Pachygrapsus crassipes are native to the West Coast of North America between 24°20' and 45° latitude. In the 1890's sightings in Japan and Korea were reported between the 34° and the 37° latitudes. The theory for P. crassipes movement into Japan and Korea is that they were carried over by ships carrying zoea larvae (first stage of the crab development)in the water ballast (Morris,et al 1980;Hui 1992).
Pachygrapsus crassipes lives on the rocky coastal shores. They live in the region that extends from upper low tidal zone to the highest-high intertidal zone. They like areas of hard substrate where there are many crevices, loose stones, sand, or mud (the mud cannot be too fine or it will suffocate the crab). The most important requirement is that there needs to be enough food in the area (Hiatt, 1948; Shanks 1995).
Pachygrapsus crassipes can be red, purple, or green. The carapace (the back shell) is a boxy shape, it is broader than it is long. A distinctive feature P. crassipes is the series of horizontal lines across the carapace.
In this species, the males are larger than the females. The size difference is noticeable after the crab's carapace reaches the width of 22 mm. After they reach 22 mm sexual dimorphism is noticeable. The female's carapace becomes narrower and shorter than that of a male. The difference of the brachyuran on the abdomen is apparent. Other features that occur is the male chelipeds are 8 percent longer. And the propodite and dactylopodite in males are larger by 10%.
Pachygrapsus crassipes can reach the size of 47.8 mm for males and females the carapace can reach the size of 40.8 mm (Morris,et.al 1980; Mohler,et.al 1997; Hiatt 1984).
Pachygrapsus crassipes reproduces sexually. The males and females come to sexual maturity at different stages. The males reaches sexual maturity when the breadth of the carapace is 12 mm, this is about seven months after hatching. The female reaches sexual maturity when the carapace is 15 mm, this occurs between 11 and 12 months after hatching.
The females become ovigerous between the months of April to September. Usually the mating only occurs once a year. However, they have been known to reproduce twice a year. It is unknown why a second reproductive period would occur.
When the eggs are fertilized, they are held under the belly of the female. There can be as much as 50,000 eggs. The crab eggs hatch into zoea larva, which turns into megalopae larva (the size is less than a centimeter), and then into crabs. When P. crassipes are in the crab stage there is a correlation between the size of the carapace and the age. Crabs with the carapace around 13 mm in width can be consider one year old, and crabs between 13 mm and 30 mm are considered to be in the second year. P. crassipes that are larger than this can be considered in the third or fourth year. (Hiatt, 1984; Shanks 1995).
Pachygrapsus crassipes are very well suited for the life in the intertidal zone. It is very well adapted to its a semi-terrestrial life. They will return to the water to moisten its gills, but can stay out of the water for up to 70 hours.
Pachygrapsus crassipes do not wander very far from their refuge. They stay close to protected areas. Therefore, they usually come out at night, under a little bit of protection. Their eyes are very well adapted for day and night vision.
These crabs are usually solitary animals that are aggressive towards one another, The larger crab always win the argument. They will fight over food, but not feeding territories. (Barry and Ehret 1993; Morris, et.al 1980)
Pachygrapsus crassipes is omnivorous, which means it eats both plants and animals. The main diet consists of algae such as the green algae Ulva, and Enteromorpha, or it eats red algae such as Endocladia, Rhodoglossm, and Grateloupia. Brown seaweed such as Fucus. However, Pachygrapsus crassipes also eats diatoms, worms, muscles, Hemigrapsus oregonesis , small dead fish., limpets, snails as in Littorina and Tegula, hermit crabs, and isopods. Pachygrapsus crassipes can also become cannibalistic and eat each other (usually this occurs when they still have their soft-shell after molting).
The predators of Pachygrapsus crassipes are seagulls, rats, raccoons, and humans. Other predators that juvenile and larvae crabs need to watch out for are sea anemones and other fish (Barry & Ehret 1993; Quammen 1984).
When the P. crassipes are in the megalopae stage they are very good swimmers. They orient themselves by swimming with the local ocean current. This allows the megalopae to be dispersed along the coastline. They also swim very close to the surface of the water so they can tell the current direction. They will return to shore, because they need to finish their development on shore (Shanks 1995).
Julie Stahl (author), University of Michigan-Ann Arbor, Karen Haberman (editor), Western Oregon University.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
the nearshore aquatic habitats near a coast, or shoreline.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
the area in which the animal is naturally found, the region in which it is endemic.
August 25, 2000. "Life on The Rocky Shore" (On-line). Accessed November 29,2000 at http://library.thinkquest.org/J001418/crab.html.
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Hiatt, R. 1984. The Biology of the Lined Shore Crab, Pachygrapsus crassipes Randall. Pacific Science, vol. 2, no. 3: 135-213.
Hui, C. 1992. Walking of the Shore Crab Pachygrapsus crassipes in its Two Natural Enviornments. The Journal of Experimental Biology, 165: 213-227.
Mohler, J., D. Fox, B. Hastie. 1997. Guide to Oregon's Rocky Intertidal Habitats. Newport,OR: Oregon Institute of Marine Biology.
Morris, R., D. Abbott, E. Haderie. 1980. Intertidal Invertebrates of Califoria. Standford, Califorina: Standford University Press.
Quammen, M. 1984. Predation by Shorebirds, Fish, and Crabs on Invertebrates in Intertidal Mudflats: An Experimental Test. Ecology, vol.65, no. 2: 529-537.