Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon

The sand stored in the gizzard of some freshwater snails should assist in the mechanical digestion of tough food, yet effects of gizzard sand on consumption of living macrophyte tissue are seldom addressed. We quantified the effects of gizzard sand both on consumption of plant tissue and on snail growth and reproduction with a cross-gradient, 28-day laboratory experiment. We offered apical shoot sections of Myriophyllum spicatum (initial ash content ~ 56%) or soft lettuce (ash content ~ 19%, as control) to Lymnaea stagnalis previously reared on sand or without sand. Gizzard sand had no effect on snail growth but sand-reared snails fed with lettuce initially produced more eggs than their no-sand counterparts. Snails fed M. spicatum exhibited stunted growth and produced only one small egg clutch. Shell height at time of first oviposition of M. spicatum-fed snails was two-thirds of that of lettuce-fed controls. Snails initially appeared to feed on periphyton and older, brownish M. spicatum leaflet tips, but later turned to midstem tissue. Fresh apices were not consumed. Gizzard sand had a limited effect on the ability of L. stagnalis to consume living M. spicatum, with apices likely defended chemically (polyphenols) and midstem tissue defended structurally (high ash content).