Factors affecting the distribution and abundance of two species of beach crab : Hemigrapsus oregonensis and Hemigrapsus nudus

Hemigrapsus oregonensis and Hemigrapsus nudus, two beach crabs common on the Pacific Coast of North America, show considerable variation in numbers, and species dominance from place to place. To determine why this should be so, a number of experiments were performed, and observations made to determine what are the morphological, and physiological differences between the two species, and how the crabs would be affected by the different physical conditions prevailing in different places. In general, it appeared that H. nudus could not tolerate muddiness of the substrate, while H. oregonensis tended to be eliminated from clean areas by predators. The abundance of the crabs appears to be correlated with the amount of cover available.Science, Faculty ofZoology, Department ofGraduat