Giovanni M. Marchetti

Orientation in Talitrus saltator (Montagu): trends in intrapopulation variability related to environmental and intrinsic factors

Variability in orientating behaviour of Talitrus saltator (Montagu, 1808) (Amphipoda; Crustacea) was investigated under natural conditions. Two populations were analysed from geomorphologically different Italian shores: a stable beach at Castiglione della Pescaia (Tyrrhenian Sea) and a more dynamic sandy shore intensively eroded by the sea at San Rossore (Ligurian Sea). Orientation was examined repeatedly on single animals on their native beach, releasing them on the sand with all cues available. Tests under controlled conditions were done inland, about 2 km away from the sea, in an experimental arena that permitted only vision of the sky and sun. The effect on orientation of some environmental (sun azimuth and meteorological variables) and intrinsic (sex, size, age and eye left/right asymmetry) factors was studied. Tests were done in spring and autumn, over several days in each season and at different times of the day. Using exploratory analysis and several multiple regression models, sun azimuth, wind direction and eye left/right asymmetry were found to be potential influences on orientation. These processes could explain the variation of orientation within a population that emerged from the choices of animals tested at different times of the day and under different meteorological conditions. The association with environmental variables and intrinsic factors demonstrated that the orientation adopted under natural conditions is dependent on a complex interaction of available cues and also on individual characteristics of the animal.

Use of multiple regression models in the study of sandhopper orientation under natural conditions

Abstract In sandhoppers (Amphipoda; Talitridae), typical dwellers of the supralittoral zone of sandy beaches, orientation with respect to the sun and landscape vision is adapted to the local direction of the shoreline. Variation of this behavioural adaptation can be related to the characteristics of the beach. Measures of orientation with respect to the shoreline direction can thus be made as a tool to assess beach stability versus changeability, once the sources of variation are correctly interpreted. Orientation of animals can be studied by statistical analysis of directions taken after release in nature. In this paper some new tools for exploring directional data are reviewed, with special emphasis on non-parametric smoothers and regression models. Results from a large study concerning one species of sandhoppers, Talitrus saltator (Montagu), from an exposed sandy beach in northeastern Tunisia are presented. Seasonal differences in orientation behaviour were shown with a higher scatter in autumn with respect to spring. The higher scatter shown in autumn depended both on intrinsic (sex) and external (climatic conditions and landscape visibility) factors and was related to the tendency of this species to migrate towards the dune anticipating winter conditions.

Multiple regression analysis of the sources of variation in orientation of two sympatric sandhoppers, Talitrus saltator and Talorchestia brito, from an exposed Mediterranean beach

Abstract. Two sympatric species from an exposed sandy beach in north-western Tunisia, the sandhoppers Talitrus saltator (Montagu) and Talorchestia brito Stebbing, were compared as regards orientation under natural conditions. Sets of experiments were carried out on the beach using two experimental arenas, one permitting the view of both the sky and landscape and the other of the sky only. Replicates were made at different hours of the day, in different days and period of the year and with various climatic conditions. Multiple regression models were fitted to the angular data obtained. These models allow an estimate of the trends of variation depending on a combination of factors. The simultaneous effects of environmental factors influencing orientation were quantified for each species. The sun was confirmed as the major orientation cue in these populations, but the concentration of directional choices was enhanced by the landscape vision. The climatic factors affected orientation in different ways depending on the time of day and year. When the two species were compared, Talitrus saltator showed a higher flexibility of response than Talorchestia brito under the same environmental conditions, supporting the hypothesis of a higher level of terrestrialization of the former species.

Chain graph models of multivariate regression type for categorical data

We discuss a class of chain graph models for categorical variables defined by what we call a multivariate regression chain graph Markov property. First, the set of local independencies of these models is shown to be Markov equivalent to those of a chain graph model recently defined in the literature. Next we provide a parametrization based on a sequence of generalized linear models with a multivariate logistic link function that captures all independence constraints in any chain graph model of this kind.

