Hybrid fish – freaks of nature

According to the Oxford dictionary, a hybrid is “The offspring of two plants or animals of different species or varieties, such as a mule.” Hybridisation occurs across the animal and plant kingdoms, including in fish. There are many hybrid fish in our hobby, several of which are some of the most popular fish we keep. But are they a good thing or a bad thing? 

The most famous hybrid in the hobby is the Parrot cichlid, where the words hybrid and parrot are almost always uttered in the same sentence. But there are many more hybrids available than one may first think, and in much larger numbers. Domestic swordtail and platy varieties, although different species on paper, have been crossed many times over the last hundred years. Early fish farmers found that by crossing a platy with a swordtail, a fish with black markings was produced, and from that the Wagtail platy came about. Countless crosses and backcrosses later, every coloured, non-green, wild type Xiphophorus hellerii and X. maculatus is actually a hybrid of the two species, and that’s also the reason why a Red swordtail for example has a shorter sword than a wild Green swordtail. It’s a swordtail/platy hybrid. 

So in order to create new coloured strains of many aquarium fish, crossing them with something else is one of the first ports of call. This can be seen in Flowerhorns and in Red texas cichlids. Without crossing, many species will naturally breed true, and stick to their wild type colouration, but cross them with another species, and that’s when more colour mutations may occur. Danios are crossed by the scientific community to study exactly that, changes in pigment and pattern. 

Guppies may have been crossed with Endlers, Mollies are possibly a cross of three molly species, and I’m still sticking to my guns that Electric Blue Jack Dempseys are in fact hybrids too. Calico Aulonocara do not naturally occur, so are hybrids. Not all hybrids are sterile, in fact most aquarium hybrids are fertile, and species from different genera can even produce viable offspring in some cases. 

Hiding in plain sight

Some hybrids we know about, and are sold as such, like Tiger shovelnose/Red tail cat hybrids, but so many go under the radar, and may even be in our tanks right now. African cichlids hybridise readily, and many an “assorted Malawi cichlid,” is a hybrid. But what about domesticated discus strains? They are a hybrid of at least two, possibly all three wild Discus species.

Angelfish? Again, only three wild, distinct species and although an altum/scalare cross is unlikely because Altums are so hard to breed, scalare/leopoldi crosses and backcrosses aren’t so impossible, and if you take the literal Oxford dictionary meaning to include “varieties” then lots of wild scalare varieties coming from all over their catchment like Rio Nanay, Manacapuru, Santa Isabella etc will have all been thrown into the genetic pot (knowingly or unknowingly,) over the years.

And the same with our Green and gold severums. I’ve watched two separate Severum species pair up, breed and have their hard to identify offspring sold into the market as one of the parents, Heros sp.”Rotkeil.” 

Freshwater shrimp? Definitely some hybridisation going on there. Rainbowfish? Yep, I spent years selling batches of what I thought were Sepik rainbows, Glossolepis multisquamatus, to find the very fish labelled in a book as a Melanotaenia/ Glossolepis hybrid. The true Sepik rainbow is now available and is a very different looking fish. Barbs? Yep, some are even crossed with denisoni, and Synodontis catfish crosses are well known about yet widely sold, and very difficult to I.D. What about the expensive stuff? Freshwater stingrays have been hybridised to create fashionable new patterns and Dragonfish, Scleropages “formosus” is actually a cross of up to five asian Scleropages species.

So back to the good thing/bad thing. Hybrids aren’t necessarily a bad thing if you know what they are, and once created they definitely require the same duty of care and rights as any other live animal, but it’s when you don’t know what they are that problems can arise.

Scleropages, for example, are Cites Appendix I. There are millions more Asian Arowana in captivity than in the wild, but if the wild species go extinct should you repopulate with a hybrid species? It may occupy the same niche, but you are replacing the equivalent of a Scottish wild cat with a domestic cat. A Dingo with a golden labrador. Unknowingly introduce new, hybrid stock to remaining wild stocks and you have the same problem. They may cross, and genetic purity may be lost. The same with rewilding Lake Victoria cichlids and Rainbowfish. If they are not 100% the same fish, and genetically pure, should you do it all? 

