Coprophagy: a supplementary food source for two freshwater gastropods?

1. The freshwater pulmonate snail Radix peregra voluntarily and regularly fed on its faeces, whereas Bithynia tentaculata (Prosobranchia) did not. 2. With high-quality faeces as food (faeces with C:N-values below ten) Raaix could grow and survived for 11 weeks. Growth was 24-30% of that achieved with control food (lettuce, Chlamydomonas and Tetramin), and body mass was close to the expected value. 3. As with real food, the percentage organic content of faeces was reduced during gut passage, with the exception of very low-quality faeces. 4. Faeces contained living cells of green algae (Chlamydomonas reinhardii) and diatoms (Achnanthes lanceolata, Eunotia pectinalis), which survived the gut passage of R, peregra and B. tentaculata. Faecal material was also rich in bacteria, with up to 150 x 10(6) cells mg(-1) dry mass. Bacteria increased in abundance in faeces which had been evacuated for 7 days or more. 5. A further degradation of partly digested food in the faeces was;indicated by the activity of the extracellular digestive enzymes cellobiase and chitobiase. These enzymes hydrolysed material with an activity of 0.2-14.5 pkat mg(-1) faecal dry mass in 1-12-day-old faeces. In faeces of Bithynia fed control food or incubated sycamore leaves, chitobiase activity was correlated with bacterial abundance. 6. Coprophagy is considered to be a suitable strategy for further degradation and utilization of refractory material for which one gut passage is too short and/or too inefficient.