A tiny worm-shaped organism has been caught living inside the shells of freshwater amoebas, and the researchers who found it say nothing like it has turned up in roughly two centuries of studying these creatures. The discovery came out of a Smith College lab in Northampton that spent years sampling pond water and wetland muck across New England, looking closely at amoebas that build their own protective shells.
The findings, reported by the Daily Hampshire Gazette, describe how the lab, led by biology professor Laura Katz, identified a microscopic protist species living inside the shells of amoebas, a relationship the researchers identified as a symbiosis. According to the newspaper, researchers reported that the phenomenon had not been recorded in 200 years of study of these organisms. Katz and her lab sampled shelled amoebas across New England wetlands from 2016 to 2024, casting a wide net before zeroing in on the unusual pairing.
A Worm-Shaped Hitchhiker Turns Up Again and Again in Maine
The amoebas in question belong to a group called Arcellinida, single-celled organisms that live in freshwater and wetlands across the globe and were first identified back in the 1880s, the Gazette reports. Fossils of their shells date back more than 730 million years, and scientists today track living Arcellinida as bioindicators of environmental changes and water contaminants, a role also noted in research published by pmc.ncbi.nlm.nih.gov, which describes the amoebas as top predators in microbial food webs. Separate research in Current Biology has traced seven major deep lineages within the group, probably diverging as early as 730 million years ago.
Inside this ancient lineage, the Smith team repeatedly observed the new phenomenon in amoebas along a stream in Acadia National Park, Maine, per the Gazette's reporting. Lab members watched a worm-shaped flagellate swimming inside an amoeba shell, a sight that stuck once the team saw it recur. Katz said her lab's discovery had not been seen despite centuries of study, and the newspaper reports the relationship adds evidence that interactions between microbes and other species are more common than researchers previously understood.
Genetic Detective Work in a Motel Room Lab
To figure out what the flagellate actually was, the lab turned to genetic analysis. Godwin Ani, a doctoral candidate in UMass Amherst's organismic and evolutionary biology program, analyzed the complete RNA transcripts of the amoebas and flagellates, a technique known as transcriptomics, per the Gazette. Ani used EukPhylo, an evolutionary history pipeline developed by Katz in her lab, to place the species on the tree of life and deduce its classification, concluding the flagellate is likely in the prokinetoplastid clade.
Ani and Giulia Ribeiro, a postdoctoral researcher at Smith College, identified the flagellate as a type of prokinetoplastid that appears parasitic to its host, the newspaper reports. Ribeiro corroborated Ani's data by analyzing gene expression in amoeba samples and observed that the amoeba showed signs of a stress response when the flagellate was present, concluding the flagellates are probably causing stress in the amoeba. The fieldwork itself was scrappy: researchers brought equipment to Maine and set up a makeshift lab in their motel room to process samples close to where they were collected, since amoebas cannot be grown in the lab, according to the Gazette's account.
Common But Still Mysterious
The flagellate was not rare once the team knew to look for it. Samples contained the flagellate in 8% of 125 collected samples, the Gazette reports, drawn from amoebas gathered at Hawley Bog in Hawley, Harvard Forest in Petersham, Orono Bog in Bangor, Maine, and Big Heath Bog in Acadia National Park. Smith College undergraduates helped collect data for the study over many semesters, work that stretched across the full 2016-to-2024 sampling window.
Even so, the research does not conclude a definitive relationship between the flagellate and Arcellinida, the newspaper notes, and researchers speculate the flagellate uses the amoeba shell for protection from environmental changes without confirming that idea outright. The caution echoes a broader pattern in amoeba research: a 2021 study of the shelled amoeba Hyalosphenia papilio and its associated Chlorella algae, examined across New England sites and detailed by sciencedirect.com, found two distinct clades of algae linked to host genetic diversity, but concluded that inactive algal nuclei suggested a temporary exploitation rather than a stable mutualism. Other amoeba-dwelling microbes studied elsewhere, including the Perkinsela organism described in Royal Society Open Biology and a bacterial symbiont profiled in The ISME Journal, show that amoebas have hosted hidden tenants before, though those cases involve different organisms entirely.
Whether this particular flagellate turns out to be helping its host, harming it, or something more complicated remains unresolved in the published findings. What the Smith College team has established is more basic and, in its way, more striking: a relationship hiding in plain sight inside amoeba shells for roughly two centuries of scientific observation, only now caught on camera and confirmed in the genetic record.
