H₂S-A Settlement Cue or a Toxic Substance For Capitella sp. I Larvae?

In small-scale laboratory experiments, organic-rich sediment lacking sulphide elicited settlement and metamorphosis in freshly hatched Capitella sp. I larvae, so that 90% settled within 30 min after hatching. Settlement times of somewhat older larvae were even shorter; 90% settled into the mud in less than 5 min. The addition of sulphide to these treatments (0.5 mM, 1.0 mM, and 2.0 mM) delayed settlement, so that it took several hours for 90% of the larvae to settle. Many of these larvae showed abnormal behavior and settled distant from the sediment. Sulphide alone (without sediment) enhanced settlement in a concentration-dependent manner (0.5-2.0 mM), as previously reported by Cuomo (1985). However, this response occurred over 12-24 h and abnormal larval settling behavior was observed. Hypoxia produced a similar response. Considerations of behavior and swimming capabilities of Capitella larvae, near-bottom hydrodynamic conditions in the field, and the time course of these responses to organic-rich sediment, sulphide, and hypoxia, lead to the conclusion that sulphide is not a settlement cue promoting habitat selection in Capitella sp. I larvae. The apparent enhancement of settlement by sulphide is hypothesized to be a sub-lethal toxic effect.