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This study deals with the organization of the coelom in the all the parts of the body of the vesti mentiferan Oasisia alvinae. The localization of the dissepiment between the vestimental and trunk regions in males and females differs. The histological structure and anatomy of the trophosome and the vascular, excre tory, and reproductive systems is described. Three cell types are distinguishable in each trophosome lobe. Up to four bacteriocytes could be found in a row from the central blood vessel to the surface of the organ. The main blood vessels in almost all parts of the body have a well developed muscle sheath and an endothelium that is expressed in varying degrees. In the trunk region, an intravasal body, which is represented by two mod ifications, is observed in the dorsal blood vessel. The excretory tree is located under the brain. The reproduc tive system is symmetrical in males and dissymmetrical in females. The gonoducts of females are extensive; in males, they are short and open in the front part of the reproductive coelom. The genital coeloms of both sexes perform the functions of gamete production and storage.

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Only seven frenulate species are currently known along the Eurasian coast of the Arctic Ocean. We describe a new genus and a new species of frenulates Crispabrachia yenisey, gen. nov. et sp. nov. The morphological analysis involved standard anatomical techniques, semithin sections and scanning electron microscopy (SEM). The molecular study included four markers (partial COI, 16S, 18S and 28S) and implemented Bayesian and Maximum likelihood phylogenetic approaches. The description of Crispabrachia gen. nov. is the first documented finding of frenulates in the Kara Sea at the estuary of the Yenisey River in rather shallow water (28 m). The establishment of a new genus is warranted based on the composition of morphological characters and several specific features including free, comparatively short curly tentacles, a triangular cephalic lobe with amplate base, the valvate extension of the posterior part of the forepart and prominent papillae on the nonmetameric region. The tube structure with prominent frills and the worm’s numerous tentacles, metameric papillae with cuticular plaques and segmental furrow on the forepart indicate that the new genus belongs to the polybrachiid group. Although the type locality in the Yenisey River estuary is unusual for siboglinids in general, the physical conditions here are common for other frenulates habitats, i.e. salinity ~30–33, bottom water temperature –1.5°C. This finding was made in the Yenisey Gulf in the region with the highest methane concentrations in the southern part of the Kara Sea that reflects permafrost degradation under the influence of river flow. Further study of the region would help to understand the factors influencing frenulate distributions and improve our knowledge of their biodiversity.