direction or when they are displaced to a differently orientated beach
(Ugolini and Macchi, 1988;
Ugolini et al., 1988;
Ugolini et al., 1991). Of
course, location-dependent compass cues play an important role in direction
finding when the general factors are not available (e.g. cloudy days, moonless
nights). Among local cues, the influence of natural or artificial
landscape-based compass cues on sandhopper orientation has been frequently
hypothesized and tested (Williamson,
1951; Williamson,
1954; Craig, 1973;
Hartwick, 1976;
Edwards and Naylor, 1987;
Ugolini et al., 1986;
Ugolini and Cannicci, 1991;
Scapini, 1997;
Scapini et al., 1997). The
artificial landscape usually consisted of black rectangles of different
heights occupying 180° on the periphery of a circular arena in which the
sandhoppers were released (e.g. Ugolini et
al., 1986). Therefore, tests of the landscape-based compass cues
on sandhopper orientation were based on a difference in contrast between the
two hemicycles: the landward and seaward ones.
The natural landscape, however, is coloured. Yet, we do not know very much
about the visual capabilities and spectral sensitivity of sandhoppers. In
T. saltator from Mediterranean coasts, the visual field in the
horizontal plane is quite wide (Beugnon et
al., 1987). Studies on the relationship between spectral filtering
and solar orientation capacity (Ugolini et
al., 1996), as well as preliminary electrophysiological
investigations (Ugolini et al.,
1996) (M. Lindstroem and A.U., unpublished data), suggested the
presence of at least two pigments in the eye: one in the blue range, the other
in the green.
Therefore, we decided to evaluate the influence of artificial coloured landscape-based compass cues on the sea–land axis orientation of T. saltator.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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After capture, the sandhoppers were transferred to the laboratory and kept in PlexiglasTM boxes containing wet sand. They were raised in conditions of natural temperature and with a light–dark cycle corresponding in phase and duration to the natural one. Food was constantly available and the sand was kept well aired and wet.
The experimental apparatus was similar to that used by Pardi and Papi
(Pardi and Papi, 1953),
modified by Ugolini and Macchi (Ugolini
and Macchi, 1988). A transparent Plexiglas bowl (diameter 20 cm)
was placed on a goniometer placed on a circular transparent Plexiglas plate
(diameter 30 cm). We tested a total of 867 sandhoppers. Each animal was tested
only once. Groups of 5–7 individuals were released at a time inside the
bowl. Previous experiments demonstrated that group releases do not influence
the directional choice of sandhoppers
(Scapini et al., 1981).
The sandhoppers were released in dry conditions (the empty bowl) and they were previously dehydrated for a few minutes by exposure to direct sunlight. The bowl was covered with a sheet of transparent acetate to prevent the animals from escaping. A cylindrical, opal white PlexiglasTM screen (height 5 cm, diameter 30 cm) placed around the bowl blocked vision of the surrounding landscape but allowed the individuals to see the sun and sky. The observer could read the directions assumed by the tested animals directly below the apparatus.
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The following types of releases were performed. (1) Control sandhoppers
were tested under the sun and blue sky, inside the bowl surrounded by the
white screen (without vision of the landscape). (2) Experimentals were tested
under the sun and blue sky, with vision of an artificial landscape. (a) Groups
of animals were tested with vision of a black cardboard strip, 5 cm high
(=19° from the centre of the bowl, the sandhoppers' release point), placed
on the inside wall of the screen and occupying 180° of the horizon. (b)
Sandhoppers were tested with vision of a coloured landscape. In each trial,
two differently coloured gelatine filters (blue–green or
yellow–orange) were always present inside the screen, each one occupying
a hemicycle. The spectral transmittance of each gelatine filter (produced by
Spotlight, Milan, Italy) was measured with a Perkin-Elmer 
900
spectrophotometer (Fig. 1A). We
used blue and green to simulate the natural landscape (blue=sea and
green=Mediterranean macchia), whilst yellow and orange were used as a
`nonsense' landscape since these colours are not prevalent in the natural
habitat of sandhoppers. (c) To test if the animals' reactions to the
blue–green landscape were due to real colour vision and not merely to a
difference in the perceived radiance between hemicycles (the amount of light
transmitted by the filters and perceived by the sandhoppers, see below), we
also used achromatic (grey) gelatine filters. These two filters were used as a
pair, or coupled with the blue and green filters: care was taken to substitute
a grey filter of similar sandhopper perceived radiance (SPR,
Fig. 1B) to the coloured
filter, calculated as the integral of the product of filter transmittance and
sandhopper spectral sensitivity functions
(Ugolini et al., 1996):
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) is the filter transmittance and
stalitrus() is the sandhopper spectral
sensitivity. Each filter of every pair and the black cardboard strip were alternately placed in the seaward and landward hemicycle, corresponding to the seaward and landward direction of the home beach.
