Answer to Parasite Case of the Week 557: As mentioned by Bernardino Rocha, this is either an L3 larva of the Anisakidae or Raphidascarididae - 2 families of nematodes that are commonly found in fish. I tend to agree with Blaine and Bernardino that this is most likely a Type I Anisakis or Pseudoterranova sp.; however, we would need to see its internal features to be sure (you can read the comments to learn how to clear the worm to visualize its internal structures). As Blaine mentioned, the presence of the mucron (the terminal spicule-like structure shown below) rules out Contracaecum spp. and Type II Anisakis. Other morphologic features that can be used to differentiate Anisakis, Pseudoterranova, and Contracaecum species are the features of the ventriculus, the number of intestinal cells in cross section, and the shape of the lateral chords (also seen in cross-section).
Blaine Mathison and Alex da Silva recently wrote an excellent chapter on Anisakiasis in Ortega's and Sterling's book entitled "Foodborne Parasites" (email Blaine if you want more information).
Thanks again to Florida Fan for donating this nice case of an anisakid nematode, with nice visualization of the anterior boring tooth and mucron. It serves as an excellent reminder to us all to cook your fish thoroughly before eating it! For the sushi lovers out there, you can kill all parasites in raw fish by freezing it for 7 days at -20C. Bon appetite!