Inheritance of solar direction finding in sandhoppers

Summary Three natural populations ofTalitrus saltator living on differently orientated seashores and the offspring produced by crossing two of them were analysed by means of progeny tests, with the aim of gaining some data on the genetic control of solar direction finding. Data from fullsib family-analyses showed a genetic heterogeneity within at least two of the populations tested: some families oriented towards the sea, some in a different direction and others were dispersed. Intrafamily behaviour in the first and second generations and a mass analysis of dispersed families, suggests that dispersion may arise from segregation of heterozygous parental genotypes. The data obtained generally support the hypothesis of an oligogenic rather than a polygenic control of direction finding.Abbreviations TED theorical escape direction - IED innate escape direction     

Phenotypic plasticity in sun orientation of sandhoppers

Summary Littoral sandhoppers perform zonal orientation mainly by means of sun orientation, which in Mediterranean populations was shown to be inborn, associated with a genetically determined directional tendency adapted to the shoreline of each population. In case of changes of the shoreline the genetic heterogeneity of populations and learning ability may cooperate in providing adaptation both at the population and at the individual level. This hypothesis has been tested carrying on studies on the ontogenesis of the reactionnorm of this behavioural trait inTalitrus saltator (Montagu) (Crustacea: Amphipoda) of a Tyrrhenian population which from previous results shows a well adapted seaward orientation. Sun orientation was analysed away from the sea shore, both in laboratory-raised individuals of different ages and in individuals captured in the field, by repeated releases on dry flat sand or in a glass bowl. Results showed variability within and between individuals of directional tendency, which varies in the individual life depending on age and experience. Particularly, age seems to bring canalization as variability within individuals tends to decrease with age. In order to evaluate the role of experience, we trained individuals to direct themselves towards a direction different from that genetically determined. The majority of the individuals showed a learning ability in the training situation. The importance of phenotypic plasticity in the oriented behaviour of sandhoppers is discussed.Dedicated in gratitude to Prof. Dr. Drs. h.c. M. Lindauer     

Inheritance of host finding ability on structurally complex surfaces

The connectivity of a surface (structural complexity) can have a significant effect on the host finding behavior and efficiency of parasitoids that must search the surface for hosts. We investigated whether the generalist egg parasitoid, Trichogramma nubilale Ertle and Davis (Hymenoptera: Trichogrammatidae), found hosts more efficiently on simple or complex surfaces, and evaluated the potential genetic basis of this variation using a full-sib/ half-sib mating design. Of the egg masses parasitized 63.8% were on the simple surface while only 36.2% were on the complex surface. There was significant repeatable variation among females (repeatability =0.59, n=19 females), with some better at finding hosts on simple surfaces and others better on complex surfaces. These results reinforce previous findings that structural complexity can affect host finding by parasitoids. The additive genetic variance in this character was not significant (sires =23, dams =46, progeny =92), but the maternal plus dominance variance ( V(m)+1/2 V(d)) was significant (P<0.036), and accounted for 48.8% of the total phenotypic variance. The maternal or dominance effects could have complex evolutionary consequences, causing the evolution of other foraging traits to be retarded, to overshoot their optima, or to have complex selective regimes. Thus, the evolution of foraging behavior may depend strongly on the mechanistic details of foraging behavior, including the effects of structural complexity on host finding.     

Moon orientation in adult and young sandhoppers

In the last 30 years, lunar orientation has received little attention from students of animal orientation. Even in Talitrus saltator, the first animals in which the lunar compass was demonstrated, research did not continue. Our studies have demonstrated that: 1) chronometrically compensated lunar orientation is independent of the earth's magnetic field (an ever present non-chronometric orientation reference); 2) lunar orientation is independent of the lunar shape; and 3) the lunar compass is also used by young animals born in the laboratory (without experience in nature). Accepted: 30 September 1998     

Moon orientation in adult and young sandhoppers under artificial light

Our experiments, carried out at night and during the day on adults and laboratory-born young of the sandhopper Talitrus saltator, deal with the identification and use of the moon as an orientating factor. Sandhoppers were released in an apparatus (a Plexiglas dome) that produced a scenario similar to the natural one (with artificial sky, moon or sun illuminated at different intensities).When tested at night, the adult and young sandhoppers used the artificial moon like the natural one, independently of the intensity of illumination of the artificial sky and moon. In other words, sandhoppers tested at night always identified the artificial moon as the moon and never as the sun. In daytime releases, the seaward orientation failed at low intensities of artificial sky and sun illumination (3.07 and 1.55 microW cm2, respectively), whereas the sun compass was used effectively at higher levels of artificial sun and sky illumination. The innate ability of moon compass orientation in inexpert young sandhoppers was demonstrated even under artificial light.     

Moon and sun compasses in sandhoppers rely on two separate chronometric mechanisms

The relationship between the chronometric system of compensation for the apparent movement of the sun and that for the moon has been the subject of several, never proven, hypotheses. Our studies on sandhoppers have demonstrated that the chronometric mechanism of the moon compass is separate from that of the sun compass. They show (i) that a period of seven days in constant darkness has no influence on the capacity for orientation, either solar or lunar, and indicates the presence of one or more continuously operating timing mechanisms; (ii) that two different shifts in the light–dark phase have no effect on the chronometric mechanism of lunar orientation, but they do affect that of solar orientation; and (iii) that exposure to an artificial moon delayed by seven days with respect to the natural cycle causes the expected change in the mean direction of individuals tested under the natural moon, but not of those tested under the sun.     

Orientation at night: an innate moon compass in sandhoppers (Amphipoda: Talitridae)

The supralittoral amphipod Talitrus saltator is well known for its capacity for astronomical orientation using the sun and moon as compasses. It has also been demonstrated that the sun compass is innate in this species. In our experiments, we released inexpert (naive) young born in the laboratory into a confined environment under the full moon and in the absence of the horizontal component of the magnetic field. They were allowed to see the natural sky and the moon only at the moment of release. The young individuals were obtained in the laboratory by crossing adult individuals from the same and different populations of sandhoppers. The young from intrapopulation crosses were well oriented towards the directions corresponding to those of their parents, whereas the young from interpopulation crosses were oriented in an intermediate direction. Therefore, our experiments demonstrate in the sandhopper T. saltator that sea-land moon orientation relies on an innate chronometrically compensated mechanism.     

Local distributions of sandhoppers and landhoppers (Crustacea: Amphipoda: Talitridae) in the coastal zone of western Tasmania

Twelve species of talitrid amphipods were recorded from pitfall transects across the supralittoral and maritime zones at three localities on the west coast of Tasmania; three were sandhoppers, the rest were landhoppers. There was a sharp demarcation between the highest range of the sandhoppers and the lowest range of the landhoppers. Organic content and sodium content of the substrate in the range of the sandhoppers were very low, but rose sharply as the sand was colonised by plants, and landhoppers replaced sandhoppers. Coastal group landhopper species were restricted to a zone about 40–70 m above the high tide mark. Cooler, wetter weather increased the activity of sandhoppers, but only affected landhopper activity slightly. These differences support the idea that landhoppers did not evolve directly from sandhopper ancestors.