All About Mosquitofish

Our pond is full of Eastern mosquitofish (scientific name Gambusia holbrooki).  They were introduced to the pond to help eat aquatic mosquito larvae in order to reduce the number of mosquitoes in our outdoor classroom.  The mosquitofish are definitely helping to limit mosquitoes, but we'll have to convince the mosquitofish to jump out of the pond and eat mosquito larvae that grow in little pockets of water in mulch, soil, tree bark and other places too.  Very soon, we will have no mosquitoes, since night-time temperatures are getting colder, and adult mosquitoes will die off for the winter.

Hello there!  A mosquitofish says 'hi' with its pectoral fin.

Mosquitofish are generalist feeders, meaning they eat all kinds of things.  They eat algae, snail eggs, mosquito larvae, other insects, and even each other!  These little fish are good survivors, since they can eat almost anything.  Interestingly, generalist feeders in the animal world tend to be more intelligent that animals that only eat one kind of thing.  Generalists' brains must be more flexible and contain more information to remember all the different kinds of things that qualify as food.

Fish are not the brightest bulbs in the Animal Kingdom, but they do have some neat behaviors.  A neat behavior in our fish is that they respond to above-water movement.  Notice what they do if you move close to the edge of the pond.  Then sit perfectly still and quiet for two minutes and notice the fish moving back into the open.  Then wave your arms and observe again.  Why do you think the fish exhibit this behavior?  Isn't it fun to interact with a fish?  Hiding in response to moving above-water things is not a behavior fish learn, which makes sense, if you think about what would have to happen for the fish to learn this first-hand (first-fin?).  Mosquitofish are born with this behavior programmed into their DNA - it is an innate behavior.  DNA is the chemical in all organisms' cells that instructs the cells how to build the organism.  Your DNA contains instructions for things like your hair color, face shape, and possibly some behaviors, though scientists are not yet sure about how many human behaviors are innate. 

Mosquitofish with some tail fin damage, likely from hungry mosquitofish 'friends'.

If you catch a mosquitofish or two with a net and put them in a glass bowl, you can observe their fins.  All mosquitofish have a dorsal fin on their back, a tail fin, two pectoral fins (see the top picture), anal fins on their lower surface, and pelvic fins in front of the anal fins.  See labeled fish fins here.  Male mosquitofish have pointy pelvic fins, which you can see in the picture below.  Females have larger, rounded pelvic fins.  Usually females are bigger than the males.  They are more likely to eat any type of food, they grow faster, and they spend lots of their bodies' energy on producing offspring.  Next time you are at the pond, find both male and female mosquitofish by comparing the body size of the fish.

Male mosquitofish revealing its pointy pelvic fin.  Females are larger and have a rounded pelvic fin.

Mosquitofish are live-bearers, meaning they give birth to live fish instead of laying eggs.  Female mosquitofish can give birth to up to 9 broods of offspring each summer, with up to 100 baby fish in each brood!  No wonder we have so many mosquitofish in our pond! 

Can you find the two mosquitofish swimming above the algae?

Mosquitofish are incredibly tolerant organisms, and they will live just fine in almost any reasonable conditions.  This means mosquitofish are great organisms to bring back to the classroom for a day or more to observe closely.  You can keep them in a glass or plastic container with 3-4 inches of water and feed them a tiny bit of fish food.  If you only want to keep them for a day or two, you can even feed them a variety of tiny bits of people food.  If you keep them a longer time, change out about half the tank water every couple of days.  With fish in the classroom, you can look for male and female fish, observe other behaviors or figure out what they like to eat.  You could also investigate the hiding response to moving things.  You could see if they are equally scared of light or dark objects.  You could see how long it takes for them to come out from under a leaf after being scared.  You could see if they hide less after seeing the same object ten times - maybe they learn that some moving things are not harmful!  If that is the case, you would be able to say you taught a fish!

Now that we've seen several organisms in our outdoor classroom, it might be interesting to start a food web of our ecosystem on a large piece of butcher paper.  A food web shows organisms with arrows showing who eats what.  The arrows go from food to consumer, representing transfer of energy and matter from the food into the eater.  So far, we can put algae, snails, dragonflies, bees, mosquitoes, mosquitofish, squirrels and walnut trees on a food web, and we will be able to add many more as the year progresses.  Here's what a small food web looks like.