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. 

In Praise of Black Walnuts

You might want to wear a hard hat next time you go to the outdoor classroom.  There is a black walnut tree in the back corner, and it's walnut season!  If you sit there long enough, you might just witness the loud thud of a walnut hitting the ground.

Here's our black walnut tree.

Black walnuts (Juglans nigra, meaning black Jupiter nut) are beautiful trees.  I like them because they provide light shade (not too dark - their leaves are sparse and let the light through), give us delicious nuts, provide food for squirrels and are an excellent source of wood.  The dark walnut wood is so beautiful, strong and light that a full-grown walnut tree can be worth $5000 or more if it is cut down and sold for wood.  Plant a few walnut trees today and harvest them in 30 years for an excellent return on your investment.  Plus you can eat the walnuts in the mean time!  Just be sure to re-plant what you harvest.

Whole in-shell walnuts are available  in grocery stores this time of year.  They are a great snack, and since they take a while to crack open, you can't spoil your dinner with them.  The grocery store walnuts are English walnuts and ours a black walnuts, but our walnuts are still very good to eat.  In fact, their strong wal-nutty flavor makes them superior for adding to ice cream, fudge and cakes.  You just have to catch them before the animals do.

A black walnut with the husk still on.

Walnuts grow with a husk on them that makes them look like big round limes.  The husk contains a chemical called juglone that makes it smell rather pungent and stain your hands brown if you touch it too much (it won't hurt you, but you can't wash it off).  The juglone in the walnut husks also seeps into the ground under walnut trees and prevents some plants from growing, which helps prevent walnut trees from having too many competitors for sunlight and soil nutrients.  This plant warfare technique is called allelopathy, and it's fairly common among plants to have secret chemical wars going on in the soil.  Smell a walnut husk and notice the bitter juglone smell.  You can also smell the same odor in crushed walnut leaves.  Every fall, the walnut leaves fall and add a new dose of juglone to the soil.  Juglone is also a potent dye and is used by dye-makers to color fabric.  You could try this out by soaking a cloth in crushed walnut husks and water.

Black walnut with squirrel teeth marks in the husk - I bet that tasted awful!

My dad taught me a special technique for removing walnut husks: put a bunch of walnuts in your driveway and drive back and forth on them.  You should be left with a bunch of crumbled husks and intact walnuts.  You could also just wait for the husk to turn black and rot away, but by that time your walnut has probably become infested with worms or fungi.

Chewed hole in a walnut shell with the entire nut removed by a squirrel.

After you remove a walnut husk, the next challenge is to open the walnut shell.  Black walnuts have stronger shells than English walnuts.  My dad's trick for walnut shells is to wrap several walnuts in an old towel and bash them with a hammer.  Very fun.  Then you can use tweezers and a nut pick to pick out the walnut pieces from the shells.  It's a messy business, but totally worth it.

Whole walnut with husk, walnut with husk partially chewed, walnut without husk, and opened walnut.

Search around the bottom of our walnut tree and look for walnuts with the husk on, with a rotted husk, with the husk gone and with the shell open.  Look closely and you will find walnuts with teeth marks from the industrious squirrels that like walnuts for the same reasons we do.  Squirrels have gigantic, orange chisel-like teeth that you can see here.  They scrape their teeth through the husk (ick!) and the shell, then dig out the walnut with their teeth, tongue and claws.  If there are enough new green walnuts, you might want to take one with you and open it yourself - they are delicious!

Black walnut tree.  If you click on the picture, you can zoom in a see clusters of walnuts.

Walnut trees do not grow walnuts in order to provide us with snacks.  The walnuts are actually the offspring of the walnut trees.  The nut part is actually a seed.  If you put a whole in-shell walnut in the ground this fall, it will sprout and grow a new walnut tree in the spring.  The food value of the walnut - the part we like to eat - provides food for the new walnut tree until it can grow enough leaves to use the sun for food.

Here's more information on harvesting black walnuts if you'd like to do a more serious walnut harvest.

