Polyrhachis sokolova, Forel, 1902

Polyrhachis sokolova View in CoL

This species is undoubtedly the most “marine” ant, nesting in the mangrove mud from the dryer Ceriops tagal to the wet Rhizophora stylosa zones amongst the tunnels of the crabs and mudskippers.

In Darwin Harbor most nests are situated at elevations from 7.2 to 6.0 meters above Lowest Astronomical Tide (LAT) which means that the nests at the upper range of the tide are immersed for 13% of the tidal cycle and for durations up to 1.25 hours. The nests at the bottom of the tidal range are covered by 64% of the cycle and the flooding can last up to 3.75 hours, with more than 2 meters of seawater above the nest ( Fig. 2 View Fig ).

The nests are often polydomous with small volcano-like entrances made by the materials from the nest. In a study exploring the structure of the nests, polyurethane foam was injected into entrances until all the galleries were filled. Sections of the cast were removed after sawing the dense root-system and removing the mangrove mud ( Nielsen 1997a). Fig. 3 View Fig . shows the gallery cast and demonstrates how the galleries are most abundant in the upper 20 cm of the mud, but can reach down to 45 cm. The numerous small chambers and galleries are situated around the complex mangrove root systems which prevents the air-filled chambers from collapsing under the high pressure at high tide. The galleries are always excavated in materials which are quite air- and water-tight. Air caught in the convoluted gallery system will remain in small pockets, where the ants and brood can cluster to survive the flooding ( Nielsen 1997a).

The flooding of the nests and the way in which the ants and brood retreat to the air pockets were captured in the film “Life in the Undergrowth” ( Attenborough 2005).

Polyrhachis sokolova View in CoL has adapted very successfully to its environment by developing an efficient swimming behavior, which is commonly used during foraging when crossing puddles or in and out coming tide ( Fig. 4 View Fig ). The ants can walk and jump on the water surface like water striders. When swimming they break the surface with the legs, using the four front legs as oars and the two hind legs as rudders ( Kohout 1988, Nielsen 1997a).