Waiting for the Crabs to Grow

Nicole Doran, the Ohio State University

 

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This is me on one of the DNR’s harbor trawls holding a bucket of mature blue crab. Photo credit to Stevie Czwartacki

My research project this summer is looking at how juvenile blue crabs respond to different salinity treatments. As I discussed in my previous blog post, climate change is expected affect the environmental conditions of estuaries where juvenile crabs mature into adult crabs, so it is important to understand how blue crabs tolerate a wide range of conditions. We know, for instance, that male and female blue crabs can use different areas of the estuary, with male crabs often preferring lower salinity habitats than female crabs. Since males and females use these available habitats differently, they may be best adapted to these different conditions. My project is designed to investigate differences between male and female tolerance for salinity.

To answer this question I am raising juvenile crabs in a laboratory in two different treatments, high and low salinity, and measuring their growth rates over time. By comparing the crabs grown in high salinity to crabs grown in low salinity we will be able to see if salinity causes variation in growth rates. Growth is a good way to measure the well being of an animal in its environment, so faster growth rates in high salinity for example means they do well in those conditions. Previous research done on juvenile blue crabs have shown they grow at greater rates in high salinity, which is what I predict to see with my experiment. But I also predict there will be a difference between males and females’ rates because in the estuaries, adult females can prefer high salinity areas and males can prefer freshwater habitat.

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Here’s the environmental room in the DNR where I am running my experiment in 16 of the tanks.

So how do you raise 50+ crabs in a lab? As it turns out, they are pretty resilient and adapted to their new conditions quickly. In each of the sixteen tanks, there are two males and one or two females. In order to get them to grow as much as possible, I feed them four days a week. Then I change out a quarter of the water every other day to make sure they have clean water to keep them healthy. To measure their growth, I weighed each crab and measured their carapace (body) width, length, and depth in the beginning of the experiment, and I re-measure them each time they molt. Because crabs have a hard shell they only grow in size after they molt, so this change in size from one molt to another is how I will determine growth rate. I also reweigh all of the crabs once a week to measure the growth that is occurring under the shell.

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This is one of my tanks, each one has a few crabs, the sponge filter and air stone to clean and aerate the water, and a Lesser Blue Crab (bottom) that helped establish the tank’s bacteria colony to keep the water’s ammonia levels under control.

The crabs have been in their experimental tanks for three weeks now and so far we have seen great results! Many of them have already molted once so hopefully they will continue to grow and molt in the remaining three weeks. Because climate change is expected to affect precipitation levels and consequentially salinity of estuaries, it will be useful for resource managers to understand how blue crab populations may respond to these shifting environmental conditions.

Acknowledgments 

I would like to acknowledge and thank the Fort Johnson REU program and NSF DBI-1757899 for making this research possible, the South Carolina Department of Natural Resources, Jeff Brunson, Stevie Czwartacki, and my mentor Dr. Michael Kendrick.

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