Four different approaches were combined to determine the nutritional relevance of debris chambers in the burrows of two thalassinidean shrimps: (1) the natural abundance of carbon and nitrogen stable isotopes in potential food sources, (2) their nutritional value based on the content and composition of essential nutrients, (3) a dual labelling experiment with shrimp in aquaria employing 15 N-and 13 C-labelled seagrass debris and (4) ration estimates using the acquisition rate of plant debris by

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... the shrimps. The results of the four approaches confirmed the use of plant debris as a food source. Based on the natural abundance of stable isotopes, Corallianassa longiventris apparently relies on the chamber content and the burrow wall as sources of carbon and nitrogen, whereas Pestarella tyrrhena probably relies on ambient debris and on benthic foraminiferans and microphytobenthos in the surface sediment. Corallianassa longiventris obtains its essential nutrients predominantly from chamber debris and to a lesser extent from its burrow wall, P. tyrrhena from chamber debris, the burrow wall and the surface sediment. Among the essential nutrients, those amino acids commonly deficient to deposit feeders were particularly enriched in the burrow environments of the two shrimps. Highly unsaturated fatty acids (HUFAs) were lacking in all of C. longiventris potential food sources; this species may either be able to synthesize them de novo from linolic acid or may use another unknown source. For P. tyrrhena , surface sediment and chamber debris represent potential HUFA sources. The most probable thiamine and b-carotene supplier for C. longiventris is the chamber debris, for P. tyrrhena again the surface sediment. In both species, the rate of debris introduction into the burrow is sufficient to meet the nutritional demand.