Preingestive selection processes in the cockle Cerastoderma edule: Mucus production related to rejection of pseudofaeces

Abstract

Feeding behaviour of Cerastoderma edule fed diets with different organic contents was studied under conditions that induced production of pseudofaeces. Cockles were fed suspended matter composed of cells of Tetraselmis suecica and inorganic particles of ashed sediment, and we measured the rates and efficiencies with which phytoplankton but also overall organic matter was processed both at pre- and postingestive level. Preferential ingestion of microphytoplankton was observed and derived benefits computed, under a framework whose main objective was to quantify the mucus involved in pseudofaeces rejection and its effect on selection processes. Energy losses in the form of mucus depended on the rate of pseudofaeces production but also on the organic content of filtered matter. Under feeding conditions characterised by high rates of rejection of filtered matter of low organic content, the production of endogenous organic matter in the pseudofaeces represented the main organic component. Consequently, lack of quantification of this term would definitively bias all calculations derived from computing the organic fraction of rejected matter. In particular, the reduction in the efficiency with which organic matter from the diet is selected under low seston organic content, reported in previous studies, was found to be a consequence of the presence of increased mucus in pseudofaeces and not an indicator of loss of efficiency in the sorting process. We have calculated that underestimation of 'true' rates of organic ingestion and absorption of dietary organic matter may be as high as 46 and 98.6 %, respectively, as a result of the assumption that the presence of mucus in the pseudofaeces is negligible. Finally, taking into account the energy loss in the form of mucus when pseudofaeces are produced, it is concluded that this energetic investment may be a major determinant of the feeding strategy of cockles.