What's so special about the Amazon molly?

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Dr Michi Tobler of Texas A & M University looks at a livebearer with an unusual reproduction method.

What is it that makes the Amazon molly so unique?
The Amazon molly (Poecilia formosa) is one of the few asexual vertebrates. In this species, all individuals are females. When it comes to reproduction, females simply clone themselves, so that the offspring is genetically identical to the mother.

However, unlike other asexual organisms, Amazon mollies can’t do it without a little help, despite the asexuality.

Sperm from heterospecific males are needed to trigger the onset of embryonic development, though there is no genetic contribution made by males to any offspring.

This unique way of reproduction is called gynogenesis or sperm-dependent parthenogenesis.

Amazon mollies can grow up to 10cm/4”, but most individuals remain considerably smaller at 4-6cm/1.6-2.4”.

Where does this fish come from?

Amazon mollies naturally occur in the coastal habitats of southern Texas and north-eastern Mexico south to the Rio Tuxpan. In the Mexican states of Tamaulipas, Nuevo Leon and Veracruz the Amazon molly also inhabits inland rivers and streams.

They have also been introduced into some central Texan springs.

Which species are sperm donors and do they differ depending on the habitat?
Since there are no male Amazon mollies, females have to trick males of other species into mating with them.

In coastal habitats, males of the Sailfin molly (Poecilia latipinna) are used as sperm donors, whereas males of the Atlantic molly (P. mexicana) are used in the same way in inland waters.

In addition, the Tamesi molly (P. latipunctata) is called on as a donor in the headwaters of the Rio Tamesi in Mexico.

What other fish live alongside Amazon mollies?
In coastal habitats, Amazon mollies are often collected alongside pupfishes (Cyprinodon variegatus), topminnows (different Fundulus) and other estuarine species.

In inland waters, they frequently co-occur with Mexican tetras (Astyanax mexicanus), cichlids (Herichthys) and other livebearers.

What are the advantages and disadvantages of its reproductive strategies?

The advantage of asexual reproduction is undoubtedly the high reproductive output that is being created.  

Asexual females don’t have to produce dispensable males, which means that they have twice the amount of grandchildren compared to sexual mollies.

The disadvantages of asexual reproduction are less clear and subject to some intense research. Due to this process of clonal reproduction, Amazon mollies appear to have a more limited potential to adapt to changing environmental conditions. They also seem more susceptible to adverse environmental conditions than sexuals.