Opsin proteins regulate a number of photoreceptive perceptions across the metazoa, including vision. The Ecdysozoa possess a wide variation in the morphology and acuity of visual systems. However, the opsins of the basal clades have largely been unexplored, and little is known about their visual modalities.
The Nematomorpha (Gordian, or Horsehair worms) is a parasitoid phylum comprising around 320 named species. Due to their close relationship to the highly derived and much more species-rich Nematoda (roundworms), understanding the Nematomorpha is key to our understanding of both the evolutionary history of the Ecdysozoa, and vision, as the Nematoda do not utilise opsin-mediated light reception. We present new opsins from both transcriptome and genome data of the Nematomorpha and show that the loss of opsin-mediated light reception in the Nematode worms is to be considered a lineage specific phenomenon.