Effects of Color Variation on the Ecology of the Invasive Female Green Crab

Carcinus maenas, also known as the green crab, is a highly invasive species that is native to Europe but has since colonized North American coasts, Patagonia, Southern Africa, Japan, and parts of Australia. C. maenas exhibits color change where the shell is bright green after molting but gradually reddens over time. In male crabs, the two morphotypes exhibit different behaviors and competitive strategies. Green males molt more frequently than red males and are more commonly found in estuaries and the intertidal zone. Green males also have greater physiological tolerance to hypoxia, anthropogenic stress, and low salinity than red males. In contrast, red males delay or stop molting entirely and have greater mating success by diverting their energy from growth to reproduction. Red males have a thicker carapace and stronger chelae, are larger, have more carapace damage, are more likely to be fouled, have lower physiological tolerance, and are more commonly found in the subtidal zone compared to green males. A population’s color distribution can indicate whether a C. maenas population is in growth or reproductive mode. Better understanding of this highly invasive species’growth and reproductive patterns is vital for effective regulation of C. maenas populations. However, there is currently not much information available about color change in female C. maenas. My research focuses on investigating color change in female C. maenas and analyzing how it compares to color change in male C. maenas. The project involves re-examining published data on the two color morphotypes of C. maenas, conducting a systematic examination of the color phases of female crabs, and composing a literature review to explore the research available on color change in female green crabs.

Hi everyone, my name is Jackie Luu and my research mentor for my project is Dr. Lee. My research project is about the effects of color variation on the ecology of the invasive female green crab.

The European green crab, scientifically known as Carcinus maenas, is a highly invasive species. This species is native to Europe but has since colonized North American coasts, Patagonia, Southern Africa, Japan, and parts of Australia. The green crab exhibits color change where the shell is bright green after molting but gradually reddens over time. In male crabs, the two color morphotypes, green and red, exhibit different behaviors and competitive strategies.

Green males molt more frequently than red males and are more commonly found in estuaries and the intertidal zone. Green males also have greater physiological tolerance to hypoxia, anthropogenic stress, and low salinity than red males. In contrast, red males delay or stop molting entirely and have greater mating success by diverting their energy from growth to reproduction. Red males have a thicker carapace (or shell) and stronger chelae (or claws), are larger, have more carapace damage, are more likely to be fouled, have lower physiological tolerance, and are more commonly found in the subtidal zone compared to green males. A population’s color distribution can indicate whether a C. maenas population is in growth or reproductive mode, and better understanding of this highly invasive species’growth and reproductive patterns is vital for effective regulation of C. maenas populations.

However, there is currently not much information available about color change in female C. maenas. My research focuses on investigating color change in female C. maenas and analyzing how it compares to color change in male C. maenas. The project involves re-examining published data on the two color morphotypes of C. maenas, conducting a systematic examination of the color phases of female crabs, and composing a literature review to explore the research available on color change in female green crabs.

Next, I am going to go over a bit of the data analyses I conducted on my research mentor’s previously collected field data. For this example, the three locations analyzed were Cobscook Bay, Maine, from August 2004, Pleasure Beach, Connecticut, from June 2004, and Tuckerton, New Jersey, from June 2007. I analyzed these sites for the percentage of each color in the populations, the percentage of females with fouling for each color (fouling means that there’s an organism attached to the crab’s carapace), the percentage of females with damage for each color (examples of damage include a missing claw or carapace damage), and the size difference between the female color phases. For all of the data analyses, only females larger than thirty millimeters in carapace width were included, because smaller crabs, which are growing and molting more frequently, are disproportionately green.

The results showed that there was a greater percentage of red females than green females in all three of the populations in Figure 1. A greater percentage of red females was observed with damage and fouling than green females at all three sites as seen in Figures 3 and 4. Red females were also larger than green females at all three sites as seen in Figure 2. In summary, red females were larger, more fouled, and more damaged than green females, suggesting that color change in females is at least superficially similar to what has been seen in males.

We also saw that for all three locations, all ovigerous (or egg-bearing) females were red. I am also currently working on compiling more ovigerous female color data from my mentor’s previously collected field data to see if we continue to see this trend with a larger sample size.

In conclusion, my project involves examining the literature for information on color change in females, conducting data analyses on field data on color change in females, and analyzing the field data for information on the color of ovigerous females. Using these results from the literature, female color change, and ovigerous female color data, my research mentor and I are working on composing a literature review on this topic of color change in female green crabs. More insight on their population dynamics could help scientists better control the spread of this highly invasive species by improving our understanding of the importance of color change in the growth and reproduction of green crab populations.

I would like to thank my research mentor, Dr. Lee, for all her support and guidance throughout this research project. I would also like to thank George Mason University’s federal work study program and the Office of Student Scholarship, Creative Activities, and Research for the funding that helped support this project. Thank you for listening to my presentation and I hope you learned something new about the green crab!