written spring 1995

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(see the fish anatomy information if you need a refresher.)

The western mosquitofish, Gambusia affinis, is one of the easiest to keep livebearers I have had experience with. It is in fact, one of the easiest fishes I have found to keep.

Gambusia affinis is no longer considered the same species as the eastern mosquitofish, Gambusia holbrooki, which occurs east of the natural range of G. affinis.

These two fishes, the western and eastern mosquitofishes, are some of the most studied fishes. The papers written on their reproduction alone would fill several file cabinets (I've seen a couple cabinets filled with such papers). Despite this, they are probably poorly known to most people in the aquarium hobby.

I was first introduced to live G. affinis in California when my fifth grade class took a field trip to the local creek to collect some animals, plants and "creek stuff" for a class aquarium project. I was already keen on livebearers, and had kept guppies and swordtails for several years. I was able to quickly identify them and pick out the pregnant females and could show off in knowing the name of the gonopodium. I think even my teacher had to look that one up.

Gambusia affinis has been introduced around the world for mosquito control, hence the common name, which it shares with at least five other fish species, including the guppy. Unfortunately G. affinis is not as good at eating mosquitoes as the fishes it tends to replace in those new locations. Mosquitofish do tend to replace native fishes where it is introduced, but probably through competition for food, but through aggression.

Gambusia affinis are aggressive fry eaters, and they may feed on the unprepared fry of their neighbors, as well as pick up a few mosquito larvae or eggs.

The current natural range of G. affinis is from about the eastern border of Mississippi west to eastern Texas. West of the Mississippi River, the range extends at least into southern Missouri. There are also isolated populations of G. affinis in the headwaters of many of the major eastern rivers, at least as far north as South Carolina. This has led some researchers to conclude that G. holbrooki was introduced by native peoples into the range of G. affinis which was then replaced. This could be a case of "turn about is fair play."

I collected my stock of mosquitofish in a small lake near my home in northcentral Mississippi, where they were apparently introduced sometime ago for pest control. They are probably native to the area, but this is a fairly new, man-made impoundment.

As I mentioned above, mosquitofish are rather aggressive, and not just toward other fishes, but also toward each other. Quite often the fins turn up ragged. Probably in part due to their being extremely well adapted to a rough life, however, I have never seen an infection on the torn fins, unlike the other fishes I have kept.

These fish are extremely hardy. I once had to keep a small pair in a paper cup for a couple hours during transport, and they did just fine. I didn't even notice the fish gulping at the surface for oxygen, though they might have.

The mosquitofish is a surprisingly pretty fish, despite it's nasty reputation and unassuming name. Overall the body is "guppy grey", with each scale edged in black. If you look close, you can see the striking pattern which looks like a tiny piece of black lace on a silver fish.

The basic body form of the female is very like the wild-type female guppy, though they can be told apart without too much practice. The male is much slimmer than the male wild-type guppy, and the gonopodium is much more evident. There are no "fancy"colors in this species.

Since they are native to the area I live in, I keep my mosquitofish at room temperature, which is about 760F in the summer, and 720F in winter. At the moment I have three fish. One female collected in the spring of 1994 is now about 1 1/2 inches long, Though I have seen females easily two inches long. Mine big female has given birth at least three times to about twenty young each time. The young were eaten before I could get them out of their mother's tank, so I have not been successful in raising them. The fry were about 1/4 of an inch long at birth, so can probably eat anything swordtail fry can eat.

After having her for about six months, she no longer appeared to be gravid, and has not been noticed to give birth for at least two months.

Because I do want to raise the fry, I have recently collected two more fish (I'm starting small). One is a young male about 3/4 of an inch long that is just maturing - his gonopodium is not yet completely formed. The other is a second female, slightly larger than the male.

Though I've now had both the newcomers for three weeks, I see no signs that either female is gravid yet.

The three fish are housed in a gallon desk-top aquarium with an airstone and weekly partial water changes. Even in this small space, I have noted that the fish have divided up the space into three territories. The largest fish, of course, gets the surface area with first chance at any food coming in.

They all eat quite well, and greedily, anything they are offered. I have fed them several brands of live food, though they do seem to have favorites - the ones with animal protein. They don't go after the plant based formulas quite as fast (though it may be my imagination). They also eat frozen brine shrimp with great glee. Though I have made some effort, I have never been able to fill their stomachs unless I'm feeding brine shrimp, but that seems true of my swordtails as well.

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Original content copyright 1995-2007 Robert B. Hole, Jr. All Rights Reserved