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. 

Slimy Snails

Ever walked down a beach at the ocean and picked up sea shells?  These are the exoskeletons of a group of organisms known as Mollusks.  Mollusks are defined as soft-bodied invertebrate (no backbone) animals with a hard protective shell usually made of calcium carbonate (and slugs, which have no shell).  Most Mollusks live in oceans, but a few live in non-salty habitats, including ponds and damp environments like the forest floor.

Freshwater snails in our pond.

Snails are Mollusks with spiral shells, eyes on stalks that look like antennae, and a muscular foot for gliding along surfaces.  Here's a neat diagram of snail anatomy to get a sense of snails' body parts.  The pond at our outdoor classroom is teeming with snails!  If you sit and look into the pond for a minute, you'll start to notice black globs moving very, very slowly.  Pick one up - they don't bite - they really only eat plant material.  You now have a snail in your hand.  Say hello to your snail.  S/he will probably not be very happy to see you, which s/he will demonstrate by curling up into his/her shell.  S/he thinks you're going to eat him/her.

Snail recoiling into his/her shell for protection.

OK - this post is starting to get annoying!  What's with all the gender slashes?  Well, it turns out that most snails are both male and female, which is actually pretty neat. 

Snails breathe oxygen, just like we do.  Some use lungs and others use gills for breathing.  You'd think that land snails would use lungs and pond snails would use gills, but it's not that simple.  You'll find some of each kind both above and below the surface of the water.  You'll have to decide what you think about our snails' breathing method.  If they touch the surface every once in a while, they have lungs.  If they never come up, they are either dead or breathe using gills.  Since there are over 4000 species (types) of freshwater snails, and they are very difficult to tell apart, I cannot tell you what kinds we have.

Snail scaling the rocks behind our waterfall on a DIY trail of slime.

I know what you'd really like to ask is, "what's the deal with snail slime?"  Well, snails are gliders.  If you've ever tried to slide down a Slip 'N Slide without water, you know that you get stuck and it's no fun.  Well, snail slime works like water for a Slip 'N Slide, but much, much slower.  The snail constantly makes slime, slathers it across the surface of whatever it's gliding on, and makes itself a slippery trail.  Snails wave their muscles in their foot (the part of their body that contacts the ground) and push themselves along.  Whee!  If you place a snail on a piece of glass or clear plastic and watch from underneath, you can observe this phenomenon.  You may also see the snail's mouth as it drags along the glass trying to scrape up any bits of plant matter.  An interesting fact about snail slime is that you can buy a face cream made with it here.  I don't know why you would want to do such a thing, but if you really want to, it would be much cheaper just to let snails glide on your face.  In case you're not grossed out enough already, humans make a very similar substance in our bodies - mucus.  Mucus helps keep the insides of our mouths, lungs, stomachs and noses slippery so they don't get stuck together.

Blob of snail eggs on algae.

Snail populations work on a boom and bust cycle.  Where there is a lot of plant material available, such as decomposing leaves or algae, snails have enough food to produce a lot of offspring.  Our pond has lots of plant food available, so there are a lot of snails.  If you look carefully at a clump of algae from the pond, you may see a lump of clear gel balls - those are snail eggs.  When all the plant material is gone, the snails will mostly die off, but they will repopulate again when nutrients are available.  It's easy to keep snails in an aquarium, as long as you offer them some lettuce or algae from time to time.  Be careful though, if you leave too much lettuce in the tank, you will end up with hundreds of snails!

The most common type of snail in our pond.

Since our snails require fresh water to survive, and snails were not intentionally added to our pond, where do you think they came from?  There are two likely answers.  One is that snails or snail eggs were attached to the aquatic plants that were planted in our pond.  The other explanation is that snail eggs are sticky (from snail slime!), and they sometimes stick to birds' legs.  Any water birds that have visited our pond could bring or take snail eggs to or from our pond.

Another type of snail found in our pond, plus some neat clouds reflected in the water.