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Comparison of three Bucephalata miracidia. aSteringophorus furciger. bParvatrema affinis. cProsorhynchus squamatus. Yellow epithelial plates, blue hypoderm, orange muscle cells, green penetration apparatus, purple nerve cells, red germinal material, gray protonephridium
Source publication
Miracidia, the ciliated larvae of Digenea, are mostly known as active swimmers that search for the molluscan host and infect it. But many miracidia reach their host “passively”—staying inside the eggs that get eaten by the snail. This strategy is found in most major digenean lineages, and still its understanding is very limited. One of the reasons...
Citations
We performed the detailed ultrastructural reconstruction of the “passive” miracidium of Derogenes varicus - the species from Hemiurata group. The miracidium is highly miniaturized and simplified in comparison with the “active” miracidia. For the first time we elucidate the nature of the spines on the surface of hemiuroid larva: they are derivatives of the epithelial plates. The anterior end of the larva is equipped with three epithelial plates, that bear both spines and cilia. The major part of the miracidial surface is formed by tegument. The nervous and excretory systems of the D. vari cus miracidium are extremely reduced. Single undifferentiated cell comprises the germinal material of the miracidium. We discuss the trends of evolution of hemiuroid miracidia that are associated with transition to passive strategy of infection.
Digeneans of Notocotylidae family stand out of the range with their peculiar eggs. The eggs feature a pair of long filaments extending from their poles, and their content differs significantly from what we expect to observe in the eggs of digeneans. Instead of a ciliated larva miracidium, the egg of Notocotylidae contains a tiny few-celled mother sporocyst. This sporocyst infects a snail host following accidental ingestion of the egg. Previous transmission electron microscope studies elucidated the structure of the sporocyst and proposed a mechanism of the injection relying on the structure called opercular cord, but the data were insufficient to describe certain details. Here we present ultrastructural data on the egg of Paramonostomum sp., clarifying the nature of the injection apparatus and providing the missing details of the in-egg mother sporocyst structure. We found the opercular cord to be part of the eggshell associated with the vitelline membrane. We also elucidated the intertwining connection between the germ cells and the sporocyst’s tegument, and explored nuage granules present in the cytoplasm of the germ cells, providing evidence for identification of these cells as the germ ones. We discuss our findings in the context of transition of miracidia to a passive strategy of snail infection.
