BEHAVIOR AND TOLERANCE TO HYPOXIA IN JUVENILE NORWAY LOBSTER (NEPHROPS-NORVEGICUS) OF DIFFERENT AGES

The annual occurrence of hypoxia (<25% oxygen saturation) in the botto m waters along the Swedish west coast coincides with the postlarval se ttlement of Norway lobster, Nephrops norvegicus (L.). This study inves tigates behaviour and the experimental effects of low oxygen concentra tions in juvenile N. norvegicus of different ages. All experimental in dividuals were reared to the juvenile (postlarval) stage in the labora tory and then given sediment as a substratum. Behavioural responses to low oxygen concentrations were tested in early and late Postlarvae 1 exposed to normoxia (>80% oxygen saturation, pO(2) > 16.7 kPa), modera te hypoxia (30% oxygen saturation, pO(2) = 6.3 kPa) and hypoxia (25% o xygen saturation, pO(2) = 5.2 kPa). The experiments were run for a max imum period of 24 h or until individuals died. Behaviour was studied u sing sequential video recordings of four behavioural activities: diggi ng, walking, inactivity or flight (escape swimming up into the water c olumn). Behaviour and mortality changed with lowered oxygen concentrat ions; energetically costly activities (such as walking) were reduced, and activity in general declined. In normoxia, juveniles initially wal ked and then burrowed, but when exposed to hypoxia they were mainly in active with occasional outbursts of escape swimming. To increase oxyge n availability the juveniles were observed to raise their bodies on st ilted legs (similar to adults in hypoxic conditions), but oxygen satur ations of 25% were lethal within 24 h. The results suggest that the ma in gas exchanges of early postlarval stages occur over the general bod y surface. Burrowing behaviour was tested in Postlarvae 1 and 2 of dif ferent ages held in >80% oxygen saturation for 1 wk. The difference in time taken to complete a V-shaped depression or a U-shaped burrow was measured. The results showed a strong negative relationship between p ostlarval age and burrowing time, but all individuals made a burrow. J uveniles were more sensitive to hypoxia than adults. Thus, the possibl e consequences of episodic hypoxia for the recruitment of Nephrops nor vegicus and for the recolonization of severely affected areas are disc ussed.