The role of the biological clock in the sun compass orientation of free-running individuals of Talitrus saltator

We used a ‘variable-drift’ paradigm to investigate the relation between the angle of solar orientation and the circadian rhythm of locomotor activity in Talitrus saltator. We recorded the circadian rhythms of individual sandhoppers in constant darkness before testing them at a set time during the day. Because of differences in the free-running period between individuals, they were thus tested at different phase points in their circadian cycle, so that the angular measurements were distributed across the subjective scotophase and photophase. To investigate the association between sun orientation angle and circadian time we used circular correlation statistics, and constructed circular–circular regression models for comparison with the biometric data. The sun orientation angles were strongly dependent on time of day, suggesting that orientation and locomotor activity are regulated by the same time-keeping system. The model fitting the experimental data best was a sinusoidal one, and the distribution of angular orientation predicted on this basis followed closely that of the nonparametrically smoothed data. During the day this followed the arc of movement of the sun, with a phase delay of about 90° and the sign of the angle of orientation reversed around 1200 and 2400 hours. We interpret the data in terms of a mean angular velocity register and discuss their ecological significance.

Variation in zonal recovery in four Talitrus saltator populations from different coastlines: a comparison of orientation in the field and in an experimental arena

Abstract Under stressful conditions (e.g. finding themselves on dry or moisture-saturated substrates) littoral talitrids (Crustacea, Amphipoda) demonstrate zonal orientation, in which they must promptly reach the optimal zone of the beach, the wet fringe near the shoreline. A relationship might therefore exist between the use of orientation and the frequency of such stressful conditions in the natural environment. Moreover, the efficiency of orientation toward the sea could be related to the possibility of using strategies other than zonal orientation in order to avoid stress. This study analysed the actual use and efficiency of orientation under natural conditions of four Talitrus saltator (Montagu, 1808) populations from Mediterranean and northern European Atlantic coasts with different ecological features. Orientation tests were carried out on the beach with all natural cues available. Then the same individuals underwent control experiments to study their sun orientation far from the sea in an experimental arena. The following results emerge from the comparison of the circular distributions: (1) marked differences among populations in the precision of zonal recovery under natural conditions; (2) a common solar orientation capacity in the control tests far from the sea; (3) different orientation choices of the same individuals according to the test conditions, natural or controlled. The habitat diversity of the four populations (amount, distribution and kind of detritus and wrack on the beach, degree of coastal erosion, orientation of the shoreline, human use of the beach) provides an ecological interpretation for the differences in orientation observed among populations.

Triangular systems for symmetric binary variables

We introduce and study distributions of sets of binary vari- ables that are symmetric, that is each has equally probable levels. The joint distribution of these special types of binary variables, if generated by a recursiveprocess of linear main e!ects is essentially par ametrized in terms of marginal correlations. This contrasts with the log-linear formulation of joint probabilities in which parameters measure conditional associations given all remaining variables. The new formulation permits useful compar- isons of di!erent types of graphical Markov models and leads to a close approximation of Gaussian orthant probabilities.

Matrix representations and independencies in directed acyclic graphs

For a directed acyclic graph, there are two known criteria to decide whether any specific conditional independence statement is implied for all distributions factorized according to the given graph. Both criteria are based on special types of path in graphs. They are called separation criteria because independence holds whenever the conditioning set is a separating set in a graph theoretical sense. We introduce and discuss an alternative approach using binary matrix representations of graphs in which zeros indicate independence statements. A matrix condition is shown to give a new path criterion for separation and to be equivalent to each of the previous two path criteria.

Marginal parameterizations of discrete models defined by a set of conditional independencies

It is well-known that a conditional independence statement for discrete variables is equivalent to constraining to zero a suitable set of log-linear interactions. In this paper we show that this is also equivalent to zero constraints on suitable sets of marginal log-linear interactions, that can be formulated within a class of smooth marginal log-linear models. This result allows much more flexibility than known until now in combining several conditional independencies into a smooth marginal model. This result is the basis for a procedure that can search for such a marginal parameterization, so that, if one exists, the model is smooth.

Behavioural adaptation to different salinities in the sandhopper Talitrus saltator (Crustacea: Amphipoda): Mediterranean vs Baltic populations

The sandhopper Talitrus saltator is common on sandy beaches at different latitudes. Therefore, behavioural variations allowing them to cope with a wide range of environmental variation are expected. To test the hypothesis that behavioural adaptations to natural environments are characterised by different salinities, we compared two behaviours (substrate choice and escape from immersion) of two T. saltator populations from a Mediterranean (high salinity) and a Baltic Sea (low salinity) sandy beach. T. saltator preferred to burrow in the high salinity substrate, irrespective of its beach of origin. Regarding orientation to escape from immersion, Baltic sandhoppers always oriented landwards, whereas Mediterranean sandhoppers showed a significant orientation landwards only when immersed in high salinity sea water. These behavioural traits matched the different environments, and this adaptation capability can be used as a model to analyse the response of a keystone species to environmental changes.