Actually some fish, like shark, reproduce via internal fertilization, which does not really answer the question, so you might want to come down from your horse for once.
As to Vonderam, we don't know WHAT he is, considering that he is also a shapeshifting shota-god at times that can sprout wings.
A common form of reproduction in fiction is...transforming into a human and entering the land, which according to Emana is "very humiliating to Mermaids" (But what does that tell us about Shufas, who enjoys going on land?).
Alternatively they are "Sea Fey" (The Fairy Folk of the Sea), which are prankster spirits that are ageless and genderless, and thus kidnap human children or men for destroy, torture, and lulz.
Alternatively there is Logress.
A clear case of "don't think too much about it".