Hypoxia induces abnormal larval development and affects biofilm–larval interaction in the serpulid polychaete Hydroides elegans

•The effect of hypoxia on larval development and settlement of H. elegans was studied.•Larval development was delayed and more malformed larvae were found under hypoxia.•Settlement rate was reduced under hypoxia, regardless of the biofilm nature.•The competent larvae were reluctant to settle on the biofilm developed under hypoxia. Hydroides elegans, a worldwide fouling polychaete, can spawn throughout the year, but its recruitment drops during summer when hypoxia prevails. Here, the influence of hypoxia on larval development and settlement of H. elegans was investigated. Results showed that larval development was compromised at 1mg O2 l−1 with a lower proportion of competent larvae and a higher proportion of malformed larvae, probably due to reduction in clearance rate. Regarding larval settlement, although most of the larvae were reluctant to settle at 1mg O2 l−1, regardless of the biofilm nature, they settled quickly within 24h in response to the resumption of dissolved oxygen. Furthermore, only about 5% of the larvae settled on the biofilms developed under hypoxia, regardless of dissolved oxygen levels of the seawater. The delayed larval development and potential alteration of biofilm nature owing to hypoxia explained why the recruitment of H. elegans declines during summer.