Ricerche sull'orientamento astronomico di anfipodi litorali della zona equatoriale

Abstract

1. The equatorial population of Talorchestia martensii Weber of the shore of Mogadishu (Somaliland) (theoretical line of escape toward the sea, TLE=154°, about SSE) is capable of lunar orientation. 2. However, the mechanism of lunar orientation, as well as the solar one, shows particular time variations of its correctness, due to the local astronomic circumstances. 3. Two groups of animals were tested at the same time: collected at sunset (T), and collected during the day before the night of experiments (G). Both the sets of animals (T and G) show systematic differences from the correct orientation between the period when the moon culminates South (series of experiments with negative lunar declination: Figs. 1A—B), and that one when the moon culminates North (series of experiments with positive lunar declination: Fig. 15A–B). 4. When the moon culminates South, the animals are actually well orientated: Figs. 2, 3, 4, 5. They show, therefore, during this period, an ability to compensate the apparent clockwise motion of the moon, which is good in the range of limits of the mechanism of lunar orientation of the Amphipods (see: Papi and Pardi, 1953 and 1959): Figs. 6, 7, 8, 9, 10. 5. When the moon culminates North, and so it has an apparent counter-clockwise motion, the animals are sometimes well orientated when the moon has its azimut about on the E (Fig. 16 A). But they show a strongly disturbed orientation when the moon is near to its culmination (Fig. 17). Under the circumstances, the animals are found to be more widely scattered, and they often show at the same time a phototactic positive tendency as well as a correct directional one. Generally, the animals tested when the moon culminates North, do not show the same behavior, either as to the angles of orientation, or, least of all, as to degree of dispersion. 6. No prominent differences can be recognized in regard to the behavior of animals T and G (series of Figs. A and B; Figs. 6–10). This suggests that also animals kept in constant darkness for several hours before the experiment (series G) can still show a correct lunar orientation (when it is possible). This result agrees with Papi and Pardi's hypothesis that the lunar orientation might be controlled by a continuously operating endogeneous rhythm, with suitable periodicity, which is different from the one controlling solar orientation. 7. During the night of the passage of the moon at the zenith (Figs. 11, 12, 13, 14), the animals still show a correct orientation up to zenithal distances of about 8°–9°. For smaller zenithal distances of the moon, the animals become strongly disorientated, and for the least zenithal distances they show (likewise during the passage of the sun at the zenith), the characteristic reversible phenomenon to pile up on each others (Fig. 14, above). 8. The different behavior observed during the two periods of negative and positive lunar declination is discussed. Up to this day, the collected data support, in our opinion, the many a time propounded hypothesis of an interference between a positive phototactic tendency and a correct directional tendency on the ground of the astronomical photomenotaxis.