Just so you know, a variety of physical structures for internal fertilisation have evolved independently among a number of fish lineages over the ages (such as among cartilaginous fish, modern bony fish groups and ancient placoderms)..
http://www.newscientist.com/article/dn22023-zoologger-the-fish-with-its-genitals-on-its-head.html
Zoologger: The fish with its genitals on its headSpecies: Phallostethus cuulong
Habitat: surface waters of the Mekong River in Vietnam
The male fish, a Phallostethus cuulong just 2 centimetres long, weaves between drifting vegetation in the sluggish waters of a canal. He closes in on a female, swims alongside her and tries to mate with her.
But to an outside observer, he seems to be doing it wrong. His head is right next to the female’s, but he’s at a 45-degree angle so his rear end is well below hers. Sounds misguided, but actually he’s doing it exactly right – it’s just that his gonads are on his head.
This is the challenge faced by all priapiumfish, a little-known group of Asian fish that have their reproductive organs on their chins, just behind their mouths. How does this Cronenbergian arrangement work?
Phallic fish
P. cuulong is only the 22nd known priapiumfish, which are named after the ancient Greek fertility deity, Priapus. They all belong to a family called Phallostethidae and live in south-east Asia.
…
Male priapiumfish don’t have a penis like humans and other mammals. Instead they have a unique organ called a priapium, which faces backwards and looks like a muscular nozzle. It’s actually a modification of the fish’s pectoral and pelvic fins.
…
No one has seen P. cuulong mating, but based on observations of other species it’s likely that the saw and rod are used for grasping the female during mating. One goes on either side of her head, holding her still while the male transfers his sperm.
To help with this, the priapium tends to be shunted to one side. The six male P. cuulong that Shibukawa found all had theirs on the right: other species tend to be left-priapiumed, or a mixture of the two.
…
(ed: interesting aspect suggesting reproductive isolation)
..
The system seems to work. The oviducts of female priapiumfish tend to be stuffed full of sperm, so pretty much every egg gets fertilised.
We don’t know why priapiumfish evolved their peculiar gonads, says Lynne Parenti of the Smithsonian Institution in Washington, DC.
They belong to a big group called the Atherinomorpha, which includes many other species with modified fins that transfer sperm. In most cases, like guppies and splitfins, it’s the anal fin that’s modified. The priapiumfish are a variation on this theme, Parenti says.
