Gemara

Questions (Rav Ashi): If a worm separated to the outside of the date, or to the outside of the date pit, or from one date to another, what is the law?

These questions are unresolved.

Version #1 (Rav Sheshes brei d'Rav Idi): Kukaini (worms in fish innards, or in the lungs or liver of an animal) are forbidden.

This is because they came from outside, i.e. they were ingested.

Objection (Rav Ashi): If they were ingested, they would be found in the end of the digestive tract!

Version #2 (Rav Sheshes brei d'Rav Idi): Kukaini are permitted.

This is because they grow from it (the fish or animal) itself.

(Rav Ashi): This is obvious. If they were ingested, they would be found in the end of the digestive tract! (end of Version #2)

The Halachah is, they are forbidden, because they enter through the Usiya (respiratory system) during sleep.

Worms underneath the skin in animals are forbidden. In fish, they are permitted.

Ravina asked his mother to mix fish worms with his fish, so he would eat them without seeing them.

64a - Question: What is the difference if eggs are of a Sheretz or a Tamei bird?

Answer (Rava): If a baby started forming and one (was warned and) ate it, he is lashed for eating a Sheretz.

Rishonim

Rif (24a): Kukaini are worms in fish innards. They are forbidden, for they come from outside.

Rambam (Hilchos Ma'achalos Asuros 2:17): If a worm is found in fish innards or in the meat, it is forbidden.

Rashi (67b DH Mahu): Rav Ashi asked whether it is considered "Shoretz Al ha'Aretz" even if it was never on the ground.

Me'iri (67b DH ul'Inyan): Kukaini are worms found in the intestines. Perhaps they came from the outside. Murna are worms found in the meat. They surely developed in the fish itself.

Poskim

Shulchan Aruch (YD 84:16): In fish, worms found in the innards are forbidden, but those between the skin and the flesh or in the flesh are permitted.

Beis Yosef (DH Kol): R. Tam forbids worms in fish only in the innards or similar places for which we can say that they came from outside. What is in the flesh of the fish is permitted, for it grows from it. Hagahos Ashri says so.

Chavas Da'as (Bi'urim 81:1): Tosfos in Bechoros (6 - I did not find this - PF) asked that a Tamei fish found in a Tahor fish should be forbidden due to 'whatever comes from Tamei is Tamei.' He should have asked that Tamei fish brine should be forbidden mid'Oraisa! I answer that 'whatever comes from Tamei is Tamei' was learned from "ha'Temei'Im". It does not apply to fish.

Shulchan ha'Levi (R. Yisrael Belsky Shlita) Birurei Halachah 22: One who cuts fish must ensure that no part of the innards (where worms are forbidden) stick to the rest of the fish. Fish sellers used to be skilled professionals. Nowadays amateurs have entered the business, so one must oversee them.

Shulchan ha'Levi (ibid.): Some say that Chazal permitted only worms that come from spontaneous generation. Nowadays we see that the worms have a life cycle, so these are different worms, and they are forbidden. Nature has not changed. Rather, they discussed worms that develop primarily (e.g. 90%) from rotting flesh. (The eggs absorb nutrients from the fish. Without this, the worm would not develop.) It is as if they arose from the fish itself. Ravina intentionally ate them to refute those who would say that since they have a life cycle, they are forbidden.

Halachically Speaking (6:9): R. Mordechai Zilber says that the worms lay eggs in the fish, and the larvae enter the meat and grow into worms, and this is considered growing from the fish.

Shulchan ha'Levi (20): The worms are swallowed by small fish such as shrimp, and bigger fish such as salmon eat the shrimp. Some say that 'whatever comes from Tamei is Tamei.' All the worms found in the meat are the same. All are permitted, or all are forbidden. We cannot forbid all, since the Gemara permits. Rather, all are permitted. Some say that the Anisakis worm is different, for it is visible in the innards, before it gets to the meat. I say that if we would forbid it, we would have to forbid all worms due to Safek, for one cannot know where they developed. If so, how did Chazal permit them without distinction? Some ask, how can worms that were forbidden in the innards become permitted when they move to the flesh? They were forbidden in the innards only lest they came from the outside. If they moved to the flesh, surely they arose from the fish itself, and they are permitted.

Shevet ha'Levi (4:83): Researchers say that worms that developed in shrimp are swallowed by other (e.g. sable) fish, and are found in the meat. This is unlike Chazal said! Chazal were experts in secrets of nature. We must say that they discussed certain kinds of worms. If Stam worms are found in the meat, we assume that they developed there. If we know otherwise, Chazal never permitted those worms. The Me'iri connotes like this, but the Tur and Shulchan Aruch connote that Stam worms found in the meat are permitted like Murna (what arises there). All agree that the opposite can occur (ingested worms can be found there). Perhaps the Rambam forbids all worms in a fish before it died due to concern lest they are Kukaini! The Rishonim argue only about Stam. If we know that they were ingested, all forbid. Researchers often lie or err. If we find that they are correct, there is no contradiction to Chazal (and the worms are forbidden).

