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Regional Specializations in the Oro-Pharyngeal Wall and Food Processing in the Carp (Cyprinus Carpio L.)

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For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

The structure of the oro-pharyngeal wall of the bottomfeeding common carp is investigated using light-and scanning electron microscopy. Densities of taste buds, mucus, club cells and the thickness of muscular layers are measured. Distribution patterns of these elements over the oro-pharyngeal surface are reconstructed from local counts. Six areas characterized by a specific combination of morphological features are distinguished and related to twelve feeding actions composing the process of food intake and handling in the carp. These areas are the lips (detection and oral manipulation of food), the orobuccal cavity (suction and resuspension chamber of ingested food and non-food particles), the most anterior pharynx (coarse selection of large food particles), the lateral pharynx (selection of small food particles), the posterior part of the anterior pharynx (formation of boluses, transport and loading of food into the chewing cavity) and the posterior pharynx (mastication and deglutition). The conical shape of the orobuccal cavity and the slit-shaped anterior pharynx reflect two different mechanisms for particle handling viz. by suction and by muscular bulging respectively. The opercular cavities serve large volume changes for suction feeding. Protruding types of taste buds and oligovillous epithelial cells may well serve mechanoreceptive functions required for steering the process. Otherwise, specialized mechanoreceptors have not been recognized. Mucous cells producing low-viscosity sialomucines occur in the anterior part of the oro-pharynx. They will serve maintaining a laminar flow during suction and lubrication of particle handling in the pharynx. Epithelial microridges may aid in holding the mucus. High-viscosity sulfomucines only appear in the posterior part of the pharynx and will aid in trapping small particles and aggregating them into boluses. The commonly accepted alarming function of club cells and their mechanism for release is questioned in view of their abundancy in the orobuccal cavity. The structure of the muscular palatal organ is discussed with respect to its role in selection between food and non-food particles. The available information on the afferent, efferent and central neural pathways of this system is briefly reviewed. Three levels of movement, related to the particle size to be handled, are proposed. Movement of the palatal organ as a whole, local outbulging of its surface into the pharyngeal slit and a possible very local movement of the muscular papillae in its anterior part. These hypotheses are based on the almost maximal taste bud densities (820/mm2) in the palatal organ, the known complex laminated cyto-architecture of the enormous vagal lobes processing its input and suggestive of a palatotopic mapping, and on the complex muscle fiber systems in this organ. The movable gill rakers of the branchial sieve, each supplied with a muscular pad and numerous taste buds (325-625/mm2), suggest their additional active role in selection. Muscle fiber systems in the posterior part of the palatal as well as in the closely appressed postlingual organ serve a peristalsis-like transport to the chewing cavity. Both are copiously supplied with sulfomucines from their deep crypts. Together these morphological and physiological features allow the carp a bottom feeding behaviour requiring the effective separation of food from soiled mixtures.

Affiliations: 1: (Department of Experimental Animal Morphology and Cell Biology, Agricultural University, Zodiac, Marijkeweg 40, 6709 PG Wageningen, The Netherlands

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