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.