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Abstract

Apple snails Pomacea lineata (SPIX 1827) are widespread in the tropical regions of Brazil as well as in the Pantanal wetland of Mato Grosso in the western part of the country. They have a key position in the Pantanal food web and serve as food for many animals e.g. fishes, birds, and caimans. However, little is known about their feeding preferences and growth rates. Stable isotopes have been used successfully on numerous studies as food source indicator. Therefore, the i15N and i13C values of snails from 0.45 to 3.03vcm in length, which were collected in the rainy season from March through May, were analyzed. Snails signatures revealed ambiguous evidence for food preferences. i15N and i13C values ranged between m2.8 and 12.4‰ and between m24.2 and m16.4‰, respectively. This range of values mirrors the highly variable isotope values of possible food sources comprising C3 and C4 macrophytes. To test whether all common food sources were similarly assimilated, feeding experiments with different diets were conducted. Snail eggs were reared in tanks and offered different but single plants. Snails fed different diets and i13C values of the food were reflected in the animal tissue. Growth varied considerably in experiments with different diets indicating the preference for certain food sources. Also, the fractionation of nitrogen isotopes between food and animal varied from 0.1 to 17.0‰. The results are explained by different feeding habits, and it is supposed that animals fed either on the plant itself or on bacteria mats growing in the tanks. In an additional experiment juvenile snails were offered one single food with a distinctive C4 grass signature. These snails did not grow detectably, but nevertheless isotope signatures approached to values of the diet.