This week I have continued working on my predation experiment. I have been trapping most days, and although I am getting high numbers of both Dungeness and green crabs (which is why I chose these sites in particular), it is difficult to find crabs that fit into my small size class. I will be trapping at a variety of sites next week, and am predicting that sites further up the estuary will have smaller crabs because these locations may provide more nursery grounds for small adult crabs. Most red rock crabs that I catch are in the appropriate large size class. I have also been able to catch more red rocks the same way most people fish recreationally: by throwing crab rings off of the dock! This targets red rock crabs, who tend to prefer sub-tidal areas, while the Dungeness and green crabs usually follow the tide in. The main reason I need smaller Dungeness and green crabs is because a lot of my trials have no outcome. Many of my trials have gone on for 7 days and no predation has occurred. Either the red rock is not hungry, or it may be reluctant to attack a crab that is smaller but still decent sized, especially in the presence of another crab that could also pose a threat during a predation attempt.
Green crabs can easily be identified by the 5 points along the top of their carapace on either side of their eyes. By looking at the distinct carapace, one can avoid confusion with color, because green crabs aren't always green! The crab below is also a green crab, although she appears to be more yellow/orange. The coloration could be a result of location and camouflage attempts, or more likely means that the crab is almost ready to molt as the shells often begins a light green and will then darken to yellow, orange, and then red. The crab on the left is a right-handed male, while the crab on the right is a female who is missing the first three chela or legs on her right side. I can tell the sex of each crab based on “apron” at the base of the abdomen. Males have a much narrower and pointed apron, while females have a wider, rounded apron.
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AuthorHi! My name is Renee and I am a rising junior at Muhlenberg College in Pennsylvania. I am a double major, studying Biology and Dance. I am so excited to be a part of the Oregon Institute of Marine Biology’s summer REU program and to work with Shon Schooler and the South Slough National Estuarine Research Reserve. Archives
August 2019
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