Dietary Preferences and Carbon Absorption by two Grazing Gastropods, G/bbula umbilicaris (LINNÉ) and Jujubinus striatus (LINNÉ)

. Ingestion rates and absorption efficiencies in Gibbula umbilicaris and Jujubinus striatus feeding on green algal films, diatoms, macro‐epiphytes, and dead Posidonia ocean/'ca‐Ieaves have been investigated in laboratory experiments. For both gastropod species, ingestion rates were highest in diatoms and thin green algal films as food source. G. umbilicaris exhibited significant lower consumption rates when feeding on seagrass debris or macro‐epiphytes, J. striatus consumed neither Posidonia‐debris nor macro‐epiphytes. Highest carbon absorption efficiencies were found in both gastropod species when feeding on diatoms and green algal films. Significantly lower efficiencies were found in G. umbilicaris when consuming seagrass debris or macro‐epiphytes. Analyses of fecal material confirmed these findings. Highest organic carbon contents were measured in fecal pellets produced after consumption of seagrass debris or macro‐epiphytes reflecting the presence of structural carbohydrates. SEM examinations of feces revealed mainly diatom frustules, refractory parts of macroalgal and other plant material. Analyses of gut contents gave similar results suggesting a basically microphagous herbivory for both gastropod species. Since seagrass debris and macro‐epiphytes are not used very effectively, the high absorption efficiencies when animals feed on the microfloral component of the periphyton are discussed as possible adaptation to limited food availability.