• Arthurdendyus triangulatus;
  • Arthioposthia;
  • biology;
  • ecology and distribution;
  • earthworm ecology;
  • flatworms;
  • pest status and environmental impact;
  • terrestrial planarians


The indigenous terrestrial planarian fauna (three species) of the UK is outnumbered by introduced exotics, of which there are at least nine species. The New Zealand flatworm, Arthurdendyus triangularis, is one of the most widespread and apparent of these non-indigenous earthworm predators, particularly in Northern Ireland and central Scotland. Despite its having been in the UK for at least 35 yr, our knowledge of the biology and ecology of this species remains somewhat limited.

In Scotland, A. triangularis occurs predominantly in botanical and domestic gardens, and is not generally considered to be a problem on agricultural land. The situation in Northern Ireland is different; although predominantly found in domestic gardens, it appears to have colonised grass leys many localities although the impact on earthworm populations remains ambiguous. In England, records are increasing, predominantly from northern regions.

Studies have indicated that earthworm species vary in terms of their vulnerability to predation by A. triangularis; surface-active and anecic species are considered to be most at risk. However, A. triangulatus and earthworm populations are known to coexist, apparently in a state of dynamic equilibrium, in a number of localities.

Naturally-occurring planarian populations are often severely constrained by food supply, but individuals survive lengthy periods of starvation. Such behaviour may preclude the natural recolonisation of habitats by prey species. Very much higher rates of population growth can be achieved where food is not limited, and where favourable conditions result from horticultural practices. The mobility of the prey species may determine the dispersal strategy of the flatworm predator, and a propensity not to move away from centres of prey density might account for the relatively low rate of colonisation of agricultural land by A. triangulatus. Additionally, reliance on protected refuges, may explain the present, somewhat limited distribution in the UK, particularly in southern England.

It is difficult to estimate the potential distribution of A. triangulatus, because of the lack of fundamental knowledge of its ecoclimatic requirements. A better understanding is needed of the precise habitat requirements (and constraints) of A. triangulatus, particularly where this species appears to have adapted to rural conditions.