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. 

Rethinking the Canada Goose

Don't say Canadian - it's Canada.  The Canada goose (Branta canadensis) is a common sight in Lincoln Park, and I got to know their behaviors a little better as I watched them raise their young this spring.  Despite what some people say, they're really quite enjoyable neighbors.

Canada geese pair and young in Lincoln Park.

The geese started hatching in early May this year, and within a day or two, they were out of the nest walking and swimming and finding food.  Adult geese mate for life, with the average goose living around 24 years of age in the wild.  They tend and defend their young, but they do not feed them.  The goslings feed themselves as soon as they are out of the nest. 

Dabbling geese with a duck observing them, probably finding their upended tails completely hilarious.

Geese eat muck from the bottom of ponds, as you can see them doing in the photo above.  The tails-up bobbing for scum is called dabbling, and it never fails to crack me up.  Geese also eat grass, grains, berries and other plants, with the occasional bug thrown in.  Herbivores in general must eat a lot more volume of food to survive, and geese seem to eat constantly.  Such a large volume of food results in respectable quantities of goose poop, which people sometimes find annoying.  Admittedly, there are some beach areas in the US with hundreds of Canada geese where I would not want to walk barefoot or swim, but these are not the norm.  A little bit of goose poop is certainly better than a little bit of dog poop, since carnivore poop usually contains more harmful disease bacteria than herbivore poop.

Many of the geese in Lincoln Park probably flew north to Chicago earlier in the spring from the southern US or Mexico.  More Canada geese are migratory than non-migratory, though some members of the species have begun to live in the same location year-round.  The newer, non-migratory geese are an evolutionary response to the recent changes in their habitat: in the past 100 years, permanent open grassy areas with maintained, predator-free ponds have appeared everywhere.  Indeed, golf courses, airports, office complexes and neighborhoods provide goose heaven, and Canada geese have increased their numbers in response.  Once a fairly rare species, the Canada goose has become common enough to annoy annoyable people, sometimes even being called a pest.

In Lincoln Park, people mostly seem to enjoy the geese.  They watch them, photograph them and feed them, despite signs forbidding the feeding of wildlife.  The goslings provide food for the black-crowned night herons, and the geese eat excess pond vegetation.  Canada goose behaviors are fairly simple and easy to observe, so lots of kids and adults learn about bird biology by observing geese.  Unfortunately, geese seem to be such a common sight that some people have lost their respect for the geese's size and strength.  Parents let their toddlers chase the geese and approach the young, even with the parent geese hissing and ready to bite (yes, they lunge, flap, bite and cause an unnerving ruckus in self-defense).

A wood duck (left) hanging out with Canada gees (right) at the Alfred Caldwell Lily Pond in Lincoln Park.

Where Canada geese are a considered a nuisance, wildlife management personnel have responded with extended hunting seasons, attack dogs, culling of adult birds and donation to food charities, collection of eggs for human consumption (I hear they are delicious) and addling of goose eggs.  Addling is a technical term that means preventing an egg from developing without destroying the egg, by shaking or coating with oil to prevent oxygen from diffusing into the egg.  Goose eggs are addled instead of being simply taken because the geese can lay a second clutch of eggs.  Canada geese, like most animals, are regulated.  Geese are regulated by the Migratory Bird Act, so before someone dabbles in addling, they must apply for a permit.  An easier humane solution to overpopulation of geese would be to modify habitat to make it less inviting.  Geese will not nest in an area without good sight-lines for detecting predators, so smaller patches of lawn interspersed with shrubs or potential predator hiding places will deter them.  If we take a little time to appreciate and get to know Canada geese, they seem much less like nuisances and much more like fascinating clowns of the bird variety.

Jellies at Shedd

We're in the middle of getting 6-8 inches of snow here in Chicago.  We've been putting off going to the museums until the winter weather really kicked in, and today was the perfect day to have an indoor adventure.  We spent a couple of hours at Shedd Aquarium, and we could have spent several more there.  My favorite exhibit was of the animals formerly known as jellyfish, currently known as jellies since they really don't have anything to do with fish other than the fact that they live in water.  I could watch them for hours - they are mesmerizing and fascinating.  Another museum-goer called them living lava lamps.

Northeast Pacific Sea Nettle, Chrysaora fruscescens

 I've written about jellies before, so I'll take a different approach here.  I'd like to point out some interesting anatomical structures and explain a little bit of their life cycle.  Let's start with anatomy.

The jellies above are swimming upside down.  The main body is called the bell.  There are two types of structures that trail behind the bell: tentacles and oral arms.  Tentacles are darker in the Pacific Sea Nettle, and they extend off of the edge of the bell.  Oral arms are usually whitish, four in number, and extend from the center of the bell.  Both appendages are covered in microscopic stingers.  The tentacles are for initial stinging of food and defense.  The oral arms are for more stinging and for moving food toward the center of the bell for digestion.

Jellies have only one orifice on their bodies, which has to serve for the in and out orifice of the digestive, reproductive and respiratory systems.  Food goes in, gets dissolved, is carried out through the rest of the body, and whatever isn't digestible is 'spit' back out.  Reproductive organs, visible as the four ring-shaped white structures in the moon jellies below, connect to the single orifice.  Males release sperm into the ocean water.  Females can release eggs into the ocean water, hopefully to find a sperm somewhere, or more commonly, they take in sperm, allow fertilization, and even allow development of the next generation inside their body cavities. 

Moon Jelly, Aurelia aurita

Jellies have a ring of muscle one-cell-thick around the edge of the bell that they can contract and relax to move.  Also around the edge of the bell, it is common for jellies to have light-detecting tissue for knowing when to swim up to the surface of the ocean.

Good view of the two types of appendages attached under the bell.

The jelly life cycle is pretty head-scratching.  The pictures on this page show all adult forms, but they have juvenile forms that look completely different - like butterflies and caterpillars look different.  Fertilized jelly eggs grow into larvae called planulae.  Planulae are almost microscopic and swim free in the ocean.  Some planulae can sting, and those invisible stings that sometimes get trapped inside your bathing suit could be planulae or they could be broken-off adult stingers.  Planulae swim to a hard surface, like a pebble or a rock, and they stick and grow into a polyp.  A polyp is a cup-shaped, usually transparent animal with tentacles in a ring around the top of the cup.  The bottom of the cup is stuck fast to a surface.  Polyps can grow mini-polyps off the side of themselves, which can fall off and become independent organisms (this is asexual reproduction by fragmentation).  Eventually, polyps grow a beret-like structure, let go of their substrate, and become an adult medusa to roam the ocean, sting at will, and reproduce the next generation.  Here is a nice diagram of the jellyfish life cycle.

Flower Hat Jelly, Olindias sp.

I'll end with a picture of an extremely strange jelly, the flower hat jelly.  It keeps its tentacles tucked up underneath its bell, and it extends them quickly when they are needed.  It is a benthic species, spending most of its time on the sea floor.  Those tentacles that extend off the top of the bell are mysterious, but I do know they are called exumbrella tentacles.  They might be lures to attract prey, or they could be stinging tentacles for self-defense.  If you are searching for a creature to study to make a name for yourself as a scientist, this might be a good subject, because little is known about it.

If you'll be in Chicago sometime in the next year, you're invited to come with me to the Shedd Aquarium - we have a membership!