The utility of the lined shore crab Pachygrapsus crassipes as an indicator of small-scale variation in the condition of salt marshes was determined by conducting detailed mark-recapture studies of the spatial and temporal patterns of movement in 2 marshes in northern and southern California, and by conducting a stable isotope study along a nutrient gradient at one of the marshes. Crabs (1158) were captured from creek bank and vegetated marsh habitats, tagged, and released. Most crabs were recaptured in the same trap where they were originally tagged, and on average, they traveled less than 5 m from the site of first capture. The mean distance traveled did not increase significantly with the time between captures. A high percentage of crabs (62.1%) tagged near the creek bank remained there; fewer tagged crabs moved between the creek bank and the marsh plain (18.2%) or remained on the marsh plain (12.6%). Crabs moved least along creek banks and farthest when crossing creeks, although few of them did the latter (6.2%). During spring tides, crabs moved from tidal creeks onto the marsh plain. The nitrogen isotope data mirrored the gradient in nutrient input, supporting the conclusion that P. crassipes remained and fed within a localized area. Thus, the lined shore crab can serve as an indicator of small-scale differences in contaminant exposure. Furthermore, semiterrestrial species that are readily sampled and move little are abundant around the world, making them ideal indicators of habitat condition.KEY WORDS: Movement · Indicator species · Mark-recapture · Nitrogen stable isotope · Crab · Pachygrapsus crassipes · Salt marsh Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 314: [271][272][273][274][275][276][277][278][279][280][281] 2006 understanding of crabs residing in salt marshes has developed in many parts of the world from studying ocypodid species, such as fiddler crabs (Uca spp.), which leave their burrows to forage during low tides (Crane 1975). However, our understanding of crabs occupying marshes along the west coast of North America is much more limited because the grapsid crabs Hemigrapsus oregonensis and Pachygrapsus crassipes primarily remain in burrows during daytime low tides (Hiatt 1942, Willason 1981, Ricketts et al. 1985, making them difficult to observe. The utility of crabs as an indicator of small-scale differences in habitat condition critically depends on the extent of their movements.Movement is a fundamental part of animals' lives, being essential for foraging, reproduction, and avoiding predators and competitors. However, movement receives more attention on land than in the marine environment, where sessile and sedentary lifestyles are more common due to (1) the delivery of food, (2) removal of waste, and (3) transfer of gametes by currents (Strathmann 1990). In the case of crabs, many juveniles and adults rely on heavy exoskeletons coupled with burrows, holes and crevices for protection from predators, seemingly moving little...