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Problems with Peritrichs

Virginia Sea Grant

Fishery Resource Grant 2021-2022 OBJECTIVES:

1. Identify and quantify specific stalked ciliates that are causing these infections of oyster seed.

2. Identify the seasonality of stalked ciliate infestations.


In 2020, we started to notice in our oyster hatchery a problematic pest (which we would come to find out is more common than we first guessed) , known as the stalked ciliate. While these critters don’t actually hurt the oysters directly, and are of no danger to humans, they are very good at disrupting our oyster seed growing operations.

Recently we received a Fishery Resource Grant to investigate these pests to learn more about what species were infesting our seed, and how best to manage them going forward. But first, a little background on what these pesky peritrichs really are.

For all marine species, there are numerous environmental stressors — from water quality, to predators, pathogens, and other ubiquitous threats — that can significantly impact or disrupt the complex life cycle of an oyster. Raising shellfish in a hatchery reduces many of those variables, but at the same time, holding high biomass in a small space brings on its own issues.

This is why an important part of our mission at OSH is to improve hatchery technology and participate in ongoing research to learn how hatcheries can adapt and improve their practices to continue harvesting larvae, while becoming more resilient to outside factors that cause seed mortality events.

A Different Type of "Tulip": Introduction to Stalked Ciliates

Warding off pests is a natural part of any organism’s lifecycle and some pests are more resolute than others, and the peritirchs afflicting our bottle nursery, and upwelled system can pose problems with seed. Last year, we took a look at these stalked ciliates, a type of protozoa (single-celled microscopic animal) that are found to colonize other living organisms. Take a second to picture a tulip, yes, the flower. Stalked ciliates appear tulip-like. They characteristically contain a branched or unbranched stalk with an inverted bell-shaped body. They also contain the presence of cilia (hair-like structures) near their feeding region - a fuzzy tulip protozoa, if you will.

Ciliates start their lifecycle as free-swimmers, not unlike oyster larvae, until they reach their sedentary stage. They are most commonly found in freshwater, brackish, and marine environments where bacterial populations are high. Stalked ciliates are characteristically epibionts, meaning they colonize in groups (also known as peritrich) or observed as suctorians, on the surface of living organisms, or their hosts, also known as basibionts. Still with us?

Stalked ciliate infestations have commonly been documented in commercially important crustaceans (primarily shrimp and copepods), but not traditionally bivalves. Through their crustaceous hosts, they have been known to infest gills, depriving their host of oxygen, and other times interfering with feeding, locomotion, decreasing fertility, and increasing sensitivity to contaminants.

In 2022, OSH received a Fishery Resource Grant through VA Sea Grant in partnership with VIMS to investigate infestations of various stalked ciliate species within our bottle and nursery systems. This project, completed last year, is described below.


From our observations, we learned that the stalked ciliate organisms attached themselves to the bill of the oyster, and were pulled into the mantle through the oyster’s feeding current. It has been documented before that stalked ciliates that attach to surfaces with increased water flow can enhance their own feeding rate (an opportunity placement/nutrient privilege of the epibiont pest).

Yet, in our case of young oyster seed, we observed that when the ciliate disturbed the oyster’s mantle, the oyster snapped shut. To give you an idea of this delicate stage of the oyster lifestyle, it takes two weeks for oyster larvae to finish their larval cycle where their volume can increase over 50x.

After they metamorphose into seed, they grow exponentially from 1-2mm up to 10-12 in just weeks. Feeding on phytoplankton, as you can imagine, is crucial to this rapid growth stage. Therefore, the stalked ciliates interfered with the oyster’s feeding process can pose serious issues.


We sampled stalked ciliates opportunistically at our hatchery and other sampling locations as infections cored from March 2022-August 2022. Frozen, fixed and live samples were collected for analyses conducted by Dr. Richard Snyder.

For identification analyses, we collected frozen, fixed, and live samples of infested seed. For frozen samples, a small amount of seed was wrapped in a wet paper towel inside a ziplock bag and placed in the freezer until transport. Fixed samples contained a small sample of seed placed in a vial of Lugol’s iodine and 14ml of seawater.

Live samples were collected for immediate transport (within 24 hours) to the VIMS Easter Shore Laboratory was feasible. These samples were kept in a vial of seawater at room temperature during transport. Dr. Richard Snyder at VIMS quantified and identified stalked ciliates from our samples to the genus level.


This project confirmed that we were dealing with various stalked ciliate species inhabiting our bottle and nursery systems throughout the 2022 hatchery season.

Suctorian Species

In March, 2022, the most abundant stalked ciliates in the bottle system were Suctorian species (Fig. 1) in the genus Acineta. These organisms contain two bundles of tentacles and a stalk that coils when it contracts. It is unknown if the toxin in their tentacles reacts with oyster tissue.

Zoothamnium & Vorticella

Free-swimming Scutio ciliates, branched peritrichs in the genus Zoothamnium (Fig 2.), and single single-stalked peritrichs in the genus Vorticella were also observed, but appeared to be rare compared to Suctorians. It was determined that the heavy organic load in the bottle culture system supports all the stalked and free-swimming ciliates and the bacteria they feed on.


During June, our hatchery also observed the first detrimental infestation of stalked ciliates on oyster seed in our outdoor upwellers, where the samples were considerably different from the bottle system samples. The quantity of sectarians were relatively the same and may have been carried over from the bottle system. However, there were more stalked ciliates in larger colonies, which appeared to be a different species compared to previous samples. A Folliculina species was also observed in this sample (Figure 3). Benthic diatoms and detritus common to surface waters were also identified. The stalked ciliates in this sample, other than suctorians, appear to be inoculate from surface waters.

While many of these species have likely always been present in low quantities I need grown in any users system, it appears that high concentration outbreaks can have significant impacts on rate of growth and survival of small nursery seed and reports made by various oyster farms of poor performance in nurseries accompanied by “fuzzy growth” on the seed, may actually be a problem with peretrichs.


Now that OSH has identified the problem pests, our goal is to find preventions and better management practices for hatcheries dealing with these infestations. OSH has a designated Research and Development Manager, Sam Glover, who will be presenting results and leading a discussion on this topic at the National Shellfisheries Association meeting this year.

Phase II of this project was recently funded by another Virginia Fishery Resource grant, titled “Controlling Epibiont Pests.” The overall objective will be to kill or drastically reduce epibiont infestations by testing various infestation control methods empirically.

Our current method of controlling stalked ciliate pests includes dipping infected seed in a diluted vinegar solution. However, it is not always successful at eradiating all of the pests (especially Suctorians). The first objective of phase II will be determining the tolerance of small and large seed to treatment concentrations of proposed control solutions. After we determine tolerance, we will test treatments for seed in bottle and nursery systems to determine which treatment is the most effective.

Part II blog coming soon.

In the weeds:

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