Molly/Limia Hybrid?

Photo of a possible Limia vittata/Poecilia mexicana hybrid. The upper fish is a female Limia vittata. The bottom fish is a possible female Limia vittata/Poecilia mexicana hybrid.

I have tried for years to produce a molly/Limia hybrid to no avail until maybe now. Why would I attempt this? Well…partly to disprove the idea the genus Limia should be subsumed into the genus Poecilia. For context, mollies are currently placed in the genus Poecilia. Prepare for a long digression before I get back to the possible molly (Poecilia)/Limia hybrid.

Way back last century, in the 1960s I seem to remember, the genus to which guppies belonged, Lebistes, was subsumed along with Mollienesia, the mollies, into the genus Poecilia. Why Poecilia? Because by the rules of scientific nomenclature Poecilia had precedent. Don’t ask…those are the rules. I’ve never been happy with tossing guppies into the same genus as mollies regardless of what one might name the resulting genus. Why you might ask since guppies and mollies do hybridize indicating they are closely related. Yes, they do, but so far as I know all the hybrids are sterile. This is unlike all the hybrids among the various molly species, which are all fertile. Of course, genera (the plural of genus) are artificial constructs of humans’ desire to order the world. While we’re at it, species is also an artifice. Let me explain.

We define a biological species as, “A group of closely related organisms that are very similar to each other and are usually capable of interbreeding and producing fertile offspring. The species is the fundamental category of taxonomic classification, ranking below a genus or subgenus.”

But when we apply this simple definition to messy life on this planet, we find species is a slippery concept. For example, consider ring species. From Wikipedia,

“In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds with closely sited related populations, but for which there exist at least two “end” populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each “linked” population.”

One of my favorite professors, Dr. Eric Pianka, in his evolutionary ecology class, gave this example of a ring species. In northern Greenland one finds two seabirds (I don’t remember their names all these years later). They resemble each other, differing primarily in size, one being large the other small. They nest next to each in the same areas, but do not hybridize. Each fits the definition of a species presented above; in other words, they appear to be separate species. As you move west around the Arctic Circle, the larger of the two birds disappears; we are out of its range. The smaller bird, however, is found. But as you continue around the Arctic Circle, the little bird gets larger. By the time you reach northern Greenland all the way around the Arctic Circle, the smaller bird has become the larger bird. So, is this one variable species or two? Clearly if we eradicated all the birds not residing in northern Greenland, we’d have two good species.

Let’s look at another messy species situation. Again, this was from Dr. Pianka. There is a fruitfly species in which the males and females will only mate on the plant species they ate as larvae. And females will only lay eggs on that plant. They are faithful to their plant even though their larvae can live on other plants. If one takes eggs laid on their plant and place them on a different but edible plant species, the resulting adults will only mate and lay on that new plant. This effectively genetically isolates them from the rest of the species. Is that now a new species?

What about dogs? Imagine that some dog virus eradicated all dogs but pure Great Danes and pure Chihuahuas. One species or two?

Then, for my African cichlid friends, we have Lake Malawi in Africa with several hundred “species” of cichlids, all of which will if given no other choices (and sometimes even if they have choices) mate with any other species in the Lake. In fact, many experts believe most of these species arose first as hybrids. Here’s a fairly technical paper touching on speciation and hybridization:

https://academic.oup.com/mbe/article/37/4/1100/5671705

As it turns out what is a species is often a matter of opinion. It only gets worse at the level of genus.

Digression over.

