In fact, though, the essence of these prohibitions is not the esthetic character or the palatibility of these things. We abstain from them specifically and exclusively because Hashem told us not to eat them. They are abominable because they are treif, not that they are treif because they are abominable. Indeed, there are theoretical means through which one could produce real shrimp and lobster that are 100% kosher. We're not talking about surimi, minced pollock that is extruded into ersatz shrimp-like shapes. We're talking about real swimming ten-legged crustacean antennaed cocktail shrimp, the real thing. And I'm not talking about some theoretical exception to the biblical law which is rabbinically prohibited. I'm talking what's-for-breakfast-muttar, as kosher as lox.
2 Ma'achalos Asuros 18:
So: the Torah tells us that creatures that live in the water are only kosher if they have scales and fins. But the Torah tells us that this requirement is limited to water creatures that live in oceans and natural lakes. If they live in artificial ponds or tanks, they do not require any signs of kashrus, and they would be kosher even without scales or fins. On the other hand, the Gemara (Chulin 66b-67a,b) continues, this is true only so long as they haven't left the water. The moment they leave the water, they fall under the heading of land creatures, and thus require signs of kashrus, which they don't have.
One problem is that if they are the product of eggs or fry that were produced by other crustaceans that were assur, they would be assur under the rule of היוצא מן הטמא טמא, that which comes out of a prohibited animal is prohibited as well. But in the case of shrimp and lobster, the eggs are microscopic, and so if one would put treif shrimp into a breeding tank, and they spawned, the water containing the spawn would be kosher, because the eggs and larvae are invisible. When they came into legal existence by becoming visible, they are in a tank or pond, and are automatically kosher. Thus, if they grew from microscopic size while in a tank, and so long as they are in their tanks, they are kosher. Once they leave their tanks, they become treif.
What if they are killed in the water and only are taken out of the tank after they're dead? (This is not far-fetched. Lobster is usually prepared by cooking alive in water, and shrimp is usually frozen before shipping, and they could easily be frozen or cooked while immersed.) This is the subject of an argument between two sets of Rishonim- we'll call it a machlokes Rashi vs. the Rambam. According to Rashi, if they're killed in the water and taken out afterwards, they remain kosher. The Rambam holds that they do become treif even if they're taken out after they're dead.
I think it's fascinating that the argument between Rashi et al and the Rambam et al stems from one single letter, the letter Vov. The Gemara in Chulin (67b) asks a question and does not resolve it, and therefore we are machmir.
Who do we pasken like? As you will see, the bottom line, the normative halacha, is the subject of dispute.
Aruch Hashulchan 84:41-43
and he goes on to say that he doesn't agree with the Shulchan Aruch's preference to be meikil, nor with the Shach, the Pri Chadash, and the Kreisi U'Pleisi that are meikil, and he ends by paskening that ובודאי שיש להחמיר, we certainly should be machmir.
One more point needs to be mentioned. I discussed this with my son in law, Harav Avraham Lawrence, and he said that the hetter only applies when the water spontaneously generated the Shratzim, not when they were spawned by Shratzim that are assur, because יוצא מן הטמא טמא. I said that the fact that the spawn is microscopic, and the larvae (the microscopic nauplii) are kosher, no less than the invisible protozoa that live in many foods that we eat, and יוצא מן הטמא טמא can't asser when there is a break in the chain of issur. I believe that his argument is without merit, but גברא רבה אמר מילתא לא תחיכו עליה, and if anyone tries to actually do this, I would bet that the argument will be presented. Wrong doesn't mean that someone's not going to vociferously argue it. (Rabbi Dr. Richard Resnick argued even further- that since we know that spontaneous generation is not factually true, the whole hetter should no longer be relied on, and the Gemara that was mattir Sheretz Hamayim that the Gemara was mattir if it wasn't piresh should be overturned. Again, I think he's wrong because a. we pasken like the Gemara regardless of their errors of natural science, and b. the microscopic stage breaks the chain of issur.)
Let me make this crystal clear. Many authorities within the last several hundred years (Shach, Kreisi U'Pleisi, etc.) would in actual practice fully allow such shrimp and lobster. In the mundane reality of psak, despite these opinions,because more recent poskim have tended to the side of issur, (e.g., the Aruch Hashulchan,) any current posek that wanted to rely on the mattirim would encounter significant resistance. (Pond catfish would be assur anyway, because they develop from eggs that are visible as soon as they are spawned, and, as I noted above, since their parent is assur, they are assur as well- היוצא מן הטמא טמא- despite their having been raised in a tank or pond.)
But all this does not diminish the truth of the underlying point that I am making, because most say that even the machmirim are only machmir under rabbinical law, but under Torah law everyone would agree that it is muttar. The main thing is that you see that it is not the shrimpiness of the shrimp that makes it assur, nor is it the clamminess of the clam. It is the fact that the Ribono shel Olam prohibited it. The same is true in the cases of כלאי גפנים, which has an element of intent, as we see in :בבא בתרא ב; and ערלה, which is only assur if it is owned by a person but not if it is hefker; and בשר בחלב which is בטל בששים only because it is not considered דרך בישול; and non-cloven-hooved animals, as we see in the rule of קלוט במעי פרה. These foods are abominations if and only if, when and only when, Hashem said they are assur, because we follow the law of Hashem. If and when Hashem says it's muttar, you can make a shehakol and eat it with a good appetite. It's not abominable any more. (Let's hope the Aris Greenspan and Zivotofsky don't get any ideas about this.....)
This should give us a deeper appreciation of the words we find in the Sifra (end of Kedoshim, Vayikra 20:26, brought by the Rambam in Shmoneh Prakim in the name of Rabban Shimon ben Gamliel.)
Second, great unknown pointed out that if one were to boil the lobster in its tank, together with some nice potatoes and onions and tomatoes, and after cooling the whole thing one would strain out the חתן הנשף, the resulting bisque would be both tasty and 100% kosher and muttar according to all poskim.
Finally, I suggested that, especially in light of the Gemara in Bava Metzia, this might be an excellent example of נבל ברשות התורה. I might be wrong, though, when you consider the Rambam in Shmoneh Prakim I brought above. The concept of נבל ברשות התורה, by definition, is impossible to pin down. But it would certainly feel grotesque, wouldn't it? And if any of you militant Rambam Rationalists takes this seriously and does it, I strongly suggest that you do it under cover of darkness, or you will acquire the permanent sobriquet "Lobster Fresser." Or, in proper German, Hummer Fresser.