Interaction among celestial orienting factors and their functioning in supralittoral crustaceans

Behavioural investigations conducted since the Fifties have revealed that the amphipod Talitrus saltator can rely on both the sun and the moon as compass cues in its zonal recovery; recently, evidence for discrete endogenous oscillators underlying its time-compensated solar and lunar orientation has been also given. T. saltator is the first species shown able to obtain compass information by using only the skylight intensity gradient. Instead, it does not rely on the celestial polarization pattern despite its sensitivity to polarized light. Although discrete receptors detecting UV-blue and green light have been identified within its compound eye, neither the capability of this species to use the spectral pattern of the sky nor the regionalisation of its visual pigments (eventually indicating the existence of a DRA) has been investigated. Furthermore, investigations on the structure of its compound eye conducted so far are quite scarce despite the importance of the vision in the perception of orienting stimuli. Evidence for solar and lunar orientation has been provided also in the isopod Tylos europaeus. However, as opposed to its ability to orientate to the sun, its moon compass-based orientation has not been confirmed. The aims of this work are: 1) to deepen our knowledges on the use of the celestial gradients by T. saltator, 2) to evaluate the regionalization of its visual capabilities, 3) to assess the optical and functional structure of its compound eye, 4) to investigate the anatomical localisation of the time-keepers regulating the sun and the moon compass mechanisms, 5) to assess the existence of antennal time-keepers involved in celestial orientation and 6) to confirm the capability of T. europaeus to orientate to the moon. In this work, the first evidence for the use of the celestial spectral gradient as a compass cue by T. saltator was obtained. The skylight intensity profile has also been confirmed to constitute a reliable orienting reference and it has been shown that it exists a minimum threshold of the gradient effectively recognised and used. Instead, tests carried out did not point out a clear spatial distribution of the photoreceptors within the eye of this species. However, it has been revealed that the dorsal edge of the eye plays an important role in the perception of celestial factors. These results, along with evidence of straight ommatidia occurring in this area of the eye, suggest a regionalisation of the visual capabilities in T. saltator and are in agreement with the existence of a DRA. Furthermore, it was shown that this species mainly possesses hook-shaped ommatidia (except for the dorsal region of its eye) and it was suggested that their photoreception efficiency was enhanced by reflecting pigment cells localized between them. Moreover, it was found that the oscillators underlying the sun and the moon compass mechanisms are localised in separate localities. In fact, the antennae seem to be the anatomical site of the time-keepers responsible for the lunar orientation (although our results suggest that timing inputs from these oscillators are downstream integrated), whereas those involved in solar orientation are located elsewhere (probably in the brain). Intriguingly, present work provided first molecular evidence for time-keepers in T. saltator by revealing rhythmicity in the expression of core genes in both brain and antennae (thus supporting the existence of oscillators in these appendages). Finally, it has been fully confirmed the capability of T. europaeus to orientate under the moon and provided partial evidence for discrete time-keepers underlying the functioning of the sun and the moon compass systems in this species.