Libidoclaea granaria was collected as part of the by-catch of the Zygochlamys patagonica (Bivalvia: Pectinidae) fishery, between 36° and 39°40′S and at 80 to 128 m depth. Crab densities ranged from 0 to 150 individuals 100 m−2, with a mean of 9·8±20·4 individuals 100 m−2. Mature males and females ranged from 24 to 61 mm, and from 16 to 48 mm, carapace width (CW), respectively. The largest immature female measured 22·5 mm CW. The number of eggs carried by a female ranged from 1200 to 9300. Sponges were the most frequent epizoic organisms, followed by the lepadomorph cirriped Ornatoscalpellum gibberum, Serpulidae and Spirorbidae worms. Since all these organisms were reported on Z. patagonica beds, crabs are camouflaged to match their habitat. Small crabs usually had naked carapaces, indicating a high moult frequency. Larger crabs instead, had epibionts on their carapaces, suggesting a lesser moult frequency. The erosion of carapace spines and the heavy epibiosis of large specimens suggest the existence of a terminal anecdysis.