Burrowing behavior of Chaoborus flavicans larvae and its ecological significance

The form and extent of the galleries created by burrowing invertebrates influence sediment structure, bioturbation, and the fluxes of nutrients and contaminants between the sediment and the water column. An important burrowing insect in soft sediments is the phantom midge Chaoborus flavicans. Larvae of this dipteran migrate between the water column, where they feed at night on zooplankton, and the sediment, where they find refuge from predatory fish during the day. The extent to which these larvae disturb sediment during their daily migrations and the depth to which they burrow are poorly known in large part because of a lack of adequate techniques for studying animal behavior near and below the sediment-water interface. We used 2 techniques to record larval behavior in the laboratory, that is, infrared videos to determine how larvae burrow into sediment and X-ray images of burrowed larvae to determine whether they burrow into deeper anoxic sediment and whether they maintain a tube connected to the overlying water. Our observations indicate that larval burrowing visibly disturbs surface sediments thereby contributing to bioturbation. Once buried, larvae do not remain in the thin surface oxic layer but burrow into anoxic sediment. In anoxic sediment, larvae assume a vertical, anterior-end up, S-shaped posture and do not create a tube connected to the overlying water through which they could pump oxygenated water. Thus, larvae are likely exposed to toxic hydrogen sulfide and contaminants in interstitial water, to which they are likely tolerant.