So, over the years I’ve tried to produce Limia/Poecilia hybrids mainly to confirm my opinion to the two genera shouldn’t be combined by showing they don’t hybridize. I’ve tried various species of the two genera and the results, or lack of them, seemed to support my position. Well…this one fish might dent my argument a bit. If this fish (the bottom of the two fish in the photo at the beginning of this blog, the top being a female Limia vittata) is indeed a hybrid then maybe I just didn’t try hard enough to get Limia/Poecilia hybrids. For visual reference, here are photos of the two species in question:

How did this particular female and possible hybrid come to be? Here’s the story. I had placed some juvenile Limia vittata in a vat to grow out. Apparently that vat had been housing Poecilia mexicana and someone (I blame anyone but me!) didn’t adequately clean the vat and left some P. mexicana fry in the vat. Due to reconstruction because of Hurricane Harvey (yeah, yeah, I know I blame everything on the storm!), that vat languished long enough for the fry to mature and produce fry that themselves matured. When we processed that vat, it was simple to sort out the P. mexicana and L. vittata except for this one female. In many ways, she seemed intermediate to the two species. While it’s difficult from the photo to tell, her coloration is similar to that of P. mexicana, down to the orangish caudal. But her body shape and head screams L. vittata.

While I’ve set her aside to see if she ever bears fry, she also encourages me to deliberately set up P. mexicana/L. vittata crosses. To do this properly, I should raise virgin females of both species and place them with males of the other species. Fat chance! That would require raising them in aquaria and using constant vigilance to remove developing males. First of all, we rarely raise fish in aquaria; we use vats on our large recirculating system. Aquaria require was too much maintenance such as water changes. We don’t have to do water changes on our recirculating system with plant filtration. Also raising virgins would require a few months for grow out. I’m a bit too impatient for that. So, what will I do? I’ll use brute force and simply set up two vats, one with P. mexicana males and L. vittata females and another with the reciprocal cross. Why both crosses? Just in case one pairing works and the other doesn’t. Now, those of you with experience with Poeciliidae (the family both species belong to) females will quickly see the problem with this approach: Limia and Poecilia females can store sperm and continue to produce fry from earlier matings. So, it can be difficult to tell if you are really getting hybrids. In this case, that doesn’t worry me because any fish resembling the above female will be hybrids. Another potential problem: it’s possible the males having mated with their own females might not pursue other species’ females. My experience with the males of both species makes that unlikely; they are both aggressive maters. And maybe the females will refuse to mate with the other species’ males…good luck with that, both species’ males are very persistent. I’ll probably set each vat up with a half dozen males and 40 females. Surely somebody will mate. After a few months I should know whether this will work. If it doesn’t either I’m right and the two species don’t hybridize, or I’ll have to bite the bullet and raise virgin females and try again.

Here’s the program I’ll use:

1. Set up two 55-gallon breeding vats:
   1. One vat with 6 male mexicana and 40 female L. vittata.
   2. One vat with 6 male vittata and 40 female P. mexicana.
2. After four weeks move breeding colonies to two new vats.
3. Discard any fry in the original vats.
4. Place a fry cage in each of the vats to provide cover for fry that might be potential hybrids.
5. After about three months, remove the adults.
6. After another two months, process the fry and segregate any possible hybrids.

The purpose of steps 2. and 3. is to purge fry the females were already carrying. Step 5. provides enough time for each female to go through three months of fertilization by the males from the other species. Step 6. allows the fry to mature enough to distinguish hybrids from non-hybrids resulting from prior fertilizations from the same-species males the females had been previously exposed to (I’m glad the rule against ending a sentence with a preposition is nearing extinction!). If the result is no hybrids, then I have to decide whether to abandon the project or to produce virgin females. If I decide to the produce virgins, I’ll also produce “virgin” males, males who have never seen mature females of their species just in case male preference for females is an issue.

By the way, you may have noticed the reference to a “fry cage” in step 4. We make these using aquaculture mesh. They are cylindrical with a mesh bottom and open to the top for feeding. We place one in each 55-gallon vat to provide shelter and refuge for fry. We also add Ceratophyllum demersum (hornwort) to each vat to provide cover for the fish and ammonia control (one of these days I’ll post my back of the envelope calculation of how much ammonia hornwort strips out of our systems daily).

Ceratophyllum demersum (hornwort), a rootless floating plant in one of our vats.If you of you have experience with Limia/Poecilia hybrids, please let me know. If you have an opinion about the pictured potential hybrid, let me know.

Good Fishkeeping!