Among the living creatures whose consumption is forbidden to Jews are various species that live in the water. Specifically, the Torah states that we may eat only those water species that possess fins and scales, but those that do not, including those categorized as sheretz hamayim, may not be eaten (Vayikra 11:9-11). Rashi (to passuk 10, s.v. sheretz) explains that the word “sheretz” refers to a low-lying creature that teems or creeps about along the ground, or, in this case, in the water. At the very end of the parashah (ibid. 11:41-47), the Torah again addresses the prohibition against eating a sheretz, adding that one can become contaminated through this type of creature; Rashi there (to passuk 41, s.v. lo) notes that a sheretz is something which is low-lying, has very short legs and (thus) appears to slither when it moves. Rashi later (to passuk 45, s.v. ki) demonstrates the significance of the prohibition against consuming a creature classified as a sheretz by citing the Gemara in Bava Metzia (61b) which teaches that the Jewish people’s adherence to this prohibition, in contradistinction to the behavior of other people who do indeed eat such creatures, would have sufficed by itself to warrant their being redeemed from Egypt (see also Pri Chadash to Yoreh De’ah 84:53). It is noteworthy that the requirement to eat only those water dwelling species which have fins and scales, and to refrain from eating those that do not, is actually presented again later in the Torah (Devarim 14:9-10).

Based upon the multiple formulations of this prohibition relating to a sheretz in all of the above pesukim, the Gemara in Makkot (16b) states that one who eats a certain type of small water creature (see Rashi there, s.v. achal putita) violates several prohibitions at once, and thus incurs several sets of lashes as punishment; Rashi there (s.v. lokeh arba’ah) delineates them. It should be noted at this point that although the creatures in the category of a sheretz are small (Rashi to Eiruvin 28a, s.v. tzir’ah, says that because it is so small, a sheretz is often detectable only when it moves; see also Rashi to Pesachim 24b, s.v. tzir’ah), the prohibition against consuming them is limited to those that can indeed be seen by the naked eye. The Aruch HaShulchan (Yoreh De’ah 84:36) writes that even though one who looks at water through a microscope may see many, many tiny creatures, invisible to the naked eye, there is no problem drinking that water because the Torah forbids only those species which can be seen under ordinary circumstances, as the Torah was not given to angels, but to human beings. Otherwise, he points out, it would be prohibited to breathe the air, because it too is filled with microscopic creatures. He concludes, though, that a creature which is indeed visible to the naked eye, even if barely visible and only in the proper light, is classified as a prohibited sheretz. Rav Moshe Feinstein (Shu”t Igrot Moshe, Yoreh De’ah 2:146) rules similarly, citing Rav Chaim Soloveitchik, that the Torah outlaws the consumption only of those creatures which are visible to the naked eye, because when one breathes the air one ingests any number of tiny organisms; the fact that one may be able to detect such creatures with a microscope is irrelevant. He notes in addition that regarding other halachot as well, such as the square shape needed for tefillin or the smooth blade needed for the shechitah knife, we consider only what is visible to the eye without magnification. Rav Ovadyah Yosef (see Shu”t Yechaveh Da’at 6:47 and Shu”t Yabia Omer 4, Yoreh De’ah 20:2 and 21:7) comes to the same conclusion regarding both the consumption of insects and other halachot.   

The Gemara in Chullin (67b) cites an authority who states that kukyanei, certain very small worms found inside a larger creature, are forbidden, because they come from outside the creature, that is, as Rashi there (s.v. me’alma) explains, the creature ingests them from the outside while eating and they are thus in the category of an ordinary sheretz which may not be eaten by a Jew. According to Rashi (s.v. kukyanei), these worms are found in the liver and lungs of animals, but according to Tosafot (s.v. kukyanei), they are found in fish, entering through various orifices in the fish. Another authority cited in that Gemara rules that kukyanei are permitted, because they in fact germinate from the flesh of the larger creature, and do not come from the outside; they thus do not qualify as a forbidden type of sheretz (see Rashi there, s.v. minah). The Gemara concludes, though, that kukyanei are indeed prohibited, the assumption being that they enter the larger creature from the outside while it is sleeping (see Rashi there, s.v. meinam nayem, and Ran, 24a in Rif, s.v. kavra); the Pri Megadim (Siftei Da’at to Yoreh De’ah 84:42-43) says that we are stringent here because we are actually not sure where the worms come from, and it might be that they are from outside the creature.

