Macrostomum guizhouense sp. nov. (Rhabditophora: Macrostomorpha), a new flatworm from China with a novel nematode-predating habit

The genus Macrostomum is a highly diverse group within the Platyhelminthes, with over 200 species described worldwide. However, research on Macrostomum in China remains limited. Based on the morphology of the penis stylet—a key diagnostic character for species identification in this genus—a new nematode-predating freshwater flatworm species of Macrostomum is described from southern China. The new species, Macrostomum guizhouense Zhang, n. sp., possesses a J-shaped penis stylet, with a visible lamella located at the posterior one-third of the stylet. The distal opening is pointed, with an arrow-shaped internal structure. Phylogenetic analysis based on 18S and 28S ribosomal genes was conducted to confirm the evolutionary position of the new species. Furthermore, feeding experiments revealed that this flatworm preys on nematodes (Rhabditis sp.), and when maintained on a diet of Rhabditis sp. in the laboratory, the flatworms reproduced successfully over an extended period.