Does "otherwise" mean "in addition" here?

No, it means that those traits are seen, individually, in the parent forms, and not seen together in any other case. As an example:

A sailing vessel rigged fore-and-aft has sails which are rigged along the center line of the ship. A sailing vessel which is square rigged has sails which are rigged on spars hung perpendicular to the center line of the ship. The two forms are distinct; otherwise seen together only on a hermaphrodite brig.

A fore and aft rigged ship



A square rigged ship



Hermaphrodite brig



So, hermaphrodite brigs combine the two types of sail rigging, which are otherwise not seen together. I don't feel that i'm getting the distinction across here, but this is the best i can do.

Hybridity refers in its most basic sense to mixture. The essence of a 'hybrid' is that it combines traits or features otherwise seen only separately. A hybrid car has an electric motor and also a gasoline engine which are otherwise seen separately (in electric cars and gasoline powered cars).

"Otherwise" is used here to mean "under other or separate circumstances."

That meaning is clear.

The traits are usually only seen separately, the EXCEPTION being, as (part of) a hybrid.

A fore and aft rigged ship

The pic is too small to see it clearly. Who would like to find a good one?

A square rigged ship

I found this:



Hermaphrodite brig

Is this a hermaphrodite brig?



You have to save the last one to your computer and then reopen it again, otherwise it is too big to be seen on the page.

Lovely pix, Boss . . . i was rather more interested in giving you an idea of the usage of "otherwise," however.

(Your second picture simply shows a brig, not a hermaphrodite brig.)