Alberto Ugolini

Learned component in the solar orientation of Talitrus saltator Montagu (Amphipoda: Talitridae)

Abstract In the sandhopper Talitrus saltator Montagu, both information on the home beach direction and the capacity to orient by the sun, compensating for its apparent motion, are innate. The adults and juveniles, however, are also capable of learning a new ecologically efficient escape direction. This capacity neither contrasts with, nor is redundant to, the genetic control of the sun compass. In fact, the capacity to calibrate the sun compass by learning favors survival on beaches that are oriented differently from the home beach. It also increases the chances for survival on the home beach by making the route of each sandhopper as rectilinear and perpendicular to the shoreline as possible.

The amphipod Talitrus saltator as a bioindicator of human trampling on sandy beaches

The present paper assesses the use of the supralittoral amphipod Talitrus saltator as a bioindicator of the effects of human trampling on the supralittoral sandy band. Samplings in delimited areas were carried out at sites subjected to different human impact. The results showed a strong negative correlation between the number of swimmers and the sandhopper population density, while there was no clear relationship between sandhopper abundance and the other factors considered: granulometry, compactness and organic carbon content of the sand, and trace metal contents in the sand and sandhoppers. A field test of trampling conducted in a confined space showed its direct negative effect on sandhopper survival. However, trace metal analysis confirmed the ability of T. saltator to bioaccumulate some elements (Hg, Zn, Cu, Cd). Our study demonstrates that T. saltator is a good bioindicator of human impact in the supralittoral zone of sandy shores.

Activity rhythms and orientation in sandhoppers (Crustacea, Amphipoda).

The aim of the present review is to combine knowledge of locomotor activity rhythms with that of compass orientation in littoral arthropods. Talitrid amphipods (the sandhoppers) represent a good biological model in the fields of animal orientation and biological rhythms. The paper examines the activity rhythms of different species of sandhoppers (mainly Talitrus saltator), as well as the chronometric mechanisms of compensation for the apparent motion of the sun and moon that these animals use in zonal recovery based on the two astronomical cues. The two chronometric mechanisms seem to be independent of each other and to operate throughout the 24-hour period. The speed of the chronometric mechanism of solar compensation appears to be related to the hours of light and is entrained by the same stimulus (light-dark alternation) that controls the circadian activity rhythm. Therefore, it is probable that in T. saltator the same mechanism regulates both the circadian locomotor activity and the solar compensation.

Learning in Zonal Orientation of Sandhoppers

It has long been known that amphipods living on sandy shores use a sky compass (the sun or moon) to find their way back to the damp belt of the splash zone (Pardi and Papi, 1952, 1953; Papi and Pardi, 1953). It has also been proved that the mechanism governing solar orientation in mediterranean sandhoppers is innate, and that these sandhoppers inherit the ability to assume a specific escape direction perpendicular to their own particular shore (Pardi et al., 1958; Pardi, 1960; see also Pardi and Ercolini, 1986 for a review).

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.

INHERITANCE OF SOLAR DIRECTION FINDING IN SANDHOPPERS

SummaryThe capacity ofTalitrus saltator populations to adapt to variations in shoreline directions within a limited space was evaluated by testing both adults collected at different points along continuous sandy arcs, and the 1st and 2nd generations of their laboratory-born offspring (exposed to the sun for the first time during testing). Solar orientation of the adults was adapted to the direction of the shoreline at the capture point, and significant statistical differences appeared between lots from different points. The innate directional tendency of the laboratory-born populations differed between collecting points only a few km apart. Solar direction-finding is thus genetically determined, and these results presuppose a genetic heterogeneity within the natural populations. Individually acquired components could improve orientation in nature.

Visual information acquired during displacement and initial orientation in Polistes gallicus (L.) (Hymenoptera, Vespidae)

Abstract Mechanisms of initial orientation in females of the social paper wasp Polistes gallicus (L.) were studied by releasing them 50 m away from their nest inside an arena and at a distance of 1000 m in the open, after displacing the wasps by four different methods. Results indicate that (1) the accuracy of initial orientation depends on visual information acquired during displacement, (2) this information consists of perceiving the direction of visual flow during passive displacement, and does not depend on the recognition of familiar landmarks, (3) the direction of visual flow is measured relative to a sky compass. The supplementary angle with a celestial reference point at the moment of release would give the wasp the reverse direction for the return flight home.

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.

Astronomical orientation and learning in the shrimp Palaemonetes antennarius

Adult and young individuals of the shrimp Palaemonetes antennarius (Milne Edwards) collected in the field (experienced), and laboratory-born (inexperienced) young were tested in a transparent bowl screened to hide the surrounding landscape from view. Field individuals were collected in South Tuscany, Italy, and testing was carried out in spring-summer 1987 and 1988. Results confirm that adults are able to assume an ecologically efficient escape direction (towards deep-water) by using celestial cues. They also show that: (1) orienting factors consist of the sun and polarized light in the sky; (2) the compass mechanism is time-compensated; (3) escape direction is not genetically fixed but (4) is learnt during the shrimps lifetime and allows ample modification. From the results, it is also hypothesized that genetic determination of the escape direction is related to the type of larval development (with or without planktonic phases) as well as to the vagility of the species.

Relationship between compass systems of orientation in equatorial sandhoppers

I investigated the relationship between the sun compass and the magnetic compass in zonal orientation of the equatorial sandhopper Talorchestia martensii. I carried out experiments in Kenya, in a confined environment, during the equinoctial phase, in which it is theoretically difficult to use the sun as an orienting cue because of the marked variation in the hourly azimuthal speed and zenithal distance. My results show that the sun compass and the magnetic compass coexisted but were not always used contemporaneously or, at least, did not have the same importance. At the equinox this depended in large part on the height of the sun: when the zenithal distance was <10° the magnetic compass prevailed. The relationship between the two compass mechanisms revealed in the present study overcomes the problem of the need to postulate a differential compensation for the apparent motion of the sun by equatorial sandhoppers.