Host Switching Across 400 Million Years of Mobilid Ciliate Evolution

Ciliates (Ciliophora) are unicellular organisms, most of which can live freely in aquatic environments. Mobilid ciliates, however, have tied their life histories closely to animal hosts. They commonly inhabit the body surface or gills of aquatic animals, including fishes, mollusks, annelids, and crustaceans, attaching themselves to host tissues by means of an adhesive disc on the aboral side. At the center of the adhesive disc is a ring of specialized denticles that helps anchor the cell to host tissues. A small number of species have even invaded internal organs, where they inhabit sites such as the urogenital tract and intestine.

In one study, researchers collected 12 edible frogs (Pelophylax esculentus) from ponds at two localities in Slovakia. Dissection revealed one heavily infected frog at each site, with several hundred ciliates inhabiting each urinary bladder. Detailed morphological comparisons showed that these ciliates represented a previously undescribed species, which was named Trichodina pelophylacis. The species measures approximately 66–82 μm in body height and possesses an adhesive disc about 36–47 μm in diameter on its aboral side, containing 26–31 denticles.

Morphological comparison of Trichodina pelophylacis (A–F) and five amphibian-associated congeners (G–K). (A) Aboral view showing the overall structure of the adhesive disc; (B) schematic diagram of the denticles, with the x- and y-axes serving as reference points for describing the morphology and relative positions of individual structures; (C) detailed structure of the oral ciliary pattern; (D) oral view showing the adoral ciliary spiral and nuclear apparatus; (E) nuclear apparatus consisting of a horseshoe-shaped macronucleus and an ellipsoidal to narrowly ellipsoidal micronucleus located in the −y position; (F) lateral views of different individuals showing variation in body shape. (G, H) Trichodina vesicularum and Trichodina urinicola; (I, J) Trichodina urinicola f. bohemica and Trichodina urinicola f. taeniatus; (K) Trichodina ranae. Abbreviations: F, oral fibers; HK, haplokinety; MA, macronucleus; MI, micronucleus; P1–3, peniculi 1–3; PK, polykinety; PP, peripheral pins（Image source：Vďačný P et al. (2026)， CC BY 4.0 ）

Trichodinids have long been known to inhabit the urinary bladders of amphibians, but many of these species were named during the first half of the twentieth century, when molecular sequence data were unavailable. Trichodina pelophylacis is morphologically similar to several other urinary bladder trichodinids of frogs and could easily be confused with Trichodina ranae or Trichodina lishuiensis. The new species, however, has a taller body, a C-shaped macronucleus, and a different micronuclear position. Its 18S rRNA sequence also differs from that of T. lishuiensis by one insertion/deletion (indel) and eight nucleotide substitutions.

Images of Trichodina pelophylacis. (A–G) In vivo; (H–J) after dry silver nitrate impregnation. (A, B) Aboral views showing the adhesive disc, denticle ring, and surrounding radial pins; the horseshoe-shaped macronucleus and adoral wreath carrying the oral cilia are also visible; (C) detail of the denticles showing the blade, central part, and ray; (D) optical section through the center of the body showing the macronucleus and oral ciliature at the base of the infundibulum (white box); (E) enlargement of the boxed area in D showing the oral ciliary pattern; (F) oral cilia emerging from the haplokinety and polykinety; (G) horseshoe-shaped macronucleus; (H, I) aboral views showing the overall structure of the adhesive disc; (J) oral view, with red triangles indicating the position of the adoral ciliary spiral. Abbreviations: AC, adoral cilia; AD, adhesive disc; B, blade; DR, denticle ring; F, oral fibers; HK, haplokinety; IF, infundibulum; MA, macronucleus; OC, oral cilia; P1–3, peniculi 1–3; PR, posterior projection of the central part; R, ray; RP, radial pins（Image source：Vďačný P et al. (2026)， CC BY 4.0 ）

This raises a broader evolutionary question: among a group of ciliates that today inhabit fishes, frogs, snails, planarians, and many other invertebrates, what kinds of animals served as their earliest hosts?

