We studied the effects of attached peritrich ciliates on the fitness of natural populations of Acartia hudsonica (Pinhey) in Stony Brook Harbor, Long Island Sound. Ciliate infection occurred during late spring, and ciliate load (No. ind.-') was not related to copepod age, stage, or body size. Simulated in situ experiments conducted throughout the period of abundance of A. hudsonica showed that egg production rate (No. d-l) was positively correlated to ambient water temperature, but negatively correlated to ciliate load. Salinity and the concentration of total or >8-10 pm Chl a were not significant in explaining the variation in egg production rate. Egg hatching success (O/o) was not influenced by the infection status (presence or absence) of the female. Infected nauplii had lower survival rates (d-l) compared to uninfected nauplii, but their developmental rates (molts d -I) were not significantly different. Significantly slower average sinking rates were found for infected adults compared to uninfected adults. Slower sinking rates for infected copepods may have been due to an increase in surface area which increased drag. Infected adults with slower sinking rates may be more susceptible to predation. Our findings show that peritrich ciliates can play a role in the seasonal decline and future recruitment of A. hudsonica.

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