A systemic parasitic infection of Pachygrapsus crassipes by an unknown ciliate
We encountered novel infections by an unidentified ciliate parasite in the Carpinteria Salt Marsh population of a common southern California macroinvertebrate, the lined shore crab (Pachygrapsus crassipes). This was surprising because that crab population has been extensively surveyed for parasites as part of on-going work putting parasites into food webs.
To better understand the role of this newly discovered parasite in estuarine food webs, we set about (1) determining the identity of the ciliate (using morphology, and phylogenetic analysis of 5.8S through 23S rDNA, including the highly-variable ITS1 and ITS2 regions); (2) determining how to best determine crab infection status, by drawing host blood and thoroughly examining crab tissues; (3) estimating how the parasite impacts infected hosts, by quantifying the intensity of infections in infected crabs and doing histological analyses to determine tissue pathology; (4) performing surveys to establish how widespread infections are in the crab population; and (5) investigating the transmission ecology of the parasite, using laboratory and field trials in which infected and uninfected crabs were grouped together in a variety of conditions.
The parasite’s morphology and pathology were inconsistent with other ciliates known to parasitize crustaceans and other marine invertebrates. Phylogenetic analysis suggested that the parasite is in the ciliate Order Apostomatida and belongs to a novel genus.
Infections were often intense and infected crabs were easily identifiable by the presence of swimming parasites in fresh blood smears, where there were 225 (± 40 SD) ciliates per µL blood. Further, simple blood draws from living crabs appeared to reliably detect infection, as examination of fully dissected crabs with parasite-negative blood revealed no cryptic infections. Detailed histological analysis is ongoing to better understand infection pathology. Female and heavily-injured crabs were most likely to be parasitized. Overall, infection prevalence was 13.0%. New infections never occurred in experimental trials. But that lack of infection, and the phylogenetic grouping with apostome ciliates, indicate that the parasite’s transmission may be linked to host molting. The increased infection prevalence characterizing injured crabs suggests that infection may also be facilitated by crab-crab fighting or failed predation by birds. The considerable infection prevalence and intensities, and a hint of increased mortality of infected crabs when stressed, combine to indicate that this parasite may substantially impact populations of striped shore crabs.