Did Chazal believe in Gigantic Fish Eyeballs?

My answer is "almost certainly not", though this is indeed how Rashi understands the gemara. Let us start with the post, and question, at Rationalist Judaism:

Dear Rabbi!

I'm having difficulty with a passage the gemara (Zevachim 22) quotes from a braita, stating that if the eye of a large fish were to dissolve and pool in its socket; one would be able to immerse himself in it as if it were a mikva, provided it has the 40 saah required for a mikva. And this is codified as law in Rambam and Y"D (201.33). 

Without doubting chazal for a second, can this be explained or reconciled with modern science?
From what I recall the largest eye ball is that of a horse and or a squid (while the large fish whose primary senses are scent and sound have relatively small eyes) Which is still drop in the bucket from the 25-35 cubic feet of volume required for 40 saah. 

Any information or guidance would be greatly appreciated.

It is not the brayta actually which says this, but rather an Amora, Rav Yitzchak bar Avdimi, and as interpreted by a Rishon, Rabbi Shlomo Yitzchaki (=Rashi). Rabbi Slifkin explains that this cannot be explained with modern science, if one does not doubt Chazal for a second. See inside. This may well be true, assuming that this is the correct explanation of the gemara. But I don't think it is the correct explanation of the gemara.

And while the scientific difficulties are icing on the cake, there are text-internal reasons for thinking the gemara means something else.

The largest known eyeball of an animal, the collosal squid, is 27 centimenters, which is approximately 11 inches. Make this the diameter of a sphere, and the volume will be nowhere near the volume required for a mikveh. Maybe there are larger eyeballs, of larger animals, which we don't know about. I don't know.

However, let us turn to the gemara, in Zevachim 22a:

אמר ריש לקיש כל המשלים למי מקוה משלים למי כיור לרביעית אינו משלים למעוטי מאי אילימא למעוטי טיט הנדוק היכי דמי אי דפרה שוחה ושותה ממנו אפי' לרביעית נמי ואי אין פרה שוחה ושותה ממנו אפילו למקוה נמי אין משלים אלא למעוטי יבחושין אדומין אפילו בעינייהו נמי דהא תניא רשב"ג אומר כל שתחילת ברייתו מן המים מטבילין בו ואמר רב יצחק בר אבדימי מטבילין בעינו של דג

The text I marked in red is the voice of the narrator of the gemara, the setama de-gemara. Now, what is the gemara trying to prove, in this segment? The following: אלא למעוטי יבחושין אדומין אפילו בעינייהו נמי. Well, more as a rhetorical question. That is, do you think it is to minimize יבחושין אדומין?! This cannot be so, because אפילו בעינייהו נמי!

There are two aspects that the gemara needs to prove. The first is the יבחושין אדומין, which originated in the water. The second is that this is so even if it בעין. To this end, the setama harnesses two sources. Addressing the first aspect is a brayta  about things originating in the water: דהא תניא רשב"ג אומר כל שתחילת ברייתו מן המים מטבילין בו. Addressing the second aspect is a statement by an Amora: ואמר רב יצחק בר אבדימי מטבילין בעינו של דג.

This is strange, because how does one immerse in the eyeball of a fish? It is, after all, solid. And most fish are small, and their eyeballs will likely be small as well. Rashi helps us out:

בעינו של דג - דג גדול שנימוק שומן עינו בחורו:

Thus, Rashi solves the problem for us by saying that its eye melted in its socket, and it is a big fish. But both these facts are missing from the explicit gemara. Rashi may be right, that this was the intent of the gemara. But I am fixated on this choice of word בעינו, when what it is supposed to address the second aspect, אפילו בעינייהו נמי. I dislike the odds that an eyeball just happened to have been chosen as the example, when the setama is trying to prove something about fish creatures which are be'en.

Rather, I am more than a bit inclined to posit that בעינו של דג was understood by the setama digemara as the actual substance of the fish, in its original form. The words match, and we don't have to imagine gigantic fish and melted eyeballs, not explicitly mentioned in the gemara. And I would further posit that the setama digemara got this right, and the Amora didn't intend this either, but was talking about an actually possible case, which works out with modern science.

