A team of biologists led by a malacologist from the Faculty of Biology of Moscow State University studied common tropical nudibranchs Coryphellina rubrolineata. Molecular genetic analyses, as well as the study of the external and internal structure of animals have shown that this is not one species, but at least five. As a result, the authors described four new species of nudibranchs that differ in mitochondrial and nuclear genes, as well as stripes on the body.
Molecular genetic methods have allowed scientists to better identify cryptic (similar in appearance, but genetically different) and pseudocryptic (different in appearance, but this becomes clear only after applying genetic methods) species and species complexes. With their spread, the hypothesis of the leading role of geographical barriers and spatial isolation in speciation began to prevail in taxonomy. Often, remote populations of one species with a wide range began to be divided into several species, based solely on genetic comparison data, which is not always justified. Recent studies in the field of evolution and phylogeny emphasize the great role of the ecological factor in speciation: organisms can diverge into different ecological niches within the same geographical area, acquiring new features and isolating themselves "according to lifestyle".
In modern taxonomy, an integrative approach to the study of species becomes important, when not only individual aspects (morphology, anatomy, ecology, DNA sequences) are evaluated, but a number of features in aggregate. This approach makes it possible not only to describe a new species, but also to understand its evolution, and sometimes even the history of the development of the fauna of an entire region.
The species Coryphellina rubrolineata, which interested Russian scientists, was first described from samples from the Red Sea in 1929, even before the spread of molecular methods. Then this nudibranch began to be found in the Mediterranean Sea, the Indian and Pacific Oceans. The main identifying feature of the species were three bright stripes running along the entire body. However, their color, discontinuity, width, proximity to each other, as well as other signs, varied in different individuals.
Employees of Lomonosov Moscow State University together with colleagues from the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences studied C. rubrolineata individuals from Vietnam as part of the work of the Russian-Vietnamese Tropical Center. To find out whether it is really a single species or a complex of cryptic or pseudocryptic species, the authors turned to molecular genetic methods.
In total, 28 Coryphellina individuals were selected in the waters of Vietnam: each of them was photographed, and then fixed for further research. Based on DNA sequencing data, the authors constructed phylogenetic trees reflecting the evolutionary relationship and assessed the degree of differences between the samples. The animals were also studied by microscopic methods, and scientists paid special attention to the reproductive and digestive systems. As a result, it turned out that the objects did not belong to the same species, C. rubrolineata, and to five, of which four were first described in this work: Coryphellina pseudolotos, Coryphellina pannae, Coryphellina flamma and Coryphellina aurora.
"Most of the species we have identified show extremely high divergence, that is, divergence, in mitochondrial genes and, more unusually, in nuclear genes. This may indirectly indicate that they are of some antiquity. The most significant differences were the signs of coloration and, in particular, the type of red stripes: in some species they are intermittent, in others they are extended along the entire length of the body, in others they are absent at all," explained Irina Ekimova, the project manager for the RNF grant and the first author of the study, Candidate of Biological Sciences, senior researcher at the Faculty of Biology of Moscow State University. — Their general coloration is also different, although it can vary quite a lot within the same species. At the same time, the signs of the internal structure of the new species are very conservative, which is probably due to similar food preferences and breeding strategies."
From such revisions — discoveries of new species within a group or, conversely, combining several into one — evolutionary trees of small and then large groups of organisms are built. If we look at these data through the prism of space and time — that is, where, when and how new species "budded" — scientists will be able to reconstruct the entire history of the fauna of the regions, going beyond the objects they study. Nudibranchs, due to different breeding strategies and settlement abilities, act as a convenient model object in this regard.
The study was supported by a grant from the Russian Science Foundation, the article was published in the journal Diversity
PHOTO: Collage with photos of Coryphellina lotos (left) and four newly described species:
(left to right) C. pseudolotos, C. pannae, C. flamma, C. aurora © Irina Ekimova, Faculty of Biology, Moscow State University

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