Preferences of invasive Ponto-Caspian and native European gammarids for zebra mussel (Dreissena polymorpha, Bivalvia) shell habitat

We investigated habitat preferences of two invasive Ponto-Caspian gammarids (Dikerogammarus haemobaphes and Pontogammarus robustoides) and a native European species (Gammarus fossarum) in laboratory experiments. The habitats consisted of the following objects: (1) living zebra mussels; (2) empty mussel shells (clean or coated with nail varnish) with both valves glued together using aquarium silicone sealant to imitate a living mussel; (3) stones (clean or varnished); (4) empty plates. Ten objects of the same type were glued to a plastic plate (10 x 10 cm) with methyl acrylic glue. The plates were placed in experimental tanks in various combinations. A single gammarid was put into the tank and its position was determined after 24 h. The studied species responded differently to the presence of zebra mussels. D. haemobaphes preferred living mussels rather than their empty shells and these two habitats over stones and empty plates. It responded positively to shell shape, selecting varnished shells rather than varnished stones, and to shell surface properties, selecting clean shells rather than varnished shells. It did not respond to waterborne mussel exudates. P. robustoides did not exhibit any preferences for the above-mentioned substrata. G. fossarum was attracted by empty mussel shells (but not by living mussels). It responded only to their shape, not to surface properties. The strong affinity for zebra mussels, exhibited by D. haemobaphes, might help it survive and develop stable populations in newly invaded areas.