Neale Monks helps one of our readers with a question about mixing different species of livebearers together in a community tank.
I have a 250-litre (55 gal) community tank containing six male bumblebee platies, 10 male Endler’s livebearers and – I thought – six male mollies. But one of them swelled up and yesterday I spotted two babies, so I assume that the ‘swollen’ one was a pregnant female.
I have had a close look, and it does seem as though two of the six Mollies are missing a gonopodium. Now the other female is looking pregnant.
Should I take the two females back to the fish shop? Also, is there a risk of them hybridising with the platies or the Endler’s livebearers? The fish tank also contains 10 glowlight tetras, eight lemon tetras and five bronze corys.
AUDREY FISHER
Neale's Advice on Livebearer Hybridisation
Sexing livebearers is one of those things that everyone thinks is easy but isn’t. Not all males present clearly as males, particularly when small. In the hubbub of a display tank, when swimming alongside several tank mates, it isn’t always obvious when a male fish has an anal fin that hasn’t completely modified into the proper gonopodium shape.
There's little chance of platies (Xiphophorus spp.) hybridising with either the Endler’s or the mollies – they’re just too distantly related. However, Endler’s, mollies and guppies belong to the same genus, Poecilia. So there is a chance that your Endler's and molllies could hybridise.
Unlikely Livebearer Hybrids
Guppy x molly hybrids are actually quite well documented. They’re somewhat nondescript-looking fish that tend to be longer in the body than mollies but are much bigger than most guppies.
That said, the two hybridise much less often than you might imagine, considering how often they share community aquariums. So, it seems to me that either the hybrid embryos aren’t consistently viable, so mostly don’t develop into fry, or else the two species can’t or won’t mate successfully most of the time. But for whatever reason, while mollies and guppies – and likely Endler’s – can hybridise, it isn’t common.
Since you have these fry, you may as well keep them and see what you get. My guess is that they will turn out to be mollies. You might want to return the female mollies if you don’t want to breed these fish. As they've bred already, they'll likely breed again and again, and you might soon be over-run with them.
Did you know?
In some livebearer species, males exist in ’strong’ and ‘weak’ forms that exhibit their secondary sexual characteristics, such as the sword length in swordtails, to greater or lesser degrees. This probably explains the stories often repeated about livebearers changing sex – something that has not been convincingly observed under laboratory conditions. It’s not so much that livebearers change sex; it’s more the fact that not all males look like the ones in the books, so could be easily mistaken for females.
Conversely, some females can acquire male-like traits in old age. However, they won't ever produce sperm and probably don’t even have the bones and muscles needed to form a functional gonopodium. It’s more a hormonal imbalance leading to changes in colouration, fin shape, and perhaps even behaviour. They might look a bit like males, but so far as reproduction goes, they’re females.