Importance of Mucus in Filter-Feeding of Bream (Abramis brama)

Abstract

Mucus boluses, found in the oropharyngeal cavity of a large number of freshly caught bream (Abramis brama), appear to play an important role in the aggregation and transport of food particles from the branchial sieve to the esophagus. Mucus boluses, some containing up to 900 zooplankters, were isolated from the oropharyngeal cavity of bream, and from these the filtering rate and retention ability of the filtering system were calculated. Using light microscopy and special staining methods, we identified different types of mucus cells in the oropharyngeal wall of bream that increase in number from anterior to posterior. Based on these data and on our knowledge of branchial sieving, we have hypothesized the role of mucus in the filter feeding mechanism of bream. Initially, small particles are retained mechanically in the branchial sieve, eventually becoming coated with mucus. They then act as "sticky" particles with respect to other mucus-coated particles. During recollection from the branchial sieve following closed protrusion of the mouth, the sticky particles adhere to one another forming a mucus bolus that is stored at the posterior end of the branchial chamber. After several of these cycles over a period of several minutes the bolus is swallowed.