פירוש על עבודה זרה 79:10

“Roe,” is another word for what we usually call caviar—the fish eggs. Since we cannot tell whether they came from a kosher or non-kosher fish, one should buy them only from an expert, one who is reliable to sell kosher fish. The same is true for entrails, which we discussed earlier.

The issue here is whether unclean fish have “roe,” meaning lay eggs. According to Rav, they do. Otherwise, roe could only come from kosher fish. But there is a baraita that says that unclean fish do not lay eggs.
There are two responses to this difficulty. The first is to emend the statement of Rav and remove the word “roe.”
The second is to suggest that they both lay eggs but that they lay them differently. Kosher fish lay eggs externally. Non-kosher fish have eggs but they hatch inside their bodies. The technical word for this “ovoviviparous” (cool word!). This is true of sharks and rays and a few other species of fish, all of which are not kosher.