The Curious Case of the Mean Spotted Fish:

How color, temperature, and genetics affect aggression in the eastern mosquitofish (Gambusia holbrooki)

The Big Question

I’ve always been fascinated by social behaviors – from living in groups and parenting to aggression and communication. You might already know that the brain controls behavior, but did you know that many different molecules, genes, and proteins within the brain interact together to produce the actions that we recognize as “behavior”? These interactions are affected by the genetics of the animal as well as by its physical and social environment. Thus, I’ve decided to focus my research on one big question:

What is the connection between the brain and social behaviors and how do genetics and the environment affect this connection?
A male melanistic mosquitofish

A male melanistic mosquitofish

Female and Silver Mosquitofish

Below: a large, pregnant female mosquitofish with the distinctive black spot on her belly. Above: a male silver mosquitofish


Eastern Mosquitofish (Gambusia holbrooki): The Mean Spotted Fish

Eastern mosquitofish are a small live-bearing fish that lives in Florida and the eastern United States. Females are large and silver with a black spot at the bottom of their bellies. Males, on the other hand, have fins fused into a long rod that they use to transfer sperm while mating. They also have a color polymorphism, which means that they can come in one or more distinct color patterns. Mosquitofish males are usually silver, but sometimes they are melanistic. These melanistic mosquitofish are unpigmented with black spots. A spotted color pattern is nothing new for most experienced fish lovers and aquarium hobbyists… except that these little spotted fish are a little weird.

Weird Fact #1: Melanism only occurs in males of this species. Previous work showed that melanism is linked to a male Y-chromosome; however, its expression is not well understood and may have many other genetic controls.

Weird Fact #2: Melanistic males are more aggressive than silver males. Since the melanistic color pattern and the aggressive behavior always occur together, we call this connection a “covariance.” If you live in Florida, take a look in a pond next time you’re outside and just watch these guys. Melanistic males chase more and they do not stop bothering females! Don’t live in Florida? Check out the video below:

They may be cute, but they can get really mean!

Weird Fact #3: In at least two Florida populations, the genes controlling melanism expression are different. In one population, males are born always melanistic and aggressive. In another population, they are born silver and only become melanistic and more aggressive after exposure to low temperatures. In our lab, we are breeding these fish as four genetic lines: always silver, always melanistic, temperature-sensitive silver, and temperature-sensitive melanistic.

Since the melanistic color pattern and the aggressive behavior always occur together, we call this connection a “covariance.” Even cooler? This covariance between melanism and aggression occurs in other tropical freshwater fishes AND the covariance between color and behavior exists in other animals, including owls and mice! The goal of my Ph.D. research is to understand the genetics and physiology underlying this color-behavior covariance as well as how the environment affects it.


Importance: Why Care About a Mean Spotted Fish and its Genetics?

This project will generate the first gene and physiological network for aggression with a robust, visual indicator (color). My research also aims to investigate how environment affects the frequency of melanistic males. A network of genetic, physiological, and environmental mechanisms will allow us to generate hypotheses, predictions, and models about how different factors affect behavior. This model has uses in a wide range of other scientific fields, such as climate change research, toxicology, and drug testing. For example, we could predict how temperature changes associated with climate change will alter the frequency of a behavior! Or, imagine being able to use a computer to predict model how a toxin found in our waterways affects behavior and then being able to test that in a fish just by checking what color it is! Thus, this project represents the first critical step to a model that has far-reaching applications.

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