Horsehair worms (Nematomorpha) from the Baltic island Bornholm (Denmark), with notes on the biology of Gordius albopunctatus

Abstract

Two species of horsehair worms (Nematomorpha), Gordius albopunctatus and Gordionus violaceus, are reported from eight streams on the Baltic island Bornholm. Males can be easily distinguished from each other by differing features in the posterior end, but females resemble each other and can be distinguished reliably only using scanning electron microscopy. Aquarium experiments showed that larvae and praepupae of Pomatophylax cingulatus (Trichoptera) are the host for G. albopunctatus, which is one of the few cases in which nematomorphs develop completely in aquatic animals. Both nematomorph species occur sympatrically in almost all streams, but G. albopunctatus prefers northeastern streams and G. violaceus western streams. This might be due to the distribution of hosts, because P. cingulatus is more abundant in northeastern streams.