Physiological and behavioural responses of different life stages of a serpulid polychaete to hypoxia

Hypoxia has become a major threat to coastal marine ecosystems worldwide. Among various groups of marine invertebrates, sessile ones are more susceptible to hypoxia since they cannot move away from hypoxic waters. The serpulid polychaete Hydroides elegans Haswell, 1883 is a fast-proliferating, dominant, sessile-fouling species in the tropical Pacific, which includes Hong Kong. We studied the effects of hypoxia on the fertilization success, embryogenesis and physiological and behavioural responses in both larval and adult stages of H. elegans. Results showed that fertilization success was reduced and embryogenesis was retarded at a dissolved oxygen (DO) level of 2 mg O-2 l(-1). At this DO level, the swimming velocity of the 2 d old larvae significantly increased, which may serve as an escape response. H. elegans showed a very high tolerance to hypoxia in both larval and adult stages, as indicated by the 24 and 48 h lethal DO concentation (LC50). However, the respiration rate and clearance rate of adult H. elegans were significantly reduced at 2 mg O-2 l(-1). Partial recovery was observed 2 d after normoxia was resumed. Previous studies have demonstrated that adult H. elegans suffer from high mortality and that there are no new recruits in summer when the temperature is high and DO concentration low. The results are interpreted as the effect of low salinity. Our study has strengthened the notion that low salinity contributes to adult mortality in summer; however, the lack of summer recruitment is possibly a result of the combined effect of low salinity and hypoxia.