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.

Drinking Bees

The waterfall at our outdoor classroom attracts more than just people.  It's the ideal place for some insects to stop for a drink.  There must be a honeybee hive somewhere near the school, because our waterfall has become a bee watering hole!

Honeybee drinking water on an algae-coated rock.

Honeybees are my #1 top favorite insect, which is ironic since I was scared of them as a child.  Once I learned about them, I grew to love them best of all.  Not only do honeybees make honey to feed their offspring and feed themselves, but while they are collecting nectar to make the honey, they pollinate flowers.  Most of our agricultural crops need to be pollinated by insects, and honeybees are the best pollinators.  Honeybees are so helpful to humans that farmers often have honeybee hives on their farms to make sure their crops are pollinated.  If you like nuts, berries, apples, cherries, squash, tomatoes or avocados, thank the bees for making them possible.  I love to think of our pond as helping the bees.

Honeybees drinking on our waterfall.

We've all seen bees pollinating flowers, but I bet few of us have been able to watch honeybees drinking.  A good, clean water resource is incredibly valuable to bees, especially in the city where water is hard to find.  Bee keepers make sure their bees have access to clean water, or they provide water dishes to their bees.  Bees, like all creatures, need to drink water.  The bees use the water to make the fluids in their bodies, and they also sweat like we do to keep their bodies cool.  Bees carry water back to their hives to help keep the hives cool and to dilute the honey to feed their larvae.  Bees fan wet surfaces in their hives with their wings, causing the water to evaporate, which cools the wet surface and the hive.  You can observe this phenomenon if you dangle a wet towel in front of a fan. 

The top level of wet rocks is safest for bees to drink from.

Water is a tricky substance for small creatures like bees to deal with.  It might be strange to think about, but water is sticky to most surfaces.  In fact it's so sticky, that you have to use a towel to get it off of yourself after you shower.  The more surface something has, the more water sticks to it.  Think of how difficult it is to dry your hair, which has millions of surfaces all packed together, compared to drying your skin, which is one surface.  The stickiness of water is both helpful and dangerous to bees.

The good part about the stickiness of water for bees is that it's easy for bee mouth parts to soak up water.  Bees have a feathery tongue that sticks to water like hair does.  The bee just has to touch its tongue to water, and water wicks into it.  You can see the phenomenon of wicking if you touch the edge of a paper towel to water and watch how the water climbs further on to the paper towel.  Paper towels that are thicker with more microscopic fibers to provide more surface wick better than thin, smooth paper towels.  Bee's feathery tongues have lots of surface and are good wickers.   You can see a bee tongue sipping nectar on the flower from my garden in the picture below, but the picture is not magnified enough to tell that the tongue is feathery.

Honeybee soaking up nectar and pollinating chive flowers.  Note battered wing and pollen sack.

The dangerous part about water's stickiness is that bees' bodies can easily get stuck to open water.  If a bee lands on the surface of the pond, its body will stick and it won't be strong enough to get unstuck from the water.  The bee will likely drown unless someone scoops it out and sets it on dry land (careful - it will likely be stressed and possibly sting if you use your hand to do this).  To protect themselves, bees must stand on hard surface and drink from water that has seeped onto the surface.  Leaves and sticks on the surface of a pond are good platforms for bees to drink from.  The rocks in our waterfall are perfect surfaces from which bees can safely soak up water.

What is a Leaf Skeleton?

Leaf skeleton floating just below the surface in our pond.

For some reason, floating leaf skeletons are most visible on gloomy days.  Perhaps the filtered sunlight doesn't glare as much off the surface of the pond, and the light-colored skeletons stand out against the dark background of the pond depths.  Needless to say, leaf skeletons are strikingly beautiful, and they are lovely to find on a gray day.

Leaves ranging from living to dead floating in our pond.  The dead ones are starting to decompose.

