Biology of the microsporidan parasite Pleistophora crangoni n. sp. in three species of crangonid sand shrimps

The microsporidium Pleistophora crangoni n. sp. is a common parasite of Crangon franciscorum, C. nigricauda, and C. stylirostris in the vicinity of Yaquina Bay, Oregon. Characteristics of the parasite are described. Skeletal muscle was the only host tissue infected. The seasonal prevalence and intensity of the parasite in crangonids are described, based on examination of 1556 C. franciscorum, 3877 C. nigricauda, and 1674 C. stylirostris collected at monthly intervals from June 1975 through June 1976. Prevalence in C. franciscorum and C. stylirostris increased through the autumn and reached winter peaks of 30.3 and 41.0%, respectively, then decreased in the spring. Prevalence in C. nigricauda remained below 8% through the year. Intensity increased with size of the shrimps in the three species. Infection experiments and field observations suggest that only very young shrimps are susceptible to infection during a relatively short period during the summer months. Following natural infection, the parasite spreads within the host, indicating repeated schizogonic cycles. Parasitic castration was indicated by the absence of gravid infected female shrimps and was confirmed by histological examination. Ovaries of infected shrimps did not develop beyond a very early stage. A shift in sex ratio toward females in infected shrimps indicates that the parasite may influence sex determination. Shrimps showed little cellular response to infection. Only rarely in heavily infected shrimps was encapsulation of parasite cysts observed; necrotic tissue was occasionally observed. Infected shrimps usually succumbed before uninfected shrimps when subjected to oxygen stress. The collection of infected shrimps that were unusually large may indicate that these individuals grew faster or lived longer than uninfected shrimps.