RE: NANFA-L-- Amazon Mollies

Hoover, Jan J ERDC-EL-MS (
Tue, 8 Feb 2005 12:37:24 -0600

Bob Bock:
> Why the storebought mollies aren't as large as they were many years ago, I
> don't know. What I do is check the stores regularly for exceptionally
> large breeder fish. A good male will come in to my local area about only
> about once or twice a year.

Joshua Wiegert:
>>Breeder's have selectively bred for smaller fish. Any fish which has been
domesticated for a long period of time is now smaller...
This is not purposeful selective breeding (at least, I don't think so.
of the mass production of fish, commercial breeders ("Fish Farms") need to
produce these fish fast to stay in business. THe ones that breed earliest
are the ones that breed the most, and are therefore represented more in the
next generation. They're selecting for fish that mature faster than others.
This, unfortunately, results in smaller fish.<<<

A related factor that probably contributes to smaller mollies: Less
outbreeding with wild-types. Decades ago, when store-bought mollies looked
like fresh-caught mollies, there was probably periodic mixing of locally
harvested fish with farm-raised fish. Today, most of the "standard" mollies
that I have seen in the pet stores do not look very much like wild fish.
They are smaller with smaller fins and indistinct pigmentation. My guess is
that very few wild mollies are collected by breeders and dealers now.

Also - Populations of mollies are highly variable - In the Tampa Bay Area,
landlocked freshwater mollies are smaller, slimmer, and drabber (and I
believe have fewer dorsal rays) than mollies down river in the estuaries. A
student I collected with in the 1970s studied these populations. He was
interested in differences in fecundity and size of embryos. As an
experiment, he raised mollies from one of the populations in different
salinities to try to establish relative influence of genetics and environment
(that nature/nuture thing) on size. I cannot remember how that experiment
turned out - so I will probably have to request a copy of his thesis.

- Jan Hoover
Vicksburg, MS

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