Rhizocephalan parasitism is one of the most important biotic factors affecting commercially valuable crab species in families such as Portunidae and Lithodidae. In addition to the long term and permanent effects of this parasitism on the hosts (e.g. sterilization, cessation of growth), other functional problems may arise due to the considerable size of the parasite and to its particular position inside and outside of the host. In this study, experiments with the Mexican blue crab Callinectes rathbunae parasitized by the rhizocephalan barnacle Loxothylacus texanus were conducted in the laboratory to test whether the parasite affects the host's oxygen consumption rate under changing salinity conditions, A total of 83 crabs (49 parasitized and 34 controls), all initially acclimated to a salinity of 5, were used for metabolic rate measurements over sequential 24 h periods at salinities of 5, 15 and 25, During this 3 d period, oxygen consumption of individual crabs was measured 5 times per day. Parasitized crabs were classified, according to the degree of maturation of the parasite, as: (1) crabs with internal parasites, bearing a modified abdomen
(2) crabs with virgin, immature, recently emerged parasites
and (3) crabs with mature, fully developed parasites. Crabs with internal and virgin parasites did not show differences in oxygen consumption rates with respect to control crabs, Crabs with mature parasites consumed from 57 to 139% more than other crabs under all salinity conditions, Although there is a significant metabolic cost for the host associated to the presence of mature L texanus, it is not clear what specific process is responsible for the observed results. Since the increased oxygen consumption was unrelated to salinity, it is concluded that the host's osmoregulatory capabilities are not affected
other possible explanations include the constant grooming and maintenance of the parasite, At the population level, our results suggest that hosts bearing mature parasites may move within the estuaries from low to high salinity areas in order to reduce the metabolic energy expenditure caused by the combination of the parasite and hypoosmotic conditions.