The pond in our outdoor classroom has many tall trees towering over it.  As summer turns into fall, it is going to fill up with leaves, and we will hopefully have lots more floating skeletons.  Ponds provide the perfect habitat for production of leaf skeletons, because they allow for fast decomposition of soft plant tissues.  Aquatic conditions provide lots of snails, insects and microscopic organisms that attack a leaf as soon as it falls, since a newly fallen leaf has lots of nutrients to feed pond organisms.  The organisms eat the softest tissues and leave the leaf veins behind, allowing us to marvel at an important plant tissue.

Leaf skeleton from a hackberry leaf.

Leaf veins are a part of the transportation system within the plant.  The veins in leaves connect through the leaf stems into the main plant stems and all the way down to the roots.  Veins in plants are network of tubes for moving necessary substances from anywhere in the plant to any other part of the plant.  My mind can't help but compare leaf veins to our system of roads.  All our houses are connected to them, and we can use roads to get anywhere else.  I also think of our system of blood vessels - a similar network of tubes for moving necessary nutrients from one place to another within our bodies.

Leaf skeleton showing network of veins.

Plant vascular systems (vein systems) contain two types of tubes inside each vein.  One type of tube, called xylem (pronouned zy-lum), is rigid and only runs in one direction.  Xylem carries water and nutrients from the roots in the ground up to the leaves.  The second type of tube is called phloem (pronounced flow-um).  Phloem tubes are soft and flimsy, and they carry sugar, the product of photosynthesis, to wherever energy is needed in the plant.  Phloem tubes run in two directions.

Leaf veins in living leaves.

Leaf skeletons can be preserved by drying them pressed between newspaper or paper towels.  Their patterns can be revealed by covering them with paper and making a rubbing.  Different types of leaves have different patterns of leaf veins - highly branched, long parallel lines or radiating out from a center.  All three of these patterns of leaf veins can be found at our outdoor classroom.  Another fun trick for learning about the function of plant veins is to place a white carnation in water with food coloring.  After a few hours to a day, the xylem will carry the food coloring up to the petals.  The same thing would work in a leaf, but the green color of living leaves would make it difficult to see the food coloring.

Invasion of the Blue Dasher Dragonflies

Blue dasher dragonfly.  Are those aviator sunglasses?

The outdoor classroom and back field of our school has been a-buzz with these miniature blue helicopter-like insects.  They are a type of dragonfly called blue dashers (scientific name Pachydiplax longipennis, which means double-thick long wings, by the way), and they are beautiful!  The one above is an adult male, which you can tell from the bluish color of his abdomen.  Females and young adult males have brownish-black abdomens.  Both males and females have brown and yellow wavy stripes on their mid-section, or thorax.  Blue dashers all have a white head with bluish-green eyes.  In the picture above, the white patch on the head is the upper and lower mandible, which the dragonfly uses to bite its prey.

Dragonfly eyes are amazing.  They cover most of the head and provide the dragonfly with a 360 degree view of their world.  Dragonflies need excellent vision of their surroundings because they can fly and maneuver so quickly.  If they couldn't see in all directions, they'd constantly be smashing into things.  Dragonflies fly quickly because they need to catch their prey, which consists of insects that can also fly.  Blue dashers specialize in mosquitoes, gnats and flies, but they also eat butterflies and grasshoppers.  

Blue dasher demonstrating its helicopter shape, with a long abdomen to balance the head.

Since dragonflies are so big and fast, sometimes people are afraid of them.  The good news is that dragonflies are harmless to humans.  They do not bite or attack humans, and the long, pointy abdomen does not sting.  The shape of the abdomen serves as a counter-weight to the head, keeping the body of the insect level as it is suspended by its wings in flight.  If you look at the picture above, you can see how the head and tail appear to balance each other out under the wings.  Blue dashers are particularly useful on the school grounds because they keep other biting insects away from the kids' outdoor areas.

