Mosquitofish are commonly found in lakes, ponds, and even ditches throughout Florida. “Their Latin name, Gambusia holbrooki, translates roughly to ‘useless’ or ‘nothing’ fish,” states Carrie Jessop, a recent research intern working on the Mosquitofish project at Archbold Biological Station. “However, they are widely used as baitfish and to control mosquito populations,” continues Jessop. “Mosquitofish love to munch on mosquito larva, preventing many of them from reaching adulthood and becoming the pests that bother us all.” These fish grow up to 3 inches in length and though they are typically drab light brown to silver grey in color, sometimes individuals show striking variations in color.

If you are lucky, you might find a mosquitofish that has black and white coloration. Because the black and white spotted scales resemble a Dalmatian, these fish are called ‘Dalmatian Variety’ mosquitofish. This pattern is also called ‘melanistic’, as it results from an excess of melanin in the skin. Melanin is the substance that gives your hair its darker shade or tans your skin after your latest trip to the beach. These Dalmatian mosquitofish are a rare sight in any population and can easily be mistaken for a different species of fish. Less than 1% of mosquitofish are the Dalmatian variety. “I hope I find a Dalmatian mosquitofish every time I go out to count and collect mosquitofish from ponds,” says Caitlin Weid, a field technician from Michigan State University currently working at Archbold Biological Station. “They are just so cool to see, a fishy four-leaf clover. I doubt they bring luck, but they do make a hot, humid day in the field exciting.”

“Dalmatian mosquitofish tend to be males,” continues Weid. “This is similar to how calico cats tend to be female. These color patterns are known as sex-linked traits, as they appear in one sex or the other. You might find a female Dalmatian mosquitofish, though that would be like finding two four-leaf clovers right next to one another.” Male mosquitofish can be distinguished from their female counterparts by the long slender fin on their underbellies, called a gonopodium. Female mosquitofish do not have this fin and have large black spots on their bellies called the gravid spot, which is a sign that the fish is pregnant.

<Caption: Male Dalmatian variety Eastern Mosquitofish (top) from Myakka, Florida and normal male Eastern Mosquitofish (bottom) from the Michigan State University mosquitofish experiment at Archbold Biological Station. A pink ID marking can be seen on the lower half of the normal male (bottom) which is used to keep track of each fish’s identity. Photo by Caitlin Weid.

Dr. Jessica Judson, another researcher from Michigan State University collaborating with Archbold to study mosquitofish, is surprised by how rare they really are. “In our experiment at Archbold, we are using mosquitofish to study inbreeding depression, which is the negative effect we see when close relatives produce offspring together. We want to understand how to prevent inbreeding in endangered species. Mosquitofish are a great model species to perform experiments with, as they are extremely common and easy to care for. For this experiment, we have 36 tanks and hundreds of mosquitofish, though we haven’t yet seen a melanistic mosquitofish in our tanks,” says Dr. Judson. Next time you find yourself near a lake or pond in central Florida, look around and see if you can spot your own fishy four-leaf clover. Who knows, you may even see two.

Photo by: Caitlin Weid

Caption: Tanks at Archbold Biological Station where Dr. Jessica Judson, Caitlin Weid, and Carrie Jessop tend to an experiment using mosquitofish to understand inbreeding and small populations sizes. Photo by Caitlin Weid.