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Originated from WWM Digital Magazine: Vol. 2, Issue 3, Winter 2011

Related FAQs: 
Endler's Livebearer, Swordtails 1, Swordtails 2, Swordtail Identification, Swordtail Behavior, Swordtail Compatibility, Swordtail Selection, Swordtail Systems, Swordtail Feeding, Swordtail Disease, Swordtail Reproduction, & Livebearers in General, Poeciliids, Guppies, Platies, Swordtails, Mollies GoodeidsAnablepidsLivebearer Identification, Livebearer Behavior, Livebearer Compatibility, Livebearer Selection, Livebearer Systems, Livebearer Feeding, Livebearer Disease, Livebearer Reproduction,

Related Articles: Poeciliids: Guppies, Platies, Swordtails, Mollies by Neale Monks Mollies, Platies, Guppies, & Livebearing Fishes by Bob Fenner, Anablepids (Four-Eyes), Goodeids, Halfbeaks (Hemiramphrids) 

 Endler's Game

By  Rick Novy

Pic by Rick Novy

What kind of fish is that? It's a common question for a keeper of the Endler's Livebearer. Metallic reds, greens, yellows, and oranges patterned with jet-black streaks or spots make this fish one of the most colorful of all the freshwater species. The color itself would be enough to attract anyone with an interest in keeping freshwater fish, but look a bit deeper and you'll discover a history just as colorful. We'll touch on that rich history here just a bit, then we'll turn our attention toward keeping this fish.

Lost and Found

The Endler's livebearer (hereafter referred to as endlers) were initially discovered by Franklyn F. Bond in 1937 in the Cumana region of northeastern Venezuela. Specimens ended up in the University of Michigan's Museum of Zoology, but the discovery was never published and the fish never distributed.

It took until 1975 for the fish to be rediscovered, this time by Dr. John Endler. He provided samples to Donn Eric Rosen, a noted Poeciliid taxonomist. Had Rosen been able to classify the fish before he passed away, much of the controversy might have been avoided.

A Common Name

Rosen did provide some of these fish to another friend of Professor Endler, Klaus Kallman, who was then of the New York Aquarium. Kallman introduced this fish to the German aquarium community under the name Endler's livebearer.

He apparently intended the name as a surprise for Professor Endler. As it turns out, nobody told Endler, and he did not encounter the fish as his namesake until sometime in the early 1980s.

Official Classification

As the endler made its way through the aquarium community in Europe and eventually North America, a behind-the-scenes battle raged between those who considered the endler a separate species and those who considered it simply a unique strain of guppy. Detailing this controversy would require a separate article, but suffice it to say that the disagreements got quite heated.

Finally, in 2005, Dutch ichthyologist Fred Poeser and colleagues Michael Kempkes and Isaäc Isbrücker published a paper classifying the endler taxonomically for the first time as a unique species.

They named the fish Poecilia wingei, to honor Dr Ãjvind Winge, a Dutch geneticist who performed significant early genetic work with guppies. The classification was not without controversy. Two years later, Tropical Fish Hobbyist magazine ran an article by frequent contributor Ted Coletti adamantly refuting the classification.

While there remains considerable controversy among experts, the aquarium community at large seems to have embraced the classification P. wingei.

But Are They Really Endlers?

One factor that complicated classification was the ability of the endler to breed with the guppy. Cross-breeding is so rampant that almost all enders found in retail are hybridized with guppies.

A sample of class N Endler's livebearer strains. A. Wild stock black bar. B. Wild stock peacock C: Fire red flame tail D. Fire red peacock. E: A typical female F. Lime green. Photos by AdrianHD Swamp River Aquatics.


In fact, hybridization is such a problem in endlers that John Endler, along with several important breeders, collectors, and avid hobbyists took the unusual step of creating a special classification system. This system is used on auction sites and in local clubs, and is presented here so the reader will be able to interpret the classifications if encountered.

Class N: Pure endlers directly descended from and fully traceable to fish collected in the Cumana region of Venezuela.

Class P: Any endler thought to be pure but lacking the requisite traceability for the class N category.

Class K: Any endler hybridized with any other livebearer species (most often guppies).

True or False?

