Effects on survival and mucous cell proliferation of Atlantic halibut, Hippoglossus hippoglossus L., larvae following microflora manipulation

A higher number of epidermal mucous cells was observed in Atlantic halibut ( Hippoglossus hippoglossus L.) yolk sac larvae kept in seawater from 50 m depth, containing 7.5×10 4 –10.3×10 4 bacteria/ml (DAPI), as compared to larvae kept in seawater from 200 m depth with 3.3×10 4 –5.0×10 4 bacteria/ml. The affected mucous cells were identified as saccular cells. The number of saccular cells was also higher in larvae kept in incubators with seawater that were supplemented with Lactobacillus plantarum (about 10 6 bacteria/ml). Thus, it appeared that the number of bacteria in the seawater could affect the proliferation of saccular cells in halibut larvae. Furthermore, survival of the halibut larvae was affected by pre-incubation of eggs and hatching larvae with different strains of commensal bacteria isolated from fish. A larval survival of 94.4% was obtained following pre-incubation with an apathogenic Vibrio salmonicida strain, whereas V. iliopiscarius sp. nov. reduced survival to 63.4% in the first 12 days, compared to 81% survival in the control group. In the same period, survival following incubation with L. plantarum was 95.8%. Survival 32 days post-hatching was significantly higher in the groups of halibut larvae pre-incubated with L. plantarum (68.4%) and V. salmonicida (72.8%), as compared to the control group (58.2%). Pre-incubation with V. iliopiscarius resulted in 41.8% survival at day 32, which was significantly lower than the control group. Thus, pre-incubation of halibut eggs and hatching larvae with strains of commensal bacteria may subsequently affect larval survival, and in the present study, an indigenous apathogenic strain closely resembling the marine pathogen V. salmonicida resulted in the highest survival.