Horsehair worms (Nematomorpha) of the Prosecco Hills UNESCO World Heritage site, Italy

Abstract

A year-long survey of the horsehair worms was undertaken within the broader area of the Prosecco Hills World Heritage Site, to establish their taxonomic diversity, seasonal abundance and host–parasite relations. The study was conducted during the period Oct 2022–Oct 2023 and involved direct collection of worms along with measurements of a variety of physico-chemical parameters. Four species were identified during the study using scanning electron microscopy, namely Parachordodes tolosanus, Gordius albopunctatus, G. aquaticus and Spinochordodes tellinii. The last three species were found in the Farrò Stream, which was selected as the ideal site for monthly monitoring. Tangles of worms of different size, species and even genera were found repeatedly in the natural habitat of the stream, generally in the deepest pools. G. aquaticus was by far the most abundant species and occurred year-round, while S. tellinii was restricted to the months of September and October and G. albopunctatus was found only once in August. Gordius aquaticus occurred in two forms, either as small, thin and white specimens or as larger, thicker, brown specimens. Pholidoptera griseoaptera (commonly known as “dark bush-cricket”) was the only host species that could be conclusively identified, as isolated specimens kept in water containers overnight released some G. aquaticus worms the following day. Other likely hosts were the upland green bush-cricket Tettigonia cantans, the southern saw-tailed bush-cricket Barbitistes obtusus and the brown centipede Lithobius forficatus, which were all repeatedly found freshly dead or moribund near the worms at the time of the survey. Gordius aquaticus was present in numbers during each month, except in July at the peak of summer, when the stream had a high density of amphibians, mainly tadpoles and subadults of Salamandra salamandra, Bombina variegata and Bufo bufo, as well as aquatic insects such as the Heteroptera Aquarius najas and Notonecta maculata, but also Coleoptera such as Hydraena sp. and Pomatinus substriatus. This is an interesting coincidence that should be investigated further, as it may reveal some kind of cause-and-effect relationship, either direct or indirect.