Not all fishes are egglayers; some of the most popular freshwater tropical fish give birth to live young.
Whilst these four livebearing tropical fishes are by no means the only livebearing tropical fishes, they have, for many decades, been the most popular livebearing tropical fishes for the vast majority of aquarists, including beginners to the hobby and long-standing devotees.
Many aquarists wrongly (in my opinion) consider livebearers to be for beginners only. Consider this; when did you first discover that certain fishes gave birth to live fishes? Perhaps, for a few of you, this is the first time that you have come across this interesting little fact.
We know that aquatic mammals including whales, dolphins and porpoises give birth to live young, just like any other (land) mammal but tiny, little fishes surely lay eggs, don’t they?
Livebearers vs Egglayers
Egg-layers lay or scatter their eggs, which are fertilized by the male (with any luck). The egg contains the embryo and the food that the embryo needs to grow outside the female.
Livebearers carry their eggs inside their body. The male fertilizes the eggs by “injecting” a package of sperm into the female. When the eggs hatch inside the female she ejects them into the water and the young fish (fry) carry with them a yolk sac from which they will feed until they are big enough to feed on their own account.
Egg-layers may lay many thousands of eggs that are more-or-less helpless to predation until after they hatch and become independent so losses are massive.
Livebearers omit the gestation period outside the female and the female gives birth to dozens of live fishes capable of living independently (if very vulnerably initially).
The massive scope of livebearing fishes
There are, in fact, many hundreds of species and sub-species of livebearing, freshwater tropical fishes. In truth, it would be wrong to put too much emphasis on the “freshwater” moniker when referring to such fishes as many of them live in seawater or brackish water. Brackish water is in between seawater and freshwater and is most commonly found in tidal estuaries and river deltas where the level of salt in the water varies the closer or further away from the sea the water is and whether the tide is coming in or going out.
Perhaps the best example of a fish that can live either in freshwater or seawater is the humble Guppy.
Crossbreeding and hybridization of the species
For over a century, aquarists have crossbred different species of livebearing fishes. Perhaps the most notable example is crossbreeding Platies with Swordtails. The Platy is a small fish, growing to no more than three inches in length and with an average lifespan of around three years. The Swordtail is around double the size and you can expect it to live for five years or more.
Whilst aquarists tend to claim the credit for crossbreeding different species of fishes, in fact, the fishes themselves are responsible for this successful mating.
In the wild, nature adapts to take advantage of its specific environment. For fishes, that environment may be a lake, a river, a pool or a combination of these. Rivers, by their nature, are long and relatively thin. They start at the source with a trickle of water and, as they meander along their course towards the sea, they tend to get wider and deeper. The terrain over which rivers flow may be smooth or rocky. The water may flow with a gentle, even current or may become a raging torrent as it flows rapidly downhill over rocks and boulders.
The depth of the water may have a major impact on the size of fish that can successfully inhabit that part of it.
Of the four main species of livebearers mentioned above, it is interesting that they are all part of the same family of fishes, the Poeciliidae. These four species all originate from Mexico, Central America and South America (and from some Caribbean islands). It is interesting, perhaps, that all of the African Poecillidae are, in fact, egg-layers.
When you drill down into the details of each species you will generally find that each one originates in different locations. such locations may be a specific region or even a specific part of a single river and this, I believe, gives us something of a clue as to why we have been so successful in crossbreeding some of the species and creating hybrids within single species.
To be successful, any species must adapt to the environment in which it finds itself. If it adapts to that environment then its chances of success over many generations increase dramatically. If it fails to adapt to its environment then the chances are that it will fall prey to predators or succumb to being unable to survive in its environment.
In an aquarium, the aquarist can control the environment to ensure the survival of a particular hybrid and for this reason, we see so many successful hybrid variants of the original river fish (and so few of the original river fish, perhaps).