Alas, Rashi would then be wrong. While I will say that Chazal, as well, are wrong about some scientific matters, some people will be more comfortable with Rishonim erring in matters of science than Chazal doing the same.

Now, Chazal might well have believed in enormous fish with enormous eyeballs. From Bava Batra 73b:

Rabbah b. Bar Hana further stated: Once we were travelling on board a ship and saw a fish in whose nostrils a parasite had entered. Thereupon, the water cast up the fish and threw it upon the shore. Sixty towns were destroyed thereby, sixty towns ate therefrom, and sixty towns salted [the remnants] thereof, and from one of its eyeballs three hundred kegs of oil were filled. On returning after twelve calendar months we saw that they were cutting rafters from its skeleton and proceeding to rebuild those towns.

Rabbah b. Bar Hana further stated: Once we were travelling on board a ship and saw a fish whose back was covered with sand out of which grew grass. Thinking it was dry land we went up and baked, and cooked, upon its back. When, however, its back was heated it turned, and had not the ship been nearby we should have been drowned.

and from the next amud, 74a:

R. Johanan related: Once we were travelling on board a ship and we saw a fish that raised its head out of the sea. Its eyes were like two moons, and water streamed from its two nostrils as [from] the two rivers of Sura.

Now, these gemaras in Bava Batra regarding Rabba bar bar Chana might well be allegorical. Though Tosafot in Chullin uses this to demonstrate that fish have nostrils, this is just establishing a basic biological fact about fish.

However, one thing to bear in mind is that others around that time took similar tales literally. Thus, there is the aspidochelone, mentioned by Pliny the Elder (where we find other parallels from Pliny in Chazal):

Pliny the Elder's Natural History tells the story of a giant fish, which he names pristis, of immense size; he also relates the tale of sailors landing on its back, only to discover that it was not in fact land when it submerged.

Also, in the Physiologus, an allegorical work based on "known" facts about the world.

"There is a monster in the sea which in Greek is called aspidochelone, in Latin "asp-turtle"; it is a great whale, that has what appear to be beaches on its hide, like those from the sea-shore. This creature raises its back above the waves of the sea, so that sailors believe that it is just an island, so that when they see it, it appears to them to be a sandy beach such as is common along the sea-shore. Believing it to be an island, they beach their ship alongside it, and disembarking, they plant stakes and tie up the ships. Then, in order to cook a meal after this work, they make fires on the sand as if on land. But when the monster feels the heat of these fires, it immediately submerges into the water, and pulls the ship into the depths of the sea.
Such is the fate of all who pay no heed to the Devil and his wiles, and place their hopes in him: tied to him by their works, they are submerged into the burning fire of Gehenna: for such is his guile."

Compare this with Rabba bar bar Chana's account of thinking it was dry land, lighting a fire, and the beast turning, such that they almost drowned. So it might well be intended allegorically, but maybe not.

In terms of gigantic eye-sockets, there is also the following from Niddah 24b, which also might be allegorical.

It was taught: Abba Saul stated, I was once a grave-digger and on one occasion there was opened a cave under me and I stood in the eye-ball of a corpse up to my nose. When I returned I was told that it was the eye of Absalom.

So it seems to be not outside the bounds of reason that Chazal believed in giant fish with gigantic eyeballs, in which one could immerse. Even so, I believe that my explanation in the gemara is better than the one proffered by Rashi. Of course, feel free to argue.

BTW, here is the Rambam on this, from Hilchot Mikvaot, perek 8:

 יא] כָּל שֶׁתְּחִלַּת בְּרִיָּתוֹ מִן הַמַּיִם, כְּגוֹן יַבְחוּשִׁין אֲדֻמִּין--מַטְבִּילִין בּוֹ; וּמַטְבִּילִין בְּעֵינוֹ שֶׁלַּדָּג.