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Mollusk Trinchesia ornata examines a prey polyp (left) and eats it (right) / Source: Irina Ekimova, Moscow State University. Photo: Andrey Shpatak

A team of Russian scientists from M.V. Lomonosov Moscow State University and IEE RAS studied how nudibranch molluscs of the family Fionidae steal stinging capsules from stinging coelenterates and how the choice of a robbery victim affects the anatomy of a thief. The results of the work were published in the prestigious scientific journal Frontiers in Zoology.

Nudibranch molluscs, like snails, belong to the class of gastropods. Only, unlike most of their fellows, they do not have a shell, and their gills are branched tender outgrowths exposed on the sides or on the back, which is where they got their name. These mollusks live mainly in the seas, where they crawl along the bottom in search of prey. Many nudibranch molluscs feed on cnidarians.

Cnidaria serve not only as food for nudibranchs. Mollusks digest their soft tissues, including the stinging cells, the protective weapon of the stingers. But the burning organelles contained in these cells - the cnidocyst - nudibranch mollusks are embedded in the cnidosac sacs, which end in the outgrowths of their own body. Zoologists call this phenomenon kleptocnidia. When danger occurs, the mollusk uses the stolen weapon as its own: a cnidocyst twisted into a spiral or into a spring with poison unfolds sharply inside the cnidosac, rushes with its end towards the victim and digs into it, injecting poison.

Few studies are known of kleptocnidia, and many obscure phenomena are associated with it. For example, how exactly the nudibranch mollusk distinguishes stinging cells from the rest during digestion, why it does not digest capsules, but builds them into its body, or how its immune system reacts to the presence of foreign organelles. A team of Russian zoologists led by Irina Ekimova, a senior researcher at the Department of Invertebrate Zoology at the Faculty of Biology, Moscow State University, has made significant progress in unraveling the unresolved issues related to kleptocnidia.

For this study, scientists have chosen 13 species from the Fionidae family, whose representatives are characterized by high selectivity in nutrition. The average species specializes in feeding 1-3 types of coelenterates. Scientists observed selected species in nature and in the laboratory, experimented with the diet of experimental subjects, studied in detail the structure of their cnidosacs, trying to trace the relationship between cnidosac morphology and diet or species relatedness.

“For the first time, we have identified a clear relationship between the structure of the radula, the mechanism of food production, the spectrum of nutrition and the structure of cnidosacs. Cnidosacs of different groups of fionids are characterized by great interspecies variability in microanatomical characters, which correlates both with differences in food objects (different types of cnidocysts in different cnidarians) and with feeding mechanisms (mollusks that feed on the same species of cnidarians, but on different parts of them, will have different cnidosacs), — explains the main author of the study Irina Ekimova. “Thus, the structure of cnidosacs does not reflect evolutionary changes, but depends on the ecological characteristics of the species. The transition to an unusual type of food can lead to a complete loss of the ability of kleptocnidia - for example, mollusks that feed on fish caviar have lost the ability to select stinging capsules. The same is true for molluscs that feed on coral polyps: although they also belong to the intestinal animals, corals do not have burning stinging cells. There are only those that glue the prey. The mollusk recognizes such cells as incorrect, and selection does not occur”.