Mangrove Ants adapt to nesting underwater

mangrove ants
One nest entrance is circled, and another was just behind me as I took this shot

When I was wandering along the bank of Ross River near the Bowen Road bridge a few months ago, I looked down, saw a perfectly ordinary looking ants’ nest and a moment later thought, “Hey! That’s odd! That would be under water at high tide!”

The ants were perfectly at home, however, running in and out of their nest entrance. Odd indeed.

mangrove ants
Close-up of a nest entrance, showing the ants

AntWiki, an excellent resource I found while looking them up, says:

This is the only species of Polyrhachis known to nest in marine and estuarine mud of the intertidal zone. Its nests are mound-like, with a small opening at the top, and are completely submerged at each high tide. Distribution is limited to the tidal mudflats of coastal mangrove forests. (Kohout 1988)

Polyrhachis sokolova occurs along the Australian coastline from Torres Strait to as far south as Gladstone in central Queensland (Kohout 1988). It is a quite common species that also occurs beyond the Australian mainland, with records from the Aru Islands, the southern coast of Papua and New Caledonia (Kohout, 2013).

They are certainly not uncommon here. Mike Downes, my Friendly Local Expert for all things formic, told me he knew of nests at Pallarenda’s Three Mile Creek.

Mogens Neilsen studied them in Darwin Harbour in the 1990s and found 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.

mangrove ants
The other nest entrance nearby was doubled, as described by Neilsen

Although the ants can swim when necessary, they can’t survive long under water except in their sealed nest, and they have to put a lot of effort into repairing water damage after each high tide. All this is apparently worthwhile, however, for the sake of exploiting a habitat which other ants have to avoid. Ajay Narendra found that their favourite food in captivity was pipis, and this anonymous video seems to show them feeding on a dead crab.

The ants themselves don’t have any obvious distinguishing physical characteristics. Like most of the nearly 500 species in their genus, Polyrhachis, they are smallish and dark – in this case grey and black. Like their cousins, the black weaver ants Mike discussed here on Green Path a year ago, they are identified most easily by their nesting habits.

• This post was first published, in a slightly different form, in Kapok, newsletter of Coastal Dry Tropics Landcare (FB page), in December 2017. 

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