ABSTRACT
Lithodid crabs use the last pair of pereiopods for grooming of gills and brooded embryos. Evidence based on experiments, behavioral observations, and comparative functional morphology is presented. Pereiopods 5 are structurally modified for complete withdrawal into and out of, as well as movement within, the branchial chambers. These grooming limbs terminate in highly toothed chelae. Most pereiopod 5 articles are armed with setae, including a complex setal type adapted for scraping and brushing. In grooming of brooded embryos, the carapace is lifted and the cleaner leg extracted from the branchial chamber and inserted among the egg mass, where grooming may occur in conjunction with chelipedes. Fifth pereiopods remain within the branchial chambers when inactive. Gills are cleaned on upper surfaces by sweeping motions of the cleaner leg, accumulating loose material on long brush setae. Other areas of gills are accessible by thrusting and picking movements of the chela. The average percentage of time spent in gill grooming was 12.5. When not active, cleaner legs were in a folded resting position, with many setae arranged so as to filter the incoming respiratory current passively. Grooming of gills in Lithodes maja is essential for survival. In the absence of grooming, extensive fouling of the gills occurred. Animals succumbed within about 2 weeks after grooming limbs were inactivated. Restricting grooming to gills of one branchial chamber allowed survival for extended periods of time. Removal of setae from pereiopod 5 resulted in fouling of gills, whereas gills accessible by a setiferous cleaner leg remained clean. A comparison of morphological and ultrastructural setal modifications for grooming is made within the Lithodidae and among other crustacean groups.
