Mannitol oxidase is an enzyme present in some gastropods, converting the polyalcohol mannitol into the sugar mannose. Molecular oxygen is the hydrogen acceptor and hydrogen peroxide is produced. Therefore, this enzymatic activity can be measured by monitoring H2O2 production using a horseradish peroxidase coupled assay adapted from previously published methods (Malik et al., 1987; Cablé et al., 1993; Rocha et al., 2003). This assay was used to evaluate mannitol oxidase activity in the digestive gland of gastropods, but in some slugs and snails this enzyme was also detected in digestive tract tissues (Vorhaben et al., 1984; Malik et al., 1987). This enzyme can also oxidase other polyalcohols (Large & Connock, 1994)