Making sense of the molly muddle


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There’s a whole lot more to mollies than the ‘Heinz 57’ varieties that we see in stores. Neale Monks explains.

Mollies are underappreciated. Whatever the colours of the farmed varieties, they pale into dullness compared with the wild mollies that live in caves, hypersaline lagoons, even in poisonous pools. There are even mollies that seem to break the laws of nature, but we’ll get to that later…

What is a molly?

It’s a simple enough question, but rather difficult to answer! Traditionally, guppies and mollies have been lumped into a single genus, Poecilia, but the DNA evidence reveals a more complicated picture.

The four most familiar molly species (Poecilia sphenops, Poecilia mexicana, Poecilia latipinna, and Poecilia velifera) clump together with less familiar ones like Poecilia gilli and Poecilia sulphuraria. Collectively, their closest relatives are Pamphorichthys, South American livebearers not much seen in the trade. More distantly related are Limia, and together they seem to have branched off from the guppy and Micropoecilia lineage around 25 million years ago.

Older aquarists will remember the Mollienesia name used for mollies in years past, and of course, why we call them mollies at all. Likewise, guppies have sometimes been placed in their own genus as well; Lebistes according to some, Acanthophacelus according to others. We might well see these old names come back into use before too long.

A complex question

Early on in their evolution they divided into two groups of closely related species: the shortfin molly or Poecilia sphenops complex and the sailfin molly or Poecilia latipinna complex. Some scientists even go so far as to define Poecilia sphenops in particular as a single, but very varied, species containing all the mollies with short dorsal fins, including Poecilia mexicana, Poecilia sulphuraria, and Poecilia salvatoris.

What about the sailfin mollies?

These include Poecilia latipinna and Poecilia velifera, as well as some less familiar species like the Péten molly, Poecilia kykesis. In truth though, the fish we see in the trade are usually hybrids between these and perhaps a few other species. All told, there may be something like twenty species (or at least distinctive populations) of molly in Central America, which raises interesting questions: why are there so many of them? What is it about Central America that favoured rapid speciation? Why are mollies so diverse there, but not, say, the characin and catfish families that dominate South American river systems?

The answer seems to be — they got their first. The Isthmus of Panama is relatively new, around 6 million years old, and before that, the only connections between South and Central America were through the sea. Being far more tolerant of seawater than primary freshwater fish groups like characins and catfish, mollies (as well as cichlids) were able to get into Central American rivers and lakes before anything else, and quickly radiated into all the varieties we see today.

Read the rest of the feature in the August issue, on sale from July 21, 2022, available to read instantly on our digital edition HERE  or purchase the print edition HERE.

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