Histological organisation of the bathypelagic species Protoheterokrohnia bogutskayae (Chaetognatha: Tokiokaispadellidae) in the trunk–tail region

A histological study of the unique bathypelagic species Protoheterokrohnia bogutskayae Kassatkina, 2016 (Chaetognatha: Tokiokaispadellidae) was conducted. Specimens were collected from the near-bottom layers of the Laptev Sea in the deep Arctic waters. The structure of the trunk–tail region was examined, revealing that large lateral fields are characteristic of this plankto-benthic species. The intestinal structure differs from that of other chaetognaths, with the intestinal epithelium containing secretory cells with an unusual secretion containing crystal inclusions. The composition and location of the female and male gonads were also studied. The longitudinal muscles were investigated using immunocytochemical method, which revealed the presence of type IV collagen in the extracellular matrix at the base of the muscle layer. The structure and location of the transverse muscle bundles and their attachment points were described in detail. Fins and fin rays were analysed through histological sections, showing that fin rays are located within the cytoplasm of the fin epithelial cells as cytoplasmic inclusions containing proteins. The fin rays are notably long, as illustrated in the longitudinal sections. The dense arrangement of the rays is characteristic of the species examined.