

Freshly-collected specimens of Phallostethus cuulong. Photographed and retouched by L.X. Tran and K. Shibukawa, respectively.

A bizarre penis-headed fish has been discovered in Vietnam, according to a new paper published in the journal Zootaxa.

The species, dubbed Phallostethus cuulong, is the 22nd known member of the Phallostethidae family, a group of tiny, otherwise non-descript fish characterized by the presence of copulatory organs just under their throat. The authors explain:

Male phallostethids have a unique complex copulatory organ, termed the priapium, under the throat (thus the fishes of this family are commonly called “priapiumfish”). The priapium is a bilaterally asymmetric organ for holding or clasping onto females and fertilizing their eggs internally; following internal fertilization, phallostethid females do not give birth to live young, but instead lay fertilized eggs.

Phallostethus cuulong was collected during an expedition to the Mekong delta in Vietnam. It was recognized as an undescribed species after it was compared with other known members of the Phallostethidae family.



Head and anterior part of body of Phallostethus cuulong. A) lateral view of left side of head and body of male; B) lateral view of right side of head and body of male; C) lateral view of head and body of female; D) ventral view of head and body of female. Photographed and retouched by L.X. Tran and K. Shibukawa, respectively.

Scientists don’t yet understand the evolutionary origin of the unusual placement of the fish’s reproductive organ. However Phallostethidae fish are “part of a larger group that includes many species that fertilize their eggs internally,” according to National Geographic News, which goes on to note that “the vast majority of fish species fertilize their eggs outside the body.”

Lynne Parenti, curator of fishes at the National Museum of Natural History in Washington, D.C., explained the fish’s mating behavior to National Geographic News: “As with all Phallostethus—’penis chest’ in Greek—species, the male uses its bony “priapium” to clasp a female while he inserts sperm into her urogenital opening, also located on the head.” Phallostethidae fish mate head-to-head, which is apparently “a very efficient way to do it,” according to the researcher.

The fish discovery comes less than a month after a biologist in the Amazon revealed a little know species of caecilian — a legless amphibian — that was shockingly phallic in shape.

CITATION: KOICHI SHIBUKAWA, DINH DAC TRAN, and LOI XUAN TRAN. Phallostethus cuulong, a new species of priapiumfish (Actinopterygii: Atheriniformes: Phallostethidae) from the Vietnamese Mekong. Zootaxa 3363 published: 3 Jul. 2012

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