289 ISSN 0032-9452, Journal of Ichthyology, 2016, Vol. 56, No. 2, pp. 289–297. © Pleiades Publishing, Ltd., 2016. Original Russian Text © Yu.P. Sapozhnikova, I.V. Klimenkov, I.V. Khanaev, M.M. Makarov, A.A. Belous, 2016, published in Voprosy Ikhtiologii, 2016, Vol. 56, No. 2, pp. 208–216. Ultrastructure of Saccular Epithelium Sensory Cells of Four Sculpin Fish Species (Cottoidei) of Lake Baikal in Relation to Their Way of Life Yu. P. Sapozhnikova a , I. V. Klimenkov a, b , I. V. Khanaev a , M. M. Makarov a , and A. A. Belous b a Limnological Institute, Siberian Branch, Russian Academy of Sciences, ul. Ulan-Batorskaya 3, Irkutsk, 664033 Russia b Irkutsk State University, ul. Karla Marksa 1, Irkutsk, 664003 Russia e-mail: jsap@mail.ru Received November 26, 2014 Abstract—Various structural elements of the apical region of hair cells and their location in the saccular mac- ula of four sculpin fish species (Cottoidei) of Lake Baikal—two oilfish species (big golomyanka Comephorus baicalensis and small golomyanka C. dybowski), Severobaikalsk yellowfin Cottocomephorus alexandrae, and stone sculpin Paracottus knerii—were studied by scanning electron microscopy. In stone sculpin Paracottus knerii, which inhabits the coastal areas and leads a benthic lifestyle, the diversity of hair cells (in terms of the height of kinocilium and stereocilia) is big than that in the secondary pelagic species big and small golo- myankas and in Severobaikalsk yellowfin, which inhabits the near-slope areas. Stereocilia of hair cells of stone sculpin and Severobaikalsk yellowfin are shorter than in the other species studied. The presence of such cells in the macula can ensure the sensitivity to more high-frequency acoustic signals and facilitate their per- ception by fish against the background of low-frequency noises characteristic of the coastal zone of the lake. Keywords: sculpin fishes (Cottoidei), Lake Baikal, environment, auditory system, hair cells, ultrastructure, sacculus DOI: 10.1134/S0032945216010136 INTRODUCTION Fish are characterized by a great diversity of struc- tural and functional features of the auditory apparatus (Popper et al., 1993; Rowe and Peterson, 2004). How- ever, so far, only several researchers have linked the macro- and ultrastructure of the auditory epithelium (the macula) with the functional parameters of fish hearing (Salem and Zaghloul, 2001; Xue and Peter- son, 2006; Smith et al., 2011). Important characteris- tics of hearing are the frequency of perceived sounds, their threshold amplitude, and the ability of fish to detect the direction of the sound source (i.e., direc- tional hearing) (Platt and Popper, 1981; Rowe and Peterson, 2006). The mechanisms underlying direc- tional hearing are associated with the structural char- acteristic of the sensory epithelium—that is the pres- ence of local groups of receptor (hair) cells with a sim- ilar morphological polarization (Platt and Popper, 1981; Ricci et al., 2002; Kasumyan, 2005; Popper et al., 2005; Sapozhnikova et al., 2007). Morphologi- cal polarization is understood as an acentric location of the kinocilium on the apical surface of the hair cell, where the polarization vector is directed from the ste- reocilia to the kinocilium (Fig. 1). The saccular macula of fish usually includes several zones within which all hair cells have a single polariza- tion vector. The hair cells with different directions of polarization usually differ in the lengths of stereocilia and kinocilium. The sensory sensitivity of hair cells directly depends on the length of stereocilia: the larger the length, the greater the sensitivity of fish to low-fre- quency acoustic signals (Platt and Popper, 1981; Saunders and Dear, 1983; Sugihara and Furukawa, 1989; Lombarte and Fortuño, 1992; Ricci et al., 2002; Popper and Fay, 2011; Smith et al., 2011). Extensive research of sculpin fishes (Cottoidei) has shown that they are convenient objects for the study of different types of adaptations and speciation of fishes in Lake Baikal (Sideleva, 2003). Since sculpin fishes account for as much as 70–80% of all fishes of the lake (Baikal is a golomyanka- and sculpin-rich water body), the knowledge of their characteristics is also of practical value. These fishes are characterized by a high endemism and a considerable diversity of mor- phological structure and way of life, which allowed them to populate various ecological niches of the lake (Taliev, 1955; Sideleva, 1982; Sideleva and Kozlova, 2010). Previosly, we reported the schemes of location of areas with different morphological polarization of hair