This species was featured in National Geographic’s list of the 10 strangest discoveries in the world in 2012.
Discovery of a New Species
In July 2009, Koichi Shibukawa, a scientist from the Nagao Natural Environment Foundation in Tokyo, Japan, discovered a new species while observing a small canal of the Hau River—part of the Mekong River system—in Cu Lao Dung district, Soc Trang province, Vietnam.
During his collaboration with associates from Can Tho University, including Dr. Tran Dac Dinh and Master Tran Xuan Loi, he realized that this was an unknown species. The research on this new fish was published in the scientific journal Zootaxa, issue 3363, in July 2012.
The research on this new species was published in the scientific journal Zootaxa in 2012. (Photo: VietnamNewFish)
The Cuu Long Fish (scientific name: Phallostethus cuulong) is a type of ray-finned fish belonging to the family Phallostethidae, order Atheriniformes, discovered in Vietnam in 2009 and officially named as a new species in 2012. They are considered quite a rare freshwater fish living in the Mekong River region of Southeast Asia.
The scientific name of this fish can be understood as follows: The name of the genus and family is derived from Greek, meaning penis (phallos) on the breast (stēthos): phallos + stēth + the suffix us = Phallostethus. The English name priapiumfish for this group of fish is named after the ancient Greek fertility god, Priapus, while the species name (cuulong) is derived from the river where it was first discovered.
The standard length of this species is 25mm. (Photo: Huffpost).
The genus Phallostethus currently contains three species. Besides P. cuulong, there are P. dunckeri (C. T. Regan, 1913) found in the Malay Peninsula and P. lehi (Lynne R. Parenti, 1966) in Northwestern Borneo. Phallostethus cuulong is the 22nd species in the Phallostethidae family discovered. Furthermore, the Cuu Long Fish is endemic to Vietnam and does not exist anywhere else in the world.
A Fish with a Unique Mating Mechanism
This species is characterized by having its reproductive organs located just below the chin, behind the mouth. Male fish possess a priapium, an organ resembling a penis but inverted and similar to a type of pectoral fin. This organ enhances the fertilization rate of the eggs, allowing the female’s oviduct to contain a higher quantity of sperm. Meanwhile, the female’s reproductive organ is located inside its mouth.
This fish has a unique mating mechanism where the male positions its head close to the female, creating an angle of about 45 degrees. Although there are no specific records of their mating process, scientists hypothesize based on body structure and comparisons with similar species that they use saw-like and whip-like structures to attract females during the breeding season. Male fish develop a penis that extends from the head like a saw hook to grasp females during mating.
This fish has a unique mating mechanism. (Photo: Zootaxa).
Thus, this species is among the most evolutionarily distinct fish in the world as it is one of the few species where fertilization occurs outside the female’s body.
Lynne Parenti, a researcher from the Smithsonian Institution in Washington DC, stated that scientists have yet to determine why priapiumfish have this unique reproductive structure. They belong to a family of fish called Atherinomorpha—in which the pectoral fins have evolved to function as “sperm transporters,” she added.
Eight specimens of P. cuulong have been studied, comprising five males and three females, with sizes ranging from 20 to 24.5mm. Their body parts were measured as a percentage relative to the body, including: head length from 22.1% to 24.1%, mouth length from 7.5% to 8.5%, eye diameter from 6.7% to 7.3%, interorbital width from 3.3% to 5.2%, jaw length from 8.0% to 9.4%, predorsal length from 78.0% to 82.6%, length before the anus from 46.4% to 48.7%, and the thickest part of the body from 15% to 18.7%.
