Nematomorpha spp

From Fish
A Nematomorpha spp emerging from the mouth of red-spotted masu salmon.
A nematomorpha folding in the stomach.
The nematomorpha from the fish

Nematomorpha spp and Gordiida spp ("hair worms") are a nematode parasite of Red-spotted masu salmon (Oncorhyncus masou). This relatively small group of large worms is found throughout world, but usually is restricted to areas near water. About 11 species occur in the United States. They are commonly confused with mermithid nematodes[1].

Life cycle

The adults are free-living but the juveniles are parasitic. The preparasitic worms are minute, and infect their host when they are accidentally ingested. They cannot penetrate hosts from the outside. Nematomorphs infect insects, including crickets, cockroaches, beetles, mantids, and grasshoppers, but also spiders and woodlice (sowbugs).

The species most commonly encountered, and perhaps the best studied species, is Gordius robustus. It occasionally attains high densities in its tettigoniid host Anabrus simplex Haldeman, also known as the Mormon cricket. The following information is based on G. robustus.

The body is yellowish and folded inside the stomach. The parasite possesses an external cuticle, a body cavity (pseudocoel), but no segments. The parasite is approximately 1 mm in diameter and approximately 30 cm in length. It moves slowly as if the wire is bent, e. g. it elongates or contracts poorly.

Clinical signs

Poor growth rates are a common symptom associated with this parasite, and large numbers can cause death in severely infected hosts.


The nematomorpha can be observed inside the digestive tract or emanating from the mouth. Since this parasite is not infectious to human, it is harmless in food hygiene.


Benzimadazole anthelmintics in the water are effective at eliminating this parasite in farmed fish.


  1. Poinar, GO, Jr. 1991. Nematoda and Nematomorpha Pages 249-283. In Thorp, JH and AP Covich (eds.) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, San Diego, CA.