None of this should be take to mean that there is anything wrong with endler hybrids. As long as the hybridization is done responsibly and the fish are not represented as pure endlers, hybrids can make attractive additions to a community aquarium.

Unfortunately, this is often not the case. These fish are commonly passed along to the general public under the name endler or Endler's livebearer. Close inspection by a knowledgeable eye reveals these fish to be hybrids. To the casual aquarist who takes a label at face value, a disservice has been done. Compounding the problem is that in many cases, the retailer is completely ignorant of the misrepresentation.

The main problem is that most endler hybrids available in big box or local fish stores look an awful lot like true endlers. In some cases the differences are so subtle that a trained eye is needed to distinguish a hybrid from a true endler.

How Can You Tell?

Some endler hybrids look so much like a true endler that it's nearly impossible to distinguish them. The world's most prominent breeder of the endler and of specialty strains of endler-guppy hybrids goes by the trade name AdrianHD. He has identified a number of clues to distinguish between a true endler and a hybrid.

Top image: A female Endler's Livebearer. Bottom: A gravid female guppy. The Endler's has a gold colouration while the guppy is more silver. Note also that the Endler's caudal peduncle is much narrower than that of the guppy. Photos by AdrianHD Swamp River Aquatics.

The coloration of endler males makes the job of distinguishing them from hybrids far easier than with the plain females. Black in endlers is very dark, and all other colors are metallic.

One theory behind the metallic coloration is that these fish lived in algae-filled green water, and the metallic flecks allowed females to see males. For the aquarist, it helps with identification of hybrids. If a fish displays any pastel color, it is hybrid.

Other clues are more subtle. The caudal peduncle (tail muscle area) of the true endler is narrow and elegant. The same area on many hybrids resemble that of the guppy, which is thick and meaty.

Hybrid fish often display swords on the top, bottom, or both sides of the caudal fin. While true endlers can display very short swords, these mostly consist of coloration within the caudal fin against transparent areas. These swords do not extend more than a millimeter or two past the end of the caudal fin. A sword that extends much beyond that is a sure sign of a hybrid fish.

The aquarist must be cautious if white appears on a male. Some strains of true endlers do occasionally display white, but white will never occur in some strains, such as the black bar. Knowledge of the strain in question is required to use white as a clue to detect a hybrid.

Endler X Guppy hybrids when bred in a controlled manner, can produce some stricking patterns. the top photo is a hybrid called New Red Chest Tux, the middle and bottom photos are two variations of a hybrid called Tiger Endlers. Photos by AdrianHD Swamp River Aquatics.

What About Females?

Distinguishing a true endler female from a hybrid is considerably more difficult than with males. A true endler female has a rich golden color. A guppy female is more silver in color. Distinguishing pure female endlers from guppies is easy. Unfortunately, a hybrid might be either color, or somewhere in between. That means that other clues are needed.

As with the male, the caudal peduncle of the female true endler will be narrow. Along with the silver coloration, a thick peduncle is the best direct indicator of a hybrid female endler.

Individuals from any given hybrid litter can have considerable variation. In some cases, a hybrid fish can look so much like a pure endler that the only way to distinguish it as a hybrid is through characteristics of male siblings.

Handling Hybrids Responsibly

The pervasiveness of hybrid endlers in the industry is so extreme that unless the shopkeeper can trace the fish to a known keeper of class-N fish, any endlers for sale must be assumed hybrid, or at best considered class P.

That isn't necessarily a problem. For the casual aquarist, hybrid endlers make great pets. However, keep in mind that under proper conditions any livebearer will reproduce at a surprising rate. Sometimes these offspring are given or sold to other aquarists. It is critically important to represent these fish as the hybrids they are, and not as pure endlers unless they meet the strict class N requirements.

This responsibility must be taken seriously to prevent unintentional further dilution of the P. wingei species. The fact that hybrids are advertised as endlers in the aquarium trade greatly increases the odds that hybrids will unintentionally be further crossbred with pure animals.

The Good Side of Hybrids

Hybrid endlers aren't necessarily all bad. While endlers will cross-breed with fancy guppies, these hybridized fish can display an amazing variety of interesting colors and patterns.

