Planarian (Dugesia polychroa) predation on freshwater gastropod eggs depends on prey species, clutch morphology, and egg size

Though triclad planarias could limit littoral snail recruitment by preying on eggs with their muscular ventral pharynx, planarian predation on eggs has never been quantified. Intact egg clutches encompassing eight snail species x three developmental stages were offered to Dugesia (= Schmidtea) polychroa (Paludicola: Dugesiidae) individuals (body length = 6-12 mm) in one-on-one, no-choice 24-h feeding trials to gain a first insight on snail egg palatability to dugesiid planarias. Predatory losses were quantified as differences in planaria-present trials vs. parallel planaria-absent controls. D. polychroa predation on snail eggs was generally low. Eggs in monolayered, flat clutches of bithyniids (Bithynia leachii) and planorbids (Planorbis planorbis, Helisoma trivolvis, and Planorbarius corneus) were more susceptible to planarian predation than multilayered eggs in sausage-shaped clutches of physids (Physa acuta) and lymnaeids (Lymnaea stagnalis, Radix auricularia, and Stagnicola fuscus). Planarian predation also was higher on smaller eggs, and was not related to planarian body size. Vulnerability was also markedly species-specific, with appreciable losses recorded only for pre-hatching eggs of P. planorbis and, to a lesser extent, H. trivolvis. The results suggest that, with the possible exception for the soft-clutch, small eggs of P. planorbis, predation by D. polychroa on eggs does not impact gastropod recruitment appreciably.