A recent discovery show a species of fish that have genitalia on their heads. The new fish species, known as the Phallostethus cuulong is a small fish from the Phallostethidae family. This small family of fish may look normal at the first glance, but it turns out that all members of this family bear their sex organs on their heads, just right behind their mouths.
It is an evolutionary mystery for scientists, who are having a hard time figuring out why the animals have their genitalia on the front of their bodies.
“Phallostetus” is known as “penis-chest” in Greek, which is an amazingly fitting name for this particular creature. The male of this species has a bony priapium that it uses to grasp a female and insert his sperm into her urogenital opening, also found at the front of its body.
Lynne Parenti, the curator of fishes at the National Museum of Natural History in Washington, D.C. says that she has seen another similar species from this particular fish family at a lab in Singapore. The animals were also mating, and were attached to each other by the head. The appearance of the mating fish make a distinctive V shape as their bodies touch.
She also notes that while mating for most fish species such as guppies are instantaneous, Phallostethus cuulong actually take a remarkably long time to mate.
P. cuulong is just a tiny fish, measuring less than 1 inch in length. The body of the animal is thin and small, almost transparent in appearance. More interestingly, scientists simply cannot understand how its genitals ended up on its head or why.
P. cuulong was found in the brackish river waters of the Mekong Basin in Vietnam during a field survey by scientists. The animals were published and revealed in the journal, Zootaxa. While many areas of the Mekong river are being heavily developed, the fish was found to be surprisingly adaptable and resilient, and is of no danger to go extinct any time soon.
While scientists are still unable to decipher why the fish evolved to have genitals on their head, Parenti already has a few ideas. For one thing, she notes that unlike most other types of fish that fertilize their eggs externally, the P. cuulong fertilises its eggs internally. She also notes that because of this need to fertilise eggs internally, males of the species have evolved special genitals to help them facilitate the fertilisation process more efficiently.
She says that head-to-head mating is after all, more conducive for internal fertilisation. She has dissected several female P. cuulong fish that have been preserved, and has found that their oviducts were filled with sperm. The eggs were also almost 100% fertilised, which is a higher fertilisation rate than most other types of fish that reproduce via external fertilisation.
She says that this might be a clue to how these animals developed and how their genitalia ended up on their heads.