The basic answer that most (myself included) would be tempted to give is simply that in order for a fish to be kosher, it needs to have both fins and scales.1 Since shellfish do not have these signs, they are not kosher.

In fact, I was inclined to just refer you to my article Are Bottom Feeders Kosher Fish? which, among other things, discusses the theory that bottom dwellers, including most shellfish, aren’t kosher since they’re unhealthy.

However, there’s more to the story.

Let’s delve into this by first discussing a fascinating incident that caused great controversy in the halachic world for a few hundred years.

The Story of the Stincus Marinus

Rabbi Gershon Shaul Yom-Tov Lipmann Heller (c. 1579–1654), better known as the Tosafot Yom Tov after his classic commentary on the Mishnah, writes that while he was chief rabbi of Vienna, a certain Rabbi Aharon Rofei (“Rabbi Aharon the doctor”) brought before him a Stincus marinus, described as a poisonous aquatic animal that had scales and four legs but no fins, and asked whether it was kosher.2

Now, if it had no fins, would it not be obvious that it was not kosher? Not exactly, since the Talmud states that “whatever has scales has fins as well.”3 This means that halachically, if you find a piece of fish that only has scales on it and you don’t see any fins, it is still kosher, since any fish that has scales also has fins. But it was clear that this Stincus marinus had scales but no fins.

If this creature was not kosher, then what is the meaning of the Talmudic statement? And if it was somehow kosher, as perhaps the legs themselves were considered a sort of fin, how could we say that the Torah permits a poisonous fish to be eaten?

In his final analysis, Rabbi Heller opined that there is a distinction between “fish” and “creatures of the water,” which dwell in the water but do not look like fish.

The rule of the Talmud that “whatever has scales has fins as well” only applies to regular fish, but not to other creatures of the sea. Thus, although they too need both fins and scales to be kosher, if they only have scales, no presumption can be made about them having fins as well. Therefore, Rabbi Heller ruled that the Stincus marinus was not kosher.4

However the debate about this creature raged on for many years.5

Some rabbis were of the opinion that there is no true halachic distinction between creatures of the water and regular fish, and the rule that “whatever has scales has fins as well” applied to this creature as well. Thus, in all likelihood this creature either had at one point (or would eventually grow) fins and was therefore in fact a kosher creature.6 As to the question of poisonous creatures being kosher, they argued that it wasn’t really a question, as there are many poisonous plants that are technically kosher.

Yet others were of the opinion that not only is there a distinction between fish and creatures of the water, but the kosher fish signs also do not even apply to “creatures of the water” that aren’t truly fish. Therefore, even if these creatures would have both fins and scales, they still would not be kosher.7 Consumption of these creatures would be a transgression of the prohibition “Do not eat of the swarming creatures of the waters.”8

Not Really a Creature of the Water

It should be noted that Rabbi Moshe Sofer, known as the Chatam Sofer, writes based on the opinions of various experts that what is referred to as the Stincus marinus isn’t really a creature of the water at all. Rather, it is a creature that lives next to the water and occasionally jumps in, similar to a frog or lizard. This being the case, the whole debate is essentially moot, since it isn’t in the category of fish but rather a crawling creature of the ground, which has a different criteria for it to be considered kosher.9

Thus, it would seem that what had been termed Stincus marinus, which in Latin means “aquatic skink,” is actually a misnomer (according to some, it’s actually the Scincus scincus AKA sandfish).

While the above is only a short overview of the controversy surrounding the Stincus marinus, it helps illustrate that there are basically three opinions regarding “creatures of the water”:

  1. They are just like fish. If we observe that they have fins and scales, or even just scales, they would be kosher.
  2. They are a bit different from fish. Fins and scales render them kosher, but we cannot apply the rule that “whatever has scales has fins as well.”
  3. They are not at all fish. “Creatures of the water” are in a distinct category and the kosher fish signs don’t apply to them.

Does a Kosher Sign Even Help?

We can now get back to the question about shellfish.

As we explained, the simple reason they aren’t kosher is because they do not have fins and scales. According to others, however, even if they were to have fins and scales, shellfish would still not be considered kosher, as they don’t necessarily have the halachic definition of “fish.” Therefore, according to this opinion, consuming them would actually be a transgression of the prohibition “Do not eat of the swarming creatures of the waters.”