Experimental evaluation of preferences of an invasive Ponto-Caspian gammarid Pontogammarus robustoides (Amphipoda, Gammaroidea) for mineral and plant substrata

Pontogammarus robustoides has spread across Europe since the second half of the twentieth century. It is often regarded as a psammophilous and/or phytophilous species. We studied preferences of P. robustoides for 5 mineral substrata of different grain sizes, 5 plant species and 3 artificial plants differing in leaf shape. We tested 10 gammarid individuals in 24-h pairwise choice tests in darkened tanks. Adults selected stones of 27 ± 4.8-mm grain diameter, rather than smaller or larger objects, whereas juveniles preferred particles of 15 ± 3.2-mm grain diameter. Structurally complex plants (natural and artificial) were most preferred for both age groups. Juveniles, unlike adults, preferred natural plants over artificial ones and mineral substrata. Adults preferred mineral materials over macrophytes and did not differentiate between natural and artificial plants. Juveniles did not avoid areas occupied by adults, but in their presence exhibited significantly stronger preferences for substrata providing them with suitable shelters. Habitats selected by gammarids in our study somewhat differed from those commonly used in the field, indicating their high plasticity. Different preferences of adults and juveniles may help them avoid competition and/or cannibalism in the field.