LADY CAPULET The fish lives in the sea, and ’tis much pride
For fair without the fair within to hide;
That book in many’s eyes doth share the glory
That in gold clasps locks in the golden story:
So shall you share all that he doth possess,
By having him, making yourself no less.
NURSE No less! nay, bigger women grow by men. (1.3.90-96)
As the note in the current Cambridge edition observes of the fish in line 90, ‘no satisfactory explanation has been offered’, that is, it’s completely obscure, this fishy swerve. It’s perhaps something to do with fitness, appropriateness, symbiosis, picking up on the discussion of harmony and one another lending content earlier on in the speech (although of course the fish needs the sea more than the sea needs the fish). Perhaps this whole conceit is really just Lady Capulet trying to say, you two would look so great together! but the conceit doesn’t quite work in this respect, because it’s so unbalanced: the book doesn’t really need a beautiful cover in order to be read, but the cover is nothing without the book, and the idea of locking in the story with gold clasps suggests that it’s not going to be read at all. But the gold leads, logically, to all that he doth possess: this is, after all, about money. The Nurse can contain herself no longer, and bounces back with another bawdy observation: women grow by men because they become pregnant.
Another possible explanation for the fish and what it’s doing: I’ve been keeping an eye on sonnets and not-quite-sonnets in the play. Lady Capulet’s speech is 16 lines long, 4 lines which are almost half-rhymes, ABBA, gentleman / feast / face / pen, followed by 5 rhyming couplets; there’s a turn of sorts before the final couplet. It’s the fishy couplet which takes it over 14 lines. It’s not just the conceit which doesn’t quite cohere, it’s the form, perhaps.