The Gemara (ibid.) then states regarding worms called darnei, certain tiny parasitic larvae, which are found, as explained by Rashi (s.v. darnei), between the skin and the flesh of the larger creature, are prohibited when found in meat, but permitted when found in fish. The reason is that the darnei are considered the products of the larger creature in which it germinates and grows. In the case of an animal, the host creature is forbidden at the time that the parasite develops because it needs to be slaughtered in order to be eaten; the darnei thus have the same forbidden status (see Rashi there, s.v. beshechithah). In the case of fish, however, the host creature is permitted as is, without being slaughtered; the darnei are thus similarly permitted. As for the possibility that like the kukyanei discussed above, these darnei worms may have entered the fish from outside, and should thus be forbidden, the Rashba (Chidushei HaRashba to Chullin ibid., s.v. muranei) asserts that it is for certain that such is not the case, but that these particular worms rather come from the inside of the fish, where they grow and develop. The Meiri (Beit HaBechirah to Chullin ibid., s.v. ule’inyan bi’ur), the Yam Shel Shlomo (Chullin 3:104), and others state similarly that it is known for sure that these worms emerge from inside the body of the host creature and do not come from the outside, and they may thus be consumed by a Jew. It must be noted that the Rambam (Hilchot Ma’achalot Asurot 2:17) rules that all worms found in fish are forbidden (unless they grew after the fish has died, as sometimes happens with fruit that rots after being detached from the ground), but the Ra’avad there disagrees and says, referring to the aforementioned Gemara, that worms in fish are in fact permissible. The Maggid Mishneh there, while explaining how the Rambam understands the Gemara, asserts that the majority opinion among the authorities is in fact that of the Ra’avad, and the aforementioned Meiri similarly considers the Rambam’s position very difficult.

In view of all of the above, it would seem that any worms found between the skin and the flesh of the fish do not present any Kashrut problem; the Hagahot HaAsheri (to the Rosh in Chullin 3:69, s.v. hachi), among others, states this explicitly, but adds that those worms found elsewhere in the fish are not allowed. The Shulchan Aruch (Yoreh De’ah 84:16) rules accordingly, stating without making any distinctions that all worms found in fish between the skin and the flesh of the fish or in the flesh itself are permitted, but if found elsewhere in the innards of the fish they are prohibited; the Shach (No. 43) explains that this is because those found in the former locations grow and develop there, while those found in the latter location enter from the outside. The Chochmat Adam (38:28) adds that any organisms found on the outside of the skin of the fish are forbidden because they certainly do not grow inside the fish (see also Darkei Teshuvah to Yoreh De’ah 84:184).

All this, however, leads to the question of how one can tell for sure where in the fish a particular worm originated – is it not possible that a worm found in a fish between the skin and the flesh started out elsewhere and simply moved into that part of the fish? Could one not argue that even a worm found between the skin and the flesh of a fish came in from the outside and settled in that spot? There is evidence, for example that certain worms “tunnel” from the inner organs out to the flesh; on the other hand, as noted by the Hagahot Sha’arei Dura (47:2), the worms may travel the other way too (see also Bach to Yoreh Deah 84, s.v, kol tola’im). Moreover, some of the Rishonim cited above (see also Ran to Chullin, 24a in Rif, s.v. tola’im) said that it is known with certainty that the darnei worms develop inside the fish, but what if things are not actually so certain? Finally, it is currently accepted as scientific fact that worms and other such creatures do not in fact emerge as living organisms from the insides of the creature in which they are found; there is no such thing as “spontaneous generation,” that is, of a living creature being created from within a larger host organism without procreation. Does our present scientific knowledge, which indicates that any worms located inside fish certainly did not originate independently in the fish, regardless of where in the fish they were subsequently found, impact the halachah, in light of the fact that the Gemara and the Shulchan Aruch permit the consumption of worms found between the skin and the flesh? (Regarding the issue of spontaneous generation, see the Gemara in Shabbat 107b, with Rashi to 12a there s.v. matirin, about killing certain types of lice on Shabbat, and see Shulchan Aruch Orach Chaim 316:9 with Mishnah Berurah No.41. See also the encyclopedia Pachad Yitzchak under the entry “tzayid ha’asurah” and Michtav MeEliyahu Volume 4, Letter 31B, Note 4.)

Considering the above issues, there are contemporary Rabbinic authorities who have ruled that because of what we now know to be scientifically accurate, we must assume that any worms found anywhere in fish originated outside and are thus in the category of a forbidden sheretz. Since certain species of fish, including several commonly eaten types, have been shown to be infested with worms, such as the “anasakis” worm, these worms must be removed before a Jew may eat the fish; as this is often not practical, consumption of these fish must be avoided. Some posit, along these lines, that the worms of today are different than the darnei worms expressly permitted by the Gemara and the Shulchan Aruch or that times have changed and there are now new species of worms which did not previously exist and were thus never expressly allowed. At the other extreme are certain authorities who assert that in as much as the Shulchan Aruch allows one to eat worms found between the skin and the flesh of fish, without imposing any qualifications or limitations on this ruling, we need not consider any modern scientific evidence at all; fish containing such worms thus pose no Kashrut problem whatsoever. Still others insist that the scientific evidence is not really compelling; one may maintain, for example, that although we know today that there is no such thing as spontaneous generation, it is possible that if a particular worm enters another organism while it is still microscopic in size, it is not then a halachic entity; when it grows, it appears to be growing out of that other organism, and is indeed considered by the halachah as if it is indeed so doing, and it thus does not qualify as a prohibited sheretz. One should consult one’s own Rabbinic authority to determine whether fish containing these worms may be eaten or not.