Epibiotic relationships are a widespread phenomenon in marine, estuarine and freshwater environments, and include diverse epibiont organisms such as bacteria, protists, rotifers, and barnacles. Despite its wide occurrence, epibiosis is still poorly known regarding its consequences, advantages, and disadvantages for host and epibiont. Most studies performed about epibiotic communities have focused on the epibionts' effects on host fitness, with few studies emphasizing on the epibiont itself.

The present work investigates species composition, spatial and temporal fluctuations, and aspects of the life cycle and attachment preferences of Peritrich epibionts on calanoid copepods in Chesapeake Bay, USA. Two species of Peritrich ciliates (Zoothamnium intermedium Precht, 1935, and Epistylis sp.) were identified to live as epibionts on the two most abundant copepod species (Acartia tonsa and Eurytemora affinis) during spring and summer months in Chesapeake Bay. Infestation prevalence was not significantly correlated with environmental variables or phytoplankton abundance, but displayed a trend following host abundance.

Investigation of the life cycle of Z. intermedium suggested that it is an obligate epibiont, being unable to attach to non-living substrates in the laboratory or in the field. Formation of free-swimming stages (telotrochs) occurs as a result of binary fission, as observed for other peritrichs, and is also triggered by death or molt of the crustacean host. Attachment success of dispersal stages decreased as telotroch age increased, suggesting that colonization rates in nature may be strongly dependent on intense production of telotrochs by the epibiont ciliates.

Laboratory experiments demonstrated that Z. intermedium colonizes equally adult and copepodite stages of A. tonsa and E. affinis. The epibiont is also able to colonize barnacle nauplii and a harpacticoid copepod, when these were the only living host available, but fails to colonize non-crustacean hosts, such as the rotifer Brachionus calyciflorus or polychaete larvae. When the epibiont could choose between adults of A. tonsa and alternate hosts from the zooplankton community, it always colonized preferentially its primary host, with only a few telotrochs attaching to other crustaceans (barnacle nauplii and harpacticoid copepod), and to rotifer eggs, suggesting that specific cues may be involved in host selection by this epibiotic species.