Rapid evolution through short generation gaps
We tend to think that we humans don’t really change from one generation to the next. If we think this then we are dead wrong. Take a trip to any museum and look at the size of adult clothing from one or two centuries ago. You’ll generally find that adult clothing in the past would only fit a child now. With improved food production, better living conditions and far better healthcare we grow bigger and live longer. In general, boys always tend to grow up to be taller than their mothers. As a young man, at 75 inches in height, I was usually the tallest person in my age group. Despite the fact that I am still 75 inches tall I now find that I have to look up to many younger men who are significantly taller than me.
What applies to us applies to fishes.
Imagine a shoal of Platys born halfway along the length of a sub-tropical river. Bear in mind that an adult female Platy can bear a new generation every 30 days. The shoal splits into three smaller shoals. The middle shoal stays where it is. The left-hand shoal moves away upstream whilst the right-hand shoal moves away downstream.
Thirty days later, the three shoals are now some distance apart. The upstream and downstream shoals are now in very different environments from where they were just a month ago and very different environments from each other.
Whilst the middle shoal is already perfectly adapted to its environment, the left and right shoals have to fight to survive because they are no longer perfectly adapted to their new environment.
Many of the upstream and downstream shoals fall prey to predators but some survive and, 30 days later, another brood of young arrives. In any brood, some that are born are slightly different. The colouring or size might be slightly better adapted to the new environment and this adaptation means that the future young of these fishes may become successful in the new environment. Over time, the upstream and downstream shoals may evolve to become very different from the shoal that stayed where it was and even more different from each other.
Natural selection enables fishes that are well adapted to their environment to survive and those that are poorly adapted may well perish in that environment.
With humans, we think of a generation as being around twenty to twenty-five years – the average age at which a woman will have her first child. In our modern, Western world, with the attractions of career, many women now don’t have their first child until much later – and this is a recent change in our behaviour.
For the Platy, there can be twelve generations each year so over 25 years there can be 300 generations in the time it takes a human to have one new generation. Over 300 generations, the upstream and downstream shoals of Platies will probably not only have adapted perfectly to their new environments but also will have each divided into further upstream and downstream shoals at least once, maybe several times.
Gradually, a river will have different variations of Platy along its length, each adapted or adapting to the environment in which it finds itself but each quite different in one or more aspects from that original shoal in the middle of the river.
How do these adaptations help the aquarist?
It is interesting to note that a significant majority of aquarists don’t keep and/or breed the original livebearers found in their native habitat. This is because the tropical fishes available from stockists are captive-bred and on a comparatively industrial scale.
In the wild, natural predation (including cannibalism) will see the demise of all but a few fry (young fishes) from any brood so only the fittest (or luckiest) reach maturity and this determines the characteristics passed on to the next generation.
Breeding captive fishes enable the breeder to separate the mother from its brood (especially if the adult parent will eat it’s own young, as some do) and this, in turn, ensures that most, if not all, from the brood, survive to adulthood.
Typically and as a rough ratio, the mother will give birth to three females for each male. If the female bears 80 young (each month or so) then 60 of them will be female and the other 20 will be male.
The offspring Platies will be of breeding age themselves after around four months so from those original 60 female Platies, after five to six months, each of them could give birth to their first (monthly) brood of 80 Platies of which around 60 will be female. In other words, after around six months, from the original female Platy, almost 4,000 additional female Platies and 1,000 male Platies will be born.
Platies are an example of a tropical fish where the male and female generally look very similar; the female is larger than the male amongst all of these livebearers.
In the Guppy, Molly and Swordtail, in particular, the male is generally far more ornate than the female so when aquarists are selectively breeding for specific characteristics (such as tail shape, body colour or combinations of characteristics) the aquarist will be looking, particularly, for specific characteristics in the male.
With the Platy the differences in finnage are generally less obvious but colour varies enormously.
Simple, Mendelian genetics
In the above table, a single genetic characteristic (C) is shown. It doesn’t matter to what characteristic the table refers, as the principle applies to all genetic characteristics.
You will note that the male gene pair is CC – two dominant C genes – this is a random choice on my part.