All dragonflies lay their eggs in water, and we probably have blue dasher nymphs in our pond at school.  After being deposited in the water, dragonfly eggs hatch into nymphs (underwater larvae), which grow and eat other aquatic insects and tiny fish until they are big enough to metamorphose into adults.  When a dragonfly becomes an adult, it crawls out of the water onto a stem or rock, splits its exoskeleton, and drags itself out of the shed skin as a fully-formed adult.  Blue dasher nymphs are particularly tolerant of poor water conditions, which makes them ideal for living in an urban environment.  They are widespread around the United States and in southern Canada in all variety of habitats. 

Dragonfly nymph exoskeleton. (Photo: Mary Entrekin Agee)

Green Algae

The school where I work has an outdoor classroom that invites nature into our urban schoolyard and creates a peaceful outdoor space for students to learn about life.  We have trees, wildflowers, and a pond, each hosting their share of associated organisms.  I'll be doing some nature blogging about the outdoor classroom for use by teachers at our school, and I'm posting the first outdoor classroom blog post here:

Welcome!  I'm excited about starting an outdoor classroom blog.  I can't wait to see what we find in our little urban nature oasis.  The subject of our first post is green algae.

If you look under the surface of the pond, you see a very busy ecosystem indeed!  There are plants, fish and insects, and those are just the visible organisms.  There are way more microscopic organisms than big ones, which I'll save for a future post.

Green algae growing just below the surface of our pond.

The strangest macroscopic (big enough to see) organism is the filamentous green alga that forms clouds of soggy green cotton candy.  But what on earth are green algae?  They seem a lot like plants: they photosynthesize, they have cell walls, and they have chloroplasts (green structures that photosynthesize).  There are also some features of green algae that we don't usually associate with plants: they live entirely under water; they don't form roots, leaves or stems; and some of the microscopic ones can swim!  Scientists have wavered a bit about whether algae are actually plants or protists.  The discovery of how to sequence DNA has allowed algal geneticists to confidently classify green algae as plants, though the other colors of algae (red, brown and blue-green) are not classified as plants.  I have had to re-learn my green algae taxonomy!

Interconnected strands of green algae pulled up from under the surface.

Our green alga (alga, hard g, is singular, and algae, soft g, is plural) is a filamentous type, meaning it grows in long strands.  The filaments (strands) of green algae are only one-cell thick, which is surprising considering how tough they are.  Each algal cell is cylindrical like a soup can, and the filament is arranged like an infinite strand of soup cans glued end to end.

Green algae out of the water.

The color of green algae comes from the pigment chlorophyll, just like in other plants.  Chlorophyll is the molecule that can catch light to allow plants to use its energy to build food.  Green algae cells contain structures called chloroplasts that hold the chlorophyll plus all the other machinery needed to conduct photosynthesis.  All plants' cells contain chloroplasts.

I looked at the algae from our pond using a microscope that magnified what I saw by a factor of 100.  In the picture below, you can see the cell wall between adjacent algal cells, just above the pointer.  Cell walls are rigid structures made of cellulose (a strong, rigid molecule), and they give plant cells their shape.  Paper is made by starting with plant material and getting rid of everything but the cellulose, meaning that paper is essentially squished, dried plant cell walls.

The pointer rests on a filament of green algae.  A broken alga releases its cell contents. 100x

Another interesting thing in the picture above is the broken cell.  A chloroplast is slipping out of the broken plant cell in the center of the picture.

Below is another view under the microscope, with one normal filament and one filament with shrunken cell contents.  The chloroplasts and other structures have been compressed into a central structure in each cell.  The strange filament may be undergoing reproduction or it could be stressed, but either way, you can see the beautiful cylindrical shape of each individual cell.

Shrunken cell contents in the lower filament allow you to see the cell walls.  The round object is an air bubble. 100x

In our pond, the algae seem to be growing quickly.  There is ample sun for food, plus decomposing leaves and insect/fish excrement providing nutrients, the equivalents of vitamins in our food.  When you see a pond with lots of algae in it, you can assume that there are a lot of nutrients in the water, either from natural sources or from pollution.