Mussel ( Mytilus edulis L.) and ragworm ( Nereis virens , Sars) both alleviate anaemia in common sole ( Solea solea L.)

Common sole (Solea solea L.) is a polychaetemollusc feeder (de Groot 1971). Based on the feeding ecology of sole, we assume that both ragworm (Nereis virens, Sars) and mussel (Mytilus edulis L.) reflect the dietary requirements of common sole. Common sole grows approximately 70% faster on ragworm than on commercial pellets (Ende, Kroeckel, Schrama, Schneider & Verreth 2014), the latter causing them to suffer from nutritional anaemia (Kals, Blonk, Palstra, Sobotta, Mongile, Schneider, Planas, Schrama & Verreth submitted). Changing the diet from pellet to ragworm alleviates this nutritional anaemia (Kals, Blonk, van der Mheen, Schrama & Verreth 2015). This suggests a mismatch between the feeding requirements of sole and the dietary composition of the commercial pellets. Fonds, Drinkwaard, Resink, Eysink and Toet (1989) and Ende et al. (2014) showed that the growth of sole fed mussel is comparable to the growth of sole fed ragworm. We, therefore, expect that also mussel will alleviate anaemia in sole, consequently showing comparable levels of haematocrit (Hct) and haemoglobin (Hb) for sole fed mussel or ragworm, but higher for sole fed pellet. Apart from trying to find evidence for the foregoing hypothesis, we wanted to gain insight into which dietary components could be important to alleviate nutritional anaemia in sole. This experiment was approved by the Ethical Committee for Animal Experiments and conducted at IMARES, Yerseke, the Netherlands. The experimental set-up contained three diets; raw mussel meat (mussel), fresh ragworm (ragworm) and a commercial pellet (pellet) (crude protein 63%, ether extract 14%, ash 12%). The diets were tested in triplicate for a period of 23 days with 10 fish per tank, or 90 fish in total. Mussels were collected at the Oosterschelde, the Netherlands. The meat was separated from their shell and stored at 80°C. The frozen mussel was thawed in a fridge the day before feeding. Ragworms were delivered by Topsy Baits, Wilhelminadorp, the Netherlands. Sole (220 44 g) reared on pellet and na€ıve to ragworm and mussel were randomly accommodated in 9 tanks (0.4 m, 130 L), in a flow through system using sand filtered seawater. Husbandry conditions were as follows: photoperiod 12L:12D, light intensity 11–15 lux, temperature 16.9 1.3°C, oxygen 8.3 0.5 mg L , pH 8.1 0.1, TAN 0.3 0.2 mg L , NO2 , 0.01 0 mg L , salinity 25– 30 g L 1 and flow 4.5 0.5 L min 1 kept within pre-set limits. Temperature and oxygen were measured daily; flow, pH, TAN, NO2 weekly. Fish were fed by hand twice a day (8:30 and 16:30 hours). During acclimatization, all fish were fed the pellet. The dry matter content of the diets was analysed to enable equal feeding levels (restricted to 0.54 g dm per fish per day) for all diets. One hour after feeding,