Distribution, habitat and food preferences of sympatric high intertidal isopod species Ligia occidentalis and Ligia pallasii (Ligiidae: Oniscidea)

The distribution, habitat and food preferences of two partially sympatric high intertidal isopods, Ligia pallasii and Ligia occidentalis, were investigated. A survey along the Eastern Pacific coast indicated species-specific site preferences as evidenced by a mosaic pattern of distribution with few sites of co-occurrence. Ligia pallasii preferred sites with rocky cliffs and caves that may provide a thermal refuge for the northern species, whereas L. occidentalis apparently is a habitat generalist of rocky shores. Differences in reproductive patterns exist: L. occidentalis reproduced once annually in spring and summer, but gravid females of L. pallasii were observed throughout the year. Feeding experiments did not indicate any significant differences in diets; both isopod species fed mainly on seaweed and diatoms but preferred rare protein sources when available.