What Exactly Are Killifish?


     Perhaps the one question I get asked most often when I speak to people about my hobby is “What are killifish?”  Many are familiar with a variety of tropical fish ranging from guppies to angelfish, but they have never heard about killifish.  Well, actually that’s not always true.  There are a number of fishermen who know killifish are the baitfish (Fundulus heteroclitus) they net from the tributaries along the eastern coast of the United States.  But for the most part, even many general tropical fish hobbyists have very limited knowledge of killies.

     The common name ‘killifish’ used by hobbyists is considered by many to be based on the early Dutch settlers of the United States term for a small body of water ‘kil’ and ‘vissen’ which translates as ‘to fish’.  I have not been able to verify this term as viable, but “Kil’ actually means chilly, but in either form, in the modern world ‘kilvissen’ can easily be translated in “to fish in brooks’ or ‘chilly fishing’.  It’s not a stretch to find the word can represent the small fish in cold water – or killifish.  In the area, the Dutch settled, a very common species of North American killifish Fundulus heteroclitus – the mummichug – is found and it could very well have been used as a baitfish by those settlers.

     No matter what the truth of the name may be, killifish is the most common term used today to describe a worldwide group of tropical fish.  Scientifically, killifish are part of the family Cyprinodontidae – the egg-laying toothcarps.  This group also includes live-bearing toothcarps, perhaps the best known is the common guppy. 


What Makes Killifish Interesting?

     Of all of the different varieties of freshwater tropical fish found throughout the world, there is perhaps no other group – be it the popular cichlids or livebearers – that can match the diversity of killies.  The vast majority of killifish are small, brilliantly colored fish that can be maintained in small aquariums easily.  On top of the ease of care is also the unique and sometimes amazing ways killies reproduce.  Imagine simply adding a yarn mop to a small fish tank one day and the next lifting it out to find a number of hard-shelled eggs ready to be transferred to a small tray for hatching.  These eggs are so hard that the hobbyist can use his or her fingers to pick them from the mop to the hatching container.

     Or perhaps you might find nature’s most amazing survivors – the annuals – fascinating.  To survive in nature, these fish have developed the ability to produce eggs that sleep through long dry periods where they remain in hard soil baked to cracking by the sun.  When it rains again, they hatch and the next generation of killifish is born.  The individuals force their way through the water-soaked soil and begin to swim and feed.

     There are even species of killifish that leap together out of the water and lay their eggs on the top of water lilies or other plants to incubate and hatch or perhaps … in one case – a single fish is actually a pair (we’ll get into that later).

     Within each of the varied groups are individual species that have developed their own variations on reproduction.  In other words, there is so much variation in color, breeding, and behavior to keep a hobbyist entranced for a lifetime.

     No matter how unique or interesting killies may be, killifish are not common throughout the tropical fish hobby.


Why is this?

     The reality of killifish is fairly simple.  Because of their breeding habits, it is very difficult to establish them in the commercial industry.  Killifish do not lay large numbers of eggs at one time as do cichlids, tetras, or anabantoids.  Instead, the fish of this discussion lay anywhere from 0 to 20 (occasionally more) eggs on a daily basis.  If for no other reason than this, the commercial breeders cannot be bothered to pick eggs each and every day in order to produce enough fish to sell at a reasonable price.  While there are occasionally killifish offered through the distributors, those fish often come from a local breeder who provides them to the shippers.  This is the major reason we rarely see killifish in the local aquarium store.

     Even when it comes to commercially collecting them in the wild, it is rarely easy to capture large numbers of fish in a single area.  Instead, killifish tend to be found in brooks and streams or small pools of water, not large rivers or lakes.  Therefore the professional collectors rarely go in search of killifish unless there is a specific order which can fill a box or boxes with hundreds.  So once again the general aquarist rarely has access to the fish through their local aquarium store.

     Then there is the time-worn and disappointing problem with the information in the commercial industry.  Because the name “killie’ carries a negative connotation that these fish are “killers’, many local fish store operators have limited experience or knowledge about killies, thus the negative name scares them off and unfortunate misinformation sometimes occurs.  The truth of the matter is killies are no more or no less a ‘killer’ than any other group of fish.  There are very peaceful species and there are predators – but then again fish such as neon tetras and piranhas are both characins as are bellonasox and guppies both livebearers.  Unfortunately, the most likely killifish to be found in pet shops is either the Blue Gularis or the “Golden Wonder’ Aplocheilus lineatus.  Neither of these is a particularly nice fish and their presence simply adds to the myth that killies are ‘killers’.

     When added together, all of these factors make it very difficult for the new fish keeper to find killies locally.  That is why joining an aquarium society or a major national organization becomes the most direct route to find these attractive gems.  



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