Seawater Ca2+ concentration influences solar orientation in Talitrus saltator (Crustacea, Amphipoda)

The role of salinity in the ecophysiology of many intertidal invertebrates has been extensively investigated. Calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+) are the major constituents of seawater and it has been demonstrated that sandhoppers tested under the sun in diluted seawater (3.5 ) head seaward, instead of going landward as expected. Therefore, the variation in seawater salinity (from 35 to 3.5 ) influences their directional choice. This paper investigates the contribution of different cations to the sea–land directional choice of Talitrus saltator (Crustacea, Amphipoda) by the sun compass orientation mechanism. Results of releases carried out in basic seawater selectively deprived of Ca2+, Mg2+ or K+ and containing the same concentration of Na+ indicate that only the reduction in Ca2+ concentration affects the capacity of solar orientation. The pH does not influence the directional choice of sandhoppers and nor do small variations in salinity in the range 32–39 . Moreover, the clear photopositive tendency registered in experiments of phototaxis in Ca2+-deprived seawater indicates that the absence of Ca2+ does not affect the normal functioning of the visual cells. Therefore, our results show that Ca2+ seawater concentration is important for the correct functioning of one of the principal mechanisms of orientation in supralittoral amphipods and it could affect their survival in the field.