Morpho-molecular characterization of Trichodina chlorophora Richards, 1948 (Protista: Ciliophora), a central component in the ‘snail‒ciliate‒zoochlorellae’ hyper-symbiotic system

In the mantle cavity of the heterobranch snail Physella acuta, collected from a lake in Slovakia (Central Europe), we identified the peritrich ciliate Trichodina chlorophora harboring endosymbiotic green algae. To elucidate the evolutionary origins of this tripartite consortium, we determined the phylogenetic affiliations of all three partners and conducted a detailed morpho-molecular characterization of the ciliate, a central component of this hyper-symbiotic system. The European population of T. chlorophora closely matched North American populations previously described from physinine snails. The diagnostic features of T. chlorophora include: body diameter of 41–83 μm after dry silver nitrate impregnation; denticle ring 23–39 μm wide, with 23–30 denticles and 9–11 radial pins per denticle; denticles 5.7–7.8 μm long; adoral ciliary spiral performing ~ 1.13 turns (390°–409°) around peristomial disc; and a horseshoe-shaped macronucleus. Phylogenetic analyses revealed that: (1) the host snails are closely related to North American conspecifics, reflecting the human-mediated introduction of this invasive gastropod to Europe; (2) trichodinids colonized aquatic snails multiple times independently from poikilothermic vertebrate hosts, with T. chlorophora clustering with freshwater congeners from frogs, snails, and planarians; and (3) the endosymbiotic green algae comprise two species: Chlorella sp., closely related to endosymbionts of heliozoans and cnidarians, and Jaagichlorella geometrica, which clusters with epiphytic congeners. While the algae exhibit low host specificity, snail-dwelling Trichodina species show high phylogenetic host specificity. The parallel emergence of green algae-bearing trichodinids in physinine and planorbid snails suggests co-evolutionary processes that independently gave rise to interdependent associations among aquatic snails, ciliates, and zoochlorellae.