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Background By altering their habitats, engineering species can improve their own fitness. However, the effect of this strategy on the fitness of coexisting species or on the structure of the respective food web is poorly understood. In this study, bacteria and bacterivorous nematodes with short ( Caenorhabditis elegans ) and long ( Plectus acuminatus ) life cycles were exposed to the mucus secreted by the freshwater flatworm Polycelis tenuis. The growth, reproduction, and feeding preferences of the nematodes in the presence/absence of the mucus were then determined. In addition, confocal laser scanning microscopy (CLSM) was used to examine the structural footprint of the mucus and the mucus colonization dynamics of bacteria and protozoans. Results Mucus exposure resulted in a greater reproductive output in P. acuminatus than in C. elegans . In a cafeteria experiment, both nematode species were attracted by bacteria-rich patches and were not deterred by mucus. CLSM showed that the flatworms spread a layer of polysaccharide-rich mucus ca. 15 µm thick from their tails. Subsequent colonization of the mucus by bacteria and protozoans resulted in an architecture that progressively resembled a complex biofilm. The presence of protozoans reduced nematode reproduction, presumably due to competition for their bacterial food supply. Conclusion Animal secretions such as mucus may have broader, community-level consequences and contribute to fueling microbial food webs. Electronic supplementary material The online version of this article (10.1186/s12898-019-0231-2) contains supplementary material, which is available to authorized users.

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