The world's most important breeder of the Endler's livebearer, AdrianHD, has responsibly developed several very special hybrid strains. By keeping the hybrid fish well-separated from his class-N populations, AdrianHD has line bred such strains as the tiger endler and the snakeskin endler. These are gorgeous fish that combine the most attractive aspects of the endler and fancy guppies.

Most carefully-developed hybrid endler strains are harder to find than a simple visit to the local fish store. Usually, they must be acquired through other hobbyists, fish clubs, or online auctions. They can also, of course, be purchased directly from AdrianHD through his Swamp River Aquatics business.

What About Purists?

Hybrids are one thing, but the avid aquarist most likely wants to have 'the real thing.' The question is where to find class N endlers. While occasionally somebody might bring a litter of pure endlers into the local fish store (as this author has), it doesn't happen very often. More likely, a local aquarist might bring a small collection of juveniles to a club auction.

If class-N endlers can't be found locally, the only other practical solution is mail-order. When done properly, shipping fish works surprisingly well for not only endlers, but for most freshwater species. Auction sites like Aquabid often list endlers, but the bidder must perform due diligence. Ask the seller about the lineage of the fish in question.

The best way to ensure the fish purchased are truly class-N endlers is to go directly to a known breeder or keeper of them. AdrianHD often sells pure endlers directly and through Aquabid. He also keeps a website called Endlers-usa.com. On this website you can find a list of known keepers and breeders of class-N endlers with contact information and sorted by strain. It is also a good place to network with other endler enthusiasts.


One row of the enormous fishroom where AdrianHD breeds and maintains class N Endler's Livebearers, specialty endler hybrids, and other livebearers. Photo by AdrianHD Swamp River Aquatics.

Class-N Strains

Developing hybrid endler strains is not AdrianHD's main focus. He is dedicated to the preservation and distribution of the species. He has thousands of gallons dedicated to class-N endlers, maintaining genetically diverse wild stock as well as line breeding special strains. He breeds a number of other livebearers as well.

Probably the most well-known strain of ender is the black bar. The males display a jet-black comma over the ribs, making a sharp contrast with the metallic coloration. Another well-known strain is the peacock. This strain has the jet-black color on the body, but not as consistent in shape as the black bar. However, there is always a black spot on the caudal fin that resembles the spots on the tail-feathers of a peacock bird. Other strains developed by AdrianHD include lime green and fire red flame tail.

Fishkeeping Conditions

Like most commonly available livebearers, endlers enjoy warm water. They are very comfortable at 82°F (28C), and may be able to tolerate brief periods of time in water as warm as 94°F (34C).

The endler is an excellent choice a for hard water aquarium. They prefer hard water and planted tanks. Fry will hide in floating plants, but will also hide in dense areas of plants on the substrate. Endlers of all ages will graze algae from the surface of leaves.

It is difficult to think of a more peaceful fish than the endler. They make great tank mates for other peaceful fish provided sizes are compatible.

Of course, the conscientious aquarist will ensure diversity to avoid genetic defects caused by excessive inbreeding.


Endlers display some of the most entertaining mating behavior of any freshwater fish. Males will strategically position themselves in front of a female, extend their fins to make the largest display their colors, then perform a shimmying dance.

As a rule, the females are not impressed. In response to this display, she seems to roll her eyes and swim away. The male, however, is not easily discouraged. He will reposition himself in front of the female and repeat the display.

Eventually, the female has enough and darts away. At this point, the male often realizes his display isn't working and resorts to hit-and-run mating. The female can usually maneuver through plants and eventually lose the male, who then looks for another female and starts again.

As a token to the persistence of the male endler, they breed like, well, guppies. Like their more familiar cousins, endler females will drop a litter of fry, then store sperm and continue to drop fry for upward of six months.


The very fact that females can store sperm makes it imperative to follow good fishkeeping practices. Never keep endlers and guppies in the same tank. Breeding of class-N endlers should be done with virgin females to ensure the offspring continue to be class-N.

Even if indifferent about the purity of endlers in their own tank, the fish keeper should never pass hybrid fish to others as pure endlers. The native habitat of the endler (and all other livebearers in the Cumana region) is in danger due to human activity and invasive cichlid species. That makes it especially critical that the aquarium community keep these fish responsibly, because one day it may be the only place to find them.


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