Tropical Fish Hobbyist - February 2008

Livebearers Unlimited
The Molly That Clones Itself:
The Weird, Wild World
of Poecilia formosa

The fecundity of poeciliid livebearers, and the constant mating with multiple partners, inevitably leads to radiation of a species. New color forms, finnage, and sizes pop up from time to time, even in home aquariums. The hi-fin trait seen in swordtails and platies, for example, was first discovered by hobbyist Thelma Simpson in her swordtail breeding tanks in the early 1960s.

With so many related species, it is not uncommon for a natural hybrid between two related species to emerge and develop into a separate species of its own. The most recent example of this is the swordtail Xiphophorus clemenciae, which I wrote about in my March 2007 column. Recent research suggests that the original parentage of this oddball swordtail is probably an ancestral lineage of X. hellerii (the common swordtail) and either X. maculatus or X. milleri (MEYER, SALZBURGER & SCHARTL 2006).

Ted
COLETTI

Dr. Ted Dengler Coletti is in his 25th year as an aquarist and enjoys writing and lecturing on the hobby. He resides with his patient family and guitars in the beautiful New Jersey Skylands. A Director of the American Livebearer Association, he is a founder of the Northeast Livebearer Association and the Aquarium Hobby Historical Society. Contact Dr. Ted at tedcoletti@yahoo. com with your questions, suggestions, and top 10 reasons to love lint.

The First All-Female Fish

Arguably the most bizarre of the natural hybrid Poecilia species is Poecilia formosa. The fish hails from southwestern Texas in the lower Rio Grande Valley to northeastern Mexico as far south as Tampico to the mouth of the Rio Tuxpan. There are also introduced populations in San Antonio and San Marcos, Texas—so maybe you can collect some during the collecting trip at this year’s ALA Convention (see Livebearer Newsroom, p. 46)! It prefers backwaters and quiet pools, usually over mud.

Poecilia formosa was first described by Girard in 1859. It is a gray-brown plain molly with short fins and a slightly reticulated body pattern. Reaching 3 inches

in length, it is a hardy fish that can withstand temperatures from 60° to over 80°F in both fresh and brackish water. It enjoys some vegetable matter in its diet.

Poecilia formosa become sexually active quickly in just a few months, and at full maturity can give birth to large broods (up to 100 fry) every 30 days or so.

But observations in the wild by legendary ichthyologists Carl and Laura Hubbs in 1932 took this unremarkable molly from lackluster to legendary. The Hubbses explored the geographic range of Poecilia formosa and could only find females. They had stumbled upon the first all-female fish, as well as the first unisexual vertebrate on the planet Earth! The fish was given the popular name “Amazon molly” after the race of all-female warriors in Greek mythology.

Their discovery was serendipitous. Carl Hubbs’s interest was in naturally occurring hybrid fish that result in speciation over time. Before the discovery, Poecilia formosa was considered a distinct species. But “formosa-like” hybrid fish (boys and girls) did occur in areas where the short-fin molly P. sphenops and the sailfin molly P. latipinna co-habitate. When these two species live separately, the all-female P. formosa is found. The Hubbses theorized that P. sphenops and P. latipinna were the ancestral lineage for the Amazon molly.

From these observations, the Hubbses returned home to the lab and tried to recreate Poecilia formosa by crossing P. sphenops with P. latipinna. After 12 years of experimentation, they were still getting only formosa-like hybrids of both sexes, not the fish they observed in the wild.