Nesting biology of the mangrove mud-nesting ant Polyrhachis sokolova Forel (Hymenoptera, Formicidae) in northern Australia

Summary:

The nest sites of the mud-nesting ant Polyrhachis sokolova were studied in Darwin Harbour mangroves. They were found from the Ceriops tagal zone to the Rhizophora stylosa zone at elevations ranging from 7.22 to 5.99 meters above the lowest astronomical tide (LAT), which means that the nests were inundated in 13‐61% of all high tides and for durations of up to 3.5 hours. The nest structure was studied by excavating nests and making a cast of the galleries using polyurethane foam. The nests were quite extensive, normally with two elevated nest entrances and galleries down to depths of 45 cm. The loose soil particles at the nest entrances collapsed when the tide reached them and formed a stopper which prevented water from intruding into the nest. In this way, the galleries remained dry during high tide. The ants showed a clear swimming or "walking on the surface" behaviour when they returned to the nest just before the entrance collapsed and during ebb. The tolerance of the ants to submergence was tested in the laboratory, with 50% mortality after 11 hours submergence in seawater at 23 °C, and only 3.5 hours in water at 33 °C. Therefore, the nesting behaviour with trapped air in the galleries is necessary for survival in these environments.