Sky radiance and spectral gradient are orienting cues for the sandhopper Talitrus saltator (Crustacea, Amphipoda)

The sandhopper Talitrus saltator relies on both the sun and the moon compasses to return to the belt of damp sand of the beach in which it lives buried during the day. In addition to the sun, the gradient of radiance and the spectral distribution across the sky could provide directional information that T. saltator can potentially use to orient itself during the day even when the sun is not visible (e.g. cloudy sky). The scope of this work was 1) to determine the intensity levels of sky radiance that the sandhoppers use in their zonal recovery and 2) to investigate whether this species relies on the celestial spectral gradient in its zonal recovery. Sandhoppers were tested in the laboratory under artificial radiance or spectral gradients.Our results show that under an artificial sky, simulating the natural radiance gradient on a cloudless day, sandhoppers orientated toward the correct seaward direction of their home beach, however, individuals lost their ability to use the intensity gradient as an orientation cue when the radiance was attenuated by at least 40%. Sandhoppers were also able to head in the correct seaward direction of their home beach at any time of the day by using the spectral gradient as their only source of visual orientation reference.

Regionalization in the compound eye of Talitrus saltator (Crustacea, Amphipoda)

The crustacean Talitrus saltator is known to use many celestial cues during its orientation along the sea-land axis of sandy shores. In this paper, we investigated the existence of the eye regionalization by morphological, electrophysiological and behavioural experiments. Each ommatidium possesses five radially arranged retinular cells producing a square fused rhabdom by R1-R4 cells; the smaller R5 exist between R1 and R4. The size of R5 rhabdomere is largest in dorsal part and becomes gradually smaller in median and ventral part of the eye. Spectral-sensitivity measurements were recorded from either dorsal or ventral parts of the compound eye to clarify the chromatic difference. Results show that the dorsal part is green and UV-blue dichromatic, whereas the ventral part is UV (390 nm) with a substantial population of 450 nm receptors with the responses in the longer wavelength region. To evaluate the orienting behaviour of individuals, their eyes were black painted either in the dorsal or ventral part, under natural sky or a blue filter with or without the vision of the sun. Results show that animals painted on the dorsal part of their eyes tested under the screened sun were more dispersed and in certain cases their directions deflected than other groups of individuals. Furthermore, sandhoppers subjected to the obscuring of this area met in any case high difficulties in their directional choices. Therefore, our present work indicates the existence of a regionalization of the compound eye of T. saltator.Summary statementThis work provides evidences of the morphological and electrophysiological regionalization of the compound eye and the visual capabilities for behaviour involved in the recognition of the celestial compass orienting factors in crustaceans.

Talitrus saltator (Montagu, 1808), a species complex (Amphipoda, Senticaudata, Talitroidea, Talitridae)

The frequently studied amphipod genus Talitrus Latreille, 1802, long thought to be monotypic is shown here to include three species. Two species had already been recognized as distinct species. Talitrus cloquetii Audouin, 1826 and T. platycheles Guérin, 1832, but both had long been reduced to synonymy with T. saltator. All three species are described and figured and a key is provided for their identification. Because of the extensive literature associated with the name Talitrus saltator (sensu lato) we have attempted to assign published records to their appropriate species.

Distribution and abundance of Talitridae in the southern Baltic Sea – twelve years after the first record of Platorchestia platensis (Krøyer, 1845) in 2005

Four Talitridae species have been recorded in the southern Baltic Sea, including two indigenous species – Talitrus saltator, Deshayesorchestia deshayesii, and two presumably non-indigenous ones – Cryptorchestia garbinii, Platorchestia platensis. It has been twelve years since Platorchestia platensis was recorded for the first time. The distribution and abundance of talitrids have not been studied since the 1990s. Therefore, the main objective of this research was to document the occurrence in Talitridae in the region in order to determine whether non-indigenous P. platensis has spread and whether it co-occurs with indigenous species. Talitrids were recorded at 20 out of 43 sampling sites. T. saltator occurred both along the coast of the open sea and in the Gulf of Gdańsk. The remaining species were found only around the gulf. P. platensis was more abundant than other species and its density was positively correlated with wrack biomass. Our studies have shown that the area of T. saltator occurrence has decreased during the last two decades. Non-indigenous species P. platensis co-occurred with all other Talitridae species, whereas C. garbinii co-occurred only with P. platensis.