The researchers constructed a time-calibrated phylogeny of 80 taxa based on 18S rRNA sequences and then reconstructed ancestral hosts and habitats. Because no mobilid fossils are known, divergence times could not be calibrated directly with fossils from the group itself. Instead, the analysis incorporated three temporal constraints derived from previous molecular dating studies, fossils of related ciliates, and the evolutionary histories of host groups. The analysis used an uncorrelated lognormal relaxed-clock model, which allows molecular evolutionary rates to vary among lineages, and four independent Markov Chain Monte Carlo runs of 50 million generations each. Effective sample sizes exceeded 200 for all parameters, indicating good statistical convergence among the analyses. Under this calibration scheme, the split between the two major mobilid families, Urceolariidae and Trichodinidae, was estimated to have occurred approximately 430 million years ago, during the Silurian. This estimate nevertheless carries substantial uncertainty arising from both the model and the calibration scheme and should not be treated as a fossil-determined date of origin.

Time-calibrated phylogeny of mobilid ciliates based on 18S rRNA sequences（Image source：Vďačný P et al. (2026)， CC BY 4.0 ）

Using the present-day distribution of mobilid hosts among freshwater, marine, and anadromous habitats together with the time-calibrated phylogeny, the model reconstructed the most recent common ancestor of mobilids as most likely having been associated with hosts living in freshwater. At least two major transitions from freshwater to marine environments were subsequently inferred: one within Urceolariidae and another within a lineage of Trichodinidae. After entering marine environments, some descendants later returned to freshwater or brackish habitats, indicating that the salinity barrier was crossed repeatedly during mobilid evolution.

Model-based reconstructions of (A) host groups and (B) host habitat types（Image source：Vďačný P et al. (2026)， CC BY 4.0 ）

Statistical reconstruction could not unambiguously identify the host of the most recent common ancestor of Mobilida. The ancestor of Urceolariidae was nevertheless strongly associated with invertebrates, whereas the ancestor of Trichodinidae was reconstructed as having colonized freshwater teleost fishes. Because the estimated early divergences within Mobilida predate the appearance of freshwater teleosts, the researchers proposed that the earliest mobilids were more likely associated with freshwater invertebrates, with one lineage subsequently expanding into fishes.

Once trichodinids had colonized fishes, host switching became remarkably frequent. Trichodinidae independently invaded amphibians at least twice: one lineage colonized the body surface of tadpoles, whereas another invaded the internal organs and urinary tract of adult anurans. Trichodina pelophylacis belongs to the latter lineage. The phylogeny also indicates multiple transfers from vertebrates to invertebrate hosts, including mollusks, crustaceans, and planarians, as well as at least one reversal from molluscan hosts back to a teleost fish.

The evolutionary trees of the ciliates and their hosts likewise do not show a simple pattern of one-to-one cospeciation. After testing several alternative coevolutionary models, the researchers consistently found that duplication followed by host switching was the most frequently inferred event. A substantial part of mobilid evolutionary history therefore appears to have been shaped by the colonization of new animal hosts. Mobilids lack an off-host cyst stage capable of long-term survival and can disperse independently only by swimming short distances or surviving briefly in the surrounding water. Nevertheless, whenever different animal species come into contact within the same aquatic environment, new opportunities for host colonization may arise.

Host identity may also influence how rapidly mobilid lineages accumulate species through evolutionary time. Across the diversification models, lineages associated with fishes consistently had the highest, or jointly highest, net diversification rates. Amphibian-associated lineages had the lowest rates, with estimated net diversification being negative. Mobilids associated with mollusks and other invertebrates generally fell between these extremes, although different models varied in whether their net diversification rates were positive or negative. The researchers proposed that the extraordinary species richness and greater dispersal capacity of fishes may create more opportunities for host switching and geographic expansion. By comparison, the semi-aquatic lifestyle of anurans may restrict the periods during which trichodinids can move between hosts.

Hosts were divided into non-molluscan invertebrates (dark orange), mollusks (light orange), amphibians (green), and fishes (light purple). Analysis of net diversification rates in mobilid ciliates associated with the four host groups（Image source：Vďačný P et al. (2026)， CC BY 4.0 ）

Mobilid ciliates appear to have begun their evolutionary history in freshwater environments and subsequently crossed boundaries between host groups, salinity regimes, and continents. Their intrinsic capacity for long-distance dispersal is limited, so much of their geographic expansion has depended on the animals they inhabit. A successful shift to a new host can, in some cases, become the starting point for an entirely new evolutionary lineage.

Author: Shui-Ye You

Reference:

Vďačný P et al. (2026). Evolution of mobilid ciliates in space and time, with description of the anuran-associated Trichodina pelophylacis sp. nov. (Ciliophora: Mobilida). Hydrobiologia.