The effect of peritrich ciliates on the production of Acartia hudsonica in Long Island Sound

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. Epibiotic interactions between stalked peritrich ciliates and copepod hosts have been observed in both marine and freshwater systems (Henebry and Ridgeway 1979; Nagasawa 1988; Sleigh 1989; Xu and Burns 1990). Copepod epibiosis has been reported for many sites on the eastern seaboard of the United States (Herman and Mihursky 1964; Herman et al. 197 1; Turner et al. 1979; Woodhead and Jacobson 1985; South Carolina, Lonsdale pers. obs.), such as for Acartia tonsa in Long Island Sound (Conover 1956). Acartia hudsonica, a widely distributed congener in northern estuaries along the eastern coast of North America (Conover 1956; Jeffiies 1962) and major constituent of the zooplankton population in Long Island ’ Present address: Bronx High School of Science, Biology Department, 75 West 205th Street, Bronx, New York 10468. * To whom correspondence should be sent. Acknowledgments We thank A. Durbin for suggestions on our experimental design, A. Okubo for comments on an earlier draft of the manuscript, and R. Willey for the SEM micrographs. This research was supported by the National Science Foundation (OCE 90-12540 to D. J. Lonsdale and F. C. Dobbs) and the Office of Naval Research (NO00 14-87-K0181 to J. Yen). Contribution 825 from the Marine Sciences Research Center, State University of New York at Stony Brook. Sound, was found to be infected with the solitary peritrich Rhabdostyla (identified by R. Willey; Fig. 1, above) in early summer 1989 and 1990. The attached ciliates on A. hudsonica adhered only to the surface, with no penetration of the exoskeleton (Fig. 1, below) as previously found by Herman et al. (197 1). Ciliate infestation is often considered to be a phenomenon relevant only to senescent populations of adult copepods. Moreover, it has been suggested that ciliate load (No. copepod’ ) depends on both the size of the copepod and the size of the ciliate (Herman et al. 197 1). Much of the past work on ecological interactions between copepods and attached ciliates has been to identify the ciliates, quantify the degree to which an individual is infected, and determine the percentage of the population infected. Our research examined some demographic and movement effects of peritrich ciliates on A. hudsonica on Stony Brook Harbor, Long Island Sound. Our working hypothesis was that epibiont load was inversely related to copepod fitness. It was also our intention to study ciliate interactions with A. tonsa, but no infection of this species was found in the field study year (1989-l 990). We examined egg production rate, egg-hatching success, and naupliar survival and development rate as measures of fitness. Sinking rates of adult copepods were also