The intertidal zone (sometimes referred to as the littoral zone) is an area that is exposed to the air at low tide, and goes underwater at high tide. It is one of the harshest and most unstable habitats in the tropics, yet unexpectedly, it is home to a peculiar species of menacing-looking trap-jaw ants — Odontomachus malignus (O. malignus). This was initially the only known trap-jaw ant species, exclusively found along coastlines in the Indo-Pacific, but recently a new species has been discovered in Singapore mangroves.
The new species, named Odontomachus litoralis (O. litoralis) in direct reference to its unique littoral habitat, can be found nesting in abandoned mud lobster mounds throughout local mangroves. These mounds are usually submerged during high tide, so the ants can only emerge to forage for food at low tide.
The identification of O. litoralis was made by Dr Wendy Wang, Curator of Entomological Collections at the NUS Lee Kong Chian Natural History Museum, in collaboration with Japanese researchers, including recognised Asian ant expert Professor Seiki Yamane. The results were published in the journal ZooKeys on 24 February 2020.
The research team used a combination of DNA barcodes and morphological techniques, such as examining microscopic male genitalia slides, to establish differences between O. malignus and O. litoralis. They found both species of the littoral trap-jaw ants to be similar to each other but with evident differences in their structure and sculpture. For instance, while the worker of the new species O. litoralis is similar to O. malignus workers, it has stronger body sculpture and generally darker body colour than the latter. Males of the two species, which look different from the worker ants, are also morphologically similar to each other, but O. malignus has consistently stronger body sculpture.
While nests of the new species O. litoralis appear common in local mangroves, actual nests of the extant species O. malignus have not yet been found in Singapore. Based on observations elsewhere, the researchers speculate that local O. malignus nests may be found further towards the coastline, possibly in coral rubble. Although both mangroves and the coastline are considered part of the intertidal zone, this subtle difference in nesting location may be an important feature that distinguishes the two species.
How O. litoralis thrive and even co-exist with O. malignus in the volatile intertidal zone remains an intriguing subject for future research. Much more awaits to be discovered about the strategies used by the ants especially to cope with daily cycles of inundation and drainage in a physiologically challenging environment.