The analysis of circular distributions was carried out according to
Batschelet (Batschelet, 1981).
For each distribution, we calculated the mean angle 
and the length of
the mean resultant vector, r. The V test was used to assess if the mean
vector of the data had a statistically significant vector component in the
direction of the expected direction. To quantify the goodness of orientation,
we used Batschelet's formula (Batschelet,
1981) (see p. 41) for the home
component=rcos
, where r is the length of the mean
vector and is the difference in absolute value between the mean
angle () and the reference (seaward) direction. To compare the angular
dispersion and the angular deviation between circular distributions, we used
the I and II Wallraff tests (Wallraff,
1979).
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Sandhoppers tested with the pair of blue and green hemicycles (Fig. 2G–I) showed a constant preference for the blue, independently of its seaward or landward position (Fig. 2H, I, respectively; Tables 1, 2). It should also be noted that the sandhoppers' orientation significantly improved when they were tested with the blue and green gelatine strips, as compared to the control distribution (Fig. 2G; Tables 1, 2).
The pair of grey filters had no effect on the directional choice (angular deviation) of T. saltator, independently of their position with respect to the natural sea–land axis (Fig. 3A–C; Table 3); a modest effect on the angular dispersion was present only when the medium grey filter was positioned landward (Fig. 3A vs C; Table 3). There seemed to be an increase in the goodness of orientation with respect to that of control (Tables 1, 3) when the medium grey was seaward; however, the control distribution presented a particularly high dispersion (Fig. 3A), and the two values of goodness of orientation obtained in releases with different positions of the grey filters were very similar (Table 1).
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When the blue and green filters were coupled with the medium grey and light grey filters, respectively (Fig. 3), the position of the coloured filters matched the natural disposition (blue seaward or green landward), the goodness of orientation improved (Table 1) even though the only significant difference between distributions was in the comparison of angular dispersion (Table 3D vs E, medium grey–blue filters). However, when the blue and the green filters were opposite to the natural situation, the goodness of orientation decreased or the direction of the mean vector was reversed: the comparisons between distributions for angular deviation were all highly significant (Tables 1, 3).
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Hallberg et al., 1980;
Schmitz, 1992) and the absence
of investigations of the neurobiology of vision in talitrid Amphipods prevent
us from speculating about the photoreceptors that sandhoppers use in
achromatic and colour vision (Osorio and
Vorobyev, 2005). Nevertheless, tests carried out with the black
and white landscape fully confirm the influence of vision of the artificial
landscape on the sea–land axis orientation of T. saltator, at
least as far as the difference in the sandhopper perceived radiance between
hemicycles is concerned (Ugolini et al.,
1986; Ugolini and Cannicci,
1991). It is also noteworthy that when the seaward direction
indicated by the landscape position contrasted with that indicated by the sun
compass, the sandhoppers produced a distribution where the mean vector of the
data had no statistically significant vector component in the expected
direction. This agrees well with previous findings
(Ugolini et al., 1986) that
vision of the black strip (14° high) did not invert the sandhoppers' mean
direction, despite some differences among the populations tested.
Concerning the coloured landscape, we did not attempt to reproduce the
`real colour' of the landscape sandhoppers see in the field during their jumps
along the y axis of the beach. We only tried to reproduce a scenario
with the dominant colours: blue for the sea, green for the Mediterranean
macchia. Despite this arbitrary choice, partly based on preliminary knowledge
of the spectral sensitivity of T. saltator
(Ugolini et al., 1996), it is
evident that vision of a blue and green landscape greatly influences the
direction finding: the direction of the mean vector agreed with the landscape
directional indication, even when the latter contrasted with the sun compass
indication. This did not occur when the sandhoppers were tested with the pair
of light–medium grey filters (the sandhopper perceived radiance of the
former was double that of the second). However, the inversion of the seaward
orientation was still present when T. saltator was tested with the
blue–grey or green–grey landscape (similar sandhopper perceived
radiance, different colour).
Therefore, we conclude that differences in colour between the hemicycles are important for sandhopper orientation along the sea–land axis of the beach. Nevertheless, not every colour determined the same directional choice of T. saltator, even though there was some difference when the yellow filter was positioned in the landward or seaward hemicycle. We can speculate about the reason for the effect of the pair of yellow and orange filters. Considering that these two colours are not predominant in the sandhoppers' environment, the effect could be due to different sensitivity to – and thus perception of – the two colours by the Talitrus eye. In any case, the general effect is much less than that of the blue–green landscape.
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References |
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