The Composition and Structure of a Mixed Population of Crenomytilus grayanus (Dunker, 1853) and Modiolus kurilensis (Bernard, 1983) (Bivalvia: Mytilidae) in Peter the Great Bay, Sea of Japan

The composition and structure of a mixed population of Gray’s Crenomytilus grayanus and horse mussels Modiolus kurilensis were examined in November−December 2015 in Vostok Bay (Peter the Great Bay, the Sea of Japan). In the investigated area of the bottom, in the depth range from the water’s edge to 11 m, boulders were gradually replaced by strongly silted sand and the hydrodynamic activity was reduced. The pattern of spatial organization of the Gray’s and horse mussel population varied from solitary living individuals to large aggregations of 93–100 sexually mature mollusks. The population density of Gray’s and horse mussels increased with increasing depth; at a depth of 10–11 m it was an average of 101 ± 78 and 51 ± 41 ind./m2, respectively (without taking young-of-the-year into account). At different depths, Gray’s mussel spat made up 13.3 to 42.4% of the population while horse mussel spat were 1.9 to 5.2%; the proportion of mollusks of pre-reproductive size (age) varied from 56.5 to 85.6% and from 13.2 to 31.5%, respectively. The greatest multi-age composition was found in C. grayanus aggregations on boulders at 0–1 m; the age range narrowed with depth. For M. kurilensis, the opposite trend was found. The oldest individuals of Gray’s mussel (29 years) were found on boulders at 0–1 m; while the oldest individuals of horse mussel (46 years) occurred on silty sand at 10–11 m. The differences are discussed from the standpoint of the morpho-physiological adaptations of mollusks of the genera Mytilus and Modiolus to living in different biotopes.