You will further note that in this female the gene pair is Cc – the dominant C and the recessive c this is also a random choice on my part.
The table is a simple truth table where the possible outcomes of mating are (from top to bottom and left to right) CC, CC, Cc and Cc.
Imagine that the genetic characteristic is for a red-scaled fish and a white-scaled fish and that red scales are represented by the dominant gene (C) and white scales are represented by the recessive gene (c).
According to Mendelian genetics, using the above truth table and barring genetic mutations (which fall outside the scope of the truth table) then all offspring will have red scales because all possible combinations of the genes result in offspring which carry the dominant gene (C).
In the above table, both the male and female have both a dominant and recessive gene for scale colour. When you work out the truth table options for this mating you will see that the possible combinations are CC, Cc, cC and cc.
In this instance, the fourth possible combination is where the two recessive genes combine so there is a one in four chance of an offspring having white scales despite both parents having red scales – there is a three in four chance of each offspring having red scales because three of the genetic combinations have the dominant, red scale gene.
At this point, it is worth noting that chance could determine that no offspring have white scales or no offspring have red scales because chance has no memory and cannot communicate any permutations made.
In this example, the male has a double-recessive gene for scale colour whilst the female has a dominant and recessive gene. The male has while scales whilst the female has red scales. The truth table is now cC, cc, cC and cc which means that there is a fifty per cent chance of an offspring having white scales and a fifty per cent chance of an offspring having red scales.
Where both mates have double dominant or double recessive genes for a given characteristic then all offspring will be red-scaled or white-scaled respectively.
What causes genetic mutation?
In the above examples, no mutations have occurred. Mating pairs genes according to the rules of the truth table and only red-scaled and white scaled offspring are possible.
There are, however, several reasons why a genetic mutation occurs that can cause an outcome outside the scope of the truth table. For example, the process of copying a particular gene can be faulty. A chemical or environmental change can cause a variation in the copying of a gene, as can radiation of some type.
Such a mutation, in respect of this example (scale colour), could create either a new dominant or recessive gene which might be, for example, blue scales.
More often than not, gene mutations might not survive because the gene copying process may self-rectify. Sometimes, however, the mutation is permanent and the change is handed down through the generations.
In nature, the successful mutation generally takes the form of an evolutionary change that makes the (in this case) fish better adapted to its environment.
Returning to our example of the upstream shoal and the downstream shoal then it may be that the upstream shoal needs to adapt to faster flowing, shallower waters whilst the downstream shoal may need to adopt a darker colouring the better to hide from big predators.
Using this as an example, it may be that, at some point, a male from the upstream shoal mates with a female from the downstream shoal. Some of the offspring may be stronger swimmers, some may be darker, and some may be both darker and stronger swimmers.
When you look in detail at my posts for Guppies, Mollies, Swordtails and Platies you will see that there are many hybrids of every species.
In the world of commercial fish-breeders, aquarists can seek to breed to bring out specific characteristics. For example, the guppy in its natural environment is quite an unremarkable fish (and it survives because of this). In the aquarium, however, the guppy is bred for a rich rainbow of colours and a wide range of tail shapes (only in the male). Selective breeding permits aquarists to select fishes that demonstrate particular characteristics and breed them to accentuate and even augment these characteristics.
Livebearers for beginners and experts alike
The family of livebearers is a very large family of fishes but in the freshwater tropical aquarium is largely confined to the Guppy, the Molly, the Platy and the Swordtail.
The Guppy and Molly can interbreed but, so far as we know, all of the offspring are infertile males.
The Platy and the Swordtail can also interbreed and their offspring are usually entirely viable.
In general, these livebearers are excellent community fishes and are easy to keep so are perfect for novice aquarists.
There are now innumerable variants of all of the livebearers, selectively or accidentally bred by novice and experienced aquarists alike.
Some aquarists will only ever keep Guppies whilst others may keep a wide range of different fishes. There are no rules, only your own choices and you are free to make them and to change them as your hobby develops.