Dover Shalom (Rav Mordechai Kuber, p.115): Shevet ha'Levi holds that worms almost never break through from the innards to the meat (so Stam we are not concerned for this), but if we see that they did, they are forbidden. Lehoros Nasan (9:23-25) says that if we find that worms break through, this means that nature has changed (and they are forbidden).

Suggestion: The worms are not forbidden due to 'Sheretz ha'Mayim' because they were never in water. The Tahor fish swallowed shrimp with worms inside.

Rejection (Shevet ha'Levi): The Gemara asked about a worm that separated from one date to another. This was not resolved. It is a Safek mid'Oraisa, so we are stringent. Rashi explains that perhaps it is considered "Shoretz Al ha'Aretz" even though it arose in its 'mother' (the date) and it was never on the ground. The Shulchan Aruch (84:4) forbids. From one fish to another, which is not its mother, all the more so it is (at least) a Torah Safek. Really, it is not a Safek at all. From one date to another is a Safek, for it never saw the air; it is considered one matter. From one fish to another is a different species with a different nature. The new fish is not called the worm's mother. This is called Sheritzah (swarming). In any case it is forbidden because the eggs came from a Tamei fish.

Shulchan ha'Levi (23): We have not verified the scientists' claim that the fish swallow shrimp that swallowed the worms. Even if it is true, perhaps they are not visible until they are in the Tahor fish. Also, perhaps being in the innards of a Tamei fish is not like something that exudes from a living being. Chavas Da'as proved that 'what comes from Tamei is Tamei' does not apply to fish.

Dover Shalom (p.118): The Minchas Chinuch (163) says that worms that develop in Tamei fish are forbidden, like the fish. A Teshuvah from the Mechzeh Eliyahu says that we do not forbid worms swallowed in a Tamei fish due to 'what comes from Tamei is Tamei.' He could admit regarding worms that develop in a Tamei fish. He learned from the Chavas Da'as (Bi'urim 81:2), who says that 'what comes from Tamei is Tamei' does not apply to one living being inside another. This is no proof. Even if 'what comes from Tamei is Tamei' does not apply, it is as if it grew outside. If so, a worm in a Tamei fish is forbidden because it lacks fins and scales.

Suggestion: The Rambam and Rashba argue about eggs that started forming. The Rashba obligates only for eggs of a Sheretz, but not for eggs of a Tamei bird. Likewise, he exempts regarding Tamei fish. Perhaps even the Rambam, who forbids even for eggs of a Tamei bird, would exempt here, for they never entered the air of the world.

Rejection (Shevet ha'Levi): The Rambam and Rashba argue about what formed inside the egg. It was never called a Sheretz. If it left the egg, since the egg was forbidden, there is no reason to permit the worm. In summary, if the researchers are correct, it is hard to permit the worms.

Shulchan ha'Levi: Some say that the intestines are removed from the fish in an unsanitary way, and this causes worms from the intestines to be found in the meat. Government inspectors say that this is not the case.

Chachmas Adam (38:28): Worms found on the outside of fish are forbidden. They come from the outside. In some places worms grow by the fins and in the mouth. They are not noticeable at all. They are round like lentils, and have tiny black eyes. One who recognizes them can separate them with a knife. In Prague, all recognize them. Their Isur is known. I put them in water, and one can see them swim. One must scrape the fins and near them and the tail very well, and check the mouth and behind the ears very well.

Chachmas Adam (29): Worms found in fish that never separated from the bowl are permitted. If one took it by the hand or with a knife, it separated; it is forbidden due to Mar'is Ayin. The Pri Chodosh (Sof 84:53) forbids everything (that separated).

Chachmas Adam (30): I heard from a Gaon that even though we hold that a Murna found in the lungs is Kosher, if three worms are found, the lung is Muchzak (to be wormy), and one may not eat it until checking for worms.

Chazon Ish (YD 14:8): Worms are found on carp. I heard scientists say that a pair of worms fall into a lake, multiply very rapidly, and fall on the fish to suck their blood and fat. If so, they are forbidden.

Shulchan ha'Levi: The Chazon Ish discussed lice that cling externally to the fish. The Gemara never discussed this. The scientists confirmed for him that they are what the Chachmas Adam forbade.

Halachically Speaking (ibid.): Several Rabanan of Eretz Yisrael forbade fish with the Anisakis worm, including Rav Y.S. Elyashiv Shlita, Rav Moshe Shternbuch Shlita. Rav C. Kanievsky Shlita, R. S. Wozner Shlita and R. Nisim Korelitz leaned to be stringent. Some Hechsherim, including the Edah Charedis, Rav Landau, Vaad of Flatbush and Star K forbid it. Rav Moshe Vye Shlita, Rav Eliyahu Falk Shlita and Rav Yisrael Belsky are lenient. The stringent opinion forbids whenever we know that the worm came from outside. The lenient opinion holds that Chazal permitted whenever it is found in the flesh. Some try to remove the skin in order to remove the worms. Some say that this succeeds to remove less than 10% of the worms. A Mashgi'ach must be present (if someone else will buy the fish later), for we cannot recognize fish without the skin, except for salmon (no other fish is so red). There are health concerns for eating the worms if the fish was not cooked.