Color variation in Hemigrapsus oregonensis (Arthropoda, Pancrustacea, Malacostraca, Brachyura)

crab anatomy

Fig. 1: Anterior-dorsal view of Hemigrapsus oregonensis with labeled external anatomy

Though Hemigrapsus oregonensis is commonly known as the Green Shore Crab, it does not always seem to be particularly green. This small crab (fig. 1), found along the Pacific coast from Alaska to southern California, can be any number of colors (see fig. 2). Among 67 specimens collected from Argyle Creek on San Juan Island, WA, and analyzed at FHL, the following color patterns emerged: the crabs seemed to have a solid color on their carapace with a distinct pattern of spots or shapes in another color over this. The solid colors in the sampled individuals were white, red, yellow, grey-blue and many shades of green. Often the carapace color would be a blend or mixture of two or three of these colors. The patterns were usually in black or green, often with small white spots. The setose pereopods of the crabs, which distinguishes them from the similar looking Hemigrapsus nudus, were often green with black and white spots, though they could be any color. Chelae were often white.

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Fig. 2: Photos of selected crabs’ carapaces. All collected crabs fit into one of these 28 categories. Of 67 specimens, 28 color morphs were found which suggests that there are likely many more variations out there.

The color variation among this species could be due to genetic variation, sexual dimorphism, environmental factors during or after development, or could change with age or molting stage of the crab. This last hypothesis is supported by evidence that was fortuitously gathered when one crab molted while it was being photographed. The crab was originally beige with maroon and white spots, but after ecdysis the individual emerged with a bright green carapace and the exact same spot pattern as before. This could indicate that color changes with new molts, or that as the crab approaches its molting period, the carapace fades. However, many individuals were lighter than the crab that molted, and did not molt after seven days in the lab (though the environment could have prevented them). Some molted carapaces were found of various colors, many bright red, which seems to contradict the hypothesis that carapace brightness and color are indicative of molting stage.

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Fig. 3: Crab molting. This process took about one minute.

Color also does not seem to depend on age or sex. Individuals were collected from 0.8-2.3 cm, presumably of different ages, with no evidence of a progression or trend in carapace color. There also seemed to be no difference in the colors or patterns between males, females or gravid females. As for habitat, the crabs live among rocks that are just as diversely colored as they are, which likely supports carapace color variation. Crabs did not seem to live under rocks that resembled themselves, and differently colored crabs were often found living under the same rock. Environmental factors likely influence color, but apart from habitat, it is hard to know many details of each crab’s environment, such as their developmental circumstances and diet. Genetics must have some effect on the color, and likely determines the pattern on the carapace because the molting crab kept the same pattern even when its color changed after ecdysis. Most likely, some combination of the above factors must influence carapace color and pattern of Hemigrapsus oregonensis crabs; more research is required to unravel this mystery.

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Fig. 4: Habitat of Hemigrapsus oregonensis at Argyle Creek on San Juan Island

Rachel Folz- University of Chicago