As a follow-up to this post (link), I obtained this Q&A of Daf HaKashrus with permission to publish:
The note preceding the sardine article/ R’ Belsky’s teshuva/ stated that the new scientific DNA research proves that as “R’ Belsky Shlita concludes that these are the same anisakis worms which Halachah permits as being Darni D’Cavri. These worms have migrated from the flesh of the fish to its gut. R’ Belsky’s conclusion was scientifically borne out…”. How did the scientific research prove that they migrated from the flesh to the gut and not vice-versa?
The Daf responds:
Dear Rabbi ********,
We apologize for the confusion caused by the introductory note. You are correct that the DNA research only proves that the worms in the gut of the sardines are darni d’Cavri (permitted flesh worms). It does not directly indicate the direction of the migration. However, Rav Belsky explained in a separate teshuva that anisakis enter the sardines when they eat infected fish (krill). These worms are permitted because they initially become visible inside of these very small fish and as such the worms were never rochesh in the water. Accordingly, the direction of the migration is irrelevant.
We should also point out that the fact that the worms first become visible inside of the fish is only a suggested explanation for the heter. The reason for the absolute certainty of the heter is that the Gemara says that darnei d’Cavri are mutar and the Shulchan Aruch says that worms in the flesh are mutar. If anisakis is assur then there are no worms in any fish that are mutar and this is against the Gemara and Shulchan Aruch.