Sun and landscape orientation in adult and juvenile sandhoppers Orchestoidea tuberculata (Amphipoda, Talitridae) from two beaches in south central Chile

The behaviour of a talitrid amphipod, Orchestoidea tuberculata Nicolet 1849, was investigated for populations inhabiting two exposed sandy beaches in south central Chile to evaluate orientation capabilities with respect to sun azimuthal changes and landscape. The study beaches (Calfuco and La Misión) were backed by high cliffs but differed in shoreline orientation. Under test conditions of dry substrate, both populations oriented seawards using celestial cues when landscape vision was screened off and showed a higher concentration seawards when they could see the landscape suggesting additive effects of these cues. The absence of a phototactic response to the sun implies the involvement of an inner circadian clock, as the experiments were conducted at different times of the day. The distribution of juveniles was more concentrated than adults in the trials, a result in accordance with their tendency to occupy a zone located closer to the water than adults. Our results suggest that the Calfuco population was oriented to its shoreline, while animals at La Misión tended to deviate from the expected direction and their distribution was more scattered. This observed difference between the populations may be related to the higher human impact on the latter beach, which was more intensively used for recreation and periodically cleaned, removing the wrack. The comparison of results on orientation of O. tuberculata with those of different sandhopper species contributes new phylogenetic insight concerning this important adaptation. This study not only represents an extension of contemporary knowledge of behavioural adaptations of beach amphipods to coasts differing geographically and ecologically, but also analyses orientation in a southern Pacific species of talitrid amphipod for the first time.     

Orientation of Sandhoppers Under Natural Conditions in Repeated Trials: an Analysis Using Longitudinal Directional Data

Zonal recovery is a prompt escape towards the sea performed by sandhoppers under stressful environmental conditions. Differences in habitat features, meteorological conditions during the tests and morphological characteristics of the individuals are all factors affecting orientation. This paper deals with the temporal variation of the orientation of a sandhopper sample, taking into account the effect of some environmental and morphological variables on these changes. The directions chosen by single animals in repeated trials under natural conditions were analysed, using longitudinal circular data. The longitudinal analysis highlights several details about three different kinds of intrapopulation variation due to: sun, wind and eye left/right asymmetry. A clear link between the successive directional responses is demonstrated as well as modulation of behaviour with time: the animals stabilise orientation towards a certain direction in successive trials. The use of longitudinal sets of data in orientation studies produces a good synthesis of directional behaviour, due to several advantages of this kind of analysis. These include the fact that the association between successive behaviours and variability through repeated trials is taken into account; a progressive improvement, due to the integration of information from previous experiences, can be detected; the role of the main cues and factors affecting orientation is more accurately defined; and more parsimonious statistical models can be used to fit the data.     

Importance of orientation to the sun and local landscape features in young inexpert Talitrus saltator (Amphipoda: Talitridae) from two Italian beaches differing in morphodynamics,erosion or stability

Juvenile sandhoppers (Talitrus saltator Montagu 1808; Crustacea: Amphipoda) were tested under natural conditions to examine their ability to orient to the sun and local features. Experiments were carried out on two Italian sandy beaches subject to different geomorphological dynamics, one under erosion from the sea and the other one under morphodynamic equilibrium (between erosion and accretion). Laboratory-born samples of each population were tested on their beach of origin and compared with wild individuals of the same age; other laboratory-born samples were tested on both beaches, that of origin and the other beach. The population from the beach displaying geomorphologic equilibrium, oriented seawards on both beaches, referring to the sun and the local landscape, while the population from the beach under erosion tended to orient to prominent landscape features on both beaches displaying a scototaxis. These results showed the importance of orientation to local landscape features in young inexpert talitrids, and the innateness of this behaviour in natural populations when shoreline stability allows for a genetic stabilisation of orientation mechanisms.