Identification

The only tool us fishkeepers have is our eyes, so trying to identify a mbuna or Synodontis as a hybrid is difficult. If you buy a pair of Aulonocara for example and the female is a different species, they breed, and you are none the wiser that you have hybrid offspring on your hands. You pass on the fry, and so and so on, and soon all of them are hybrids, and diversity is lost. There is an argument to say it doesn’t matter because most species never go back to the wild, but there are over 30,000 species of fish in fresh and salt waters and I’d rather have 30,000 than just one. 

So the loss of genetic diversity and identification are the main factors against, but what about for? Well, in aquaculture, hybrid Tilapia are created because they grow faster, so a faster-growing crop means faster conversion to edible adults, faster turnaround of money and more people get fed. At an ornamental level, new varieties of tropical fish generate sales and help grow and sustain the hobby. But hybrids can also be useful to science. When I visited Hull University, Alan Smith was busy creating hybrid African cichlids for his work on mate preferences in sympatric Lake Victoria cichlid species. And that led me years later to ask the scientific community for their stance on hybrids. I asked some ichthyologists…

Hybridisation and science

“Hybridisation followed by introgression (repeated backcrossing,) in fishes is far more common than people have appreciated, and it has gone on throughout time,” Peter Unmack told me in the group Ichthyology. “Many species have DNA from other species in them. People have tended to act that this is odd, but it is probably more the norm than the exception.”

Sam Payet commented on my question of how to identify a hybrid at a scientific level: “Generally hybrids can be identified by their intermediate colour patterns but this can be less obvious if the hybrid back-crosses with a parent species. In this case, genetics are a more reliable (but not always definitive) method of identification. I think a combined approach is best”         

I asked PhD student Rowan Schley what effect and/or risks hybrids could have if introduced to wild populations: “There can be many outcomes which are context-dependent. It could be that where an invasive species occupy the same habitat as a closely related narrow range endemic, the endemic species may be genetically swamped by the encroaching species, leading to the homogenization/extinction of the endemic.

Conversely, assuming two sets of species are adapted to different conditions, hybridisation may result in offspring that are poorly adapted to either parent’s habitat and are targeted by natural selection (known as reinforcement.) This is thought to maintain the divergence between species. Over millennia hybridisation can actually lead to the evolution of new species via hybrid speciation, which is thought to be common in rift valley cichlids. 

In addition to this, natural hybridisation tends to be avoided in cichlids for example by things like mate choice and preference (known as assortative mating) or even genetic barriers to reproduction/fertilization of eggs/development of larvae, in order to prevent poorly adapted offspring. 

As previously said, hybridisation and backcrossing probably occur more often than previously thought, and while a species may look like one species, that doesn’t mean that it doesn’t have hybrid ancestry, as different regions of the genome can be exchanged at different rates.”

Food for thought

So hybridisation may be a much more natural phenomenon than we first thought. It may also be much more common. A “pure” species may have hybrid genes, and a “hybrid” may go on to form a new species. You can’t always identify a hybrid on visual characteristics alone, and buying a wild fish doesn’t necessarily mean that it doesn’t have hybrid DNA. A hybrid may not always be a threat to parent species because they may not be chosen to mate with in some cases, and a hybrid fish may or may not be better adapted than its parents to survive and take over. Some say us humans are hybrids… 

Avoiding aquarium hybrids

The most common hybridisation risk you will face as a tropical fishkeeper is from cichlids. With Malawi cichlids, Lake Victoria cichlids and some Central American cichlid species, don’t mix a single male or a single female with members of the opposite sex, of different species. These fish are hard-wired to breed so they will still try to do what comes naturally. Avoid mixing very similar or closely related species, like a lone male Jade eye with a lone female Convict cichlid for example. They will be drawn to each other. 

If you can’t accurately identify a male and female of a certain species, this is where the internet comes in handy as you can post pictures and if 75 people out of a 100 comment that they are male and female of the same species, they are probably right. The catfish guys will also quickly tell you if a Synodontis decorus is a Synodontis decorus, or not.  

For egg scatterers like Rainbowfish, only raise eggs spawned by single-species groups, as its the only way of making sure that someone else’s eggs or sperm haven’t been scattered with the rest. Rainbowfish will often spawn together. My rainbows even used to spawn with my Congo tetras, although luckily, nothing came of it!