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).
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
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
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
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.