Discordant Morphological and Genetic Differentiation between Fourhorn Sculpin Triglopsis quadricornis (Teleostei: Cottidae) from the Baltic Sea and Lake Ladoga

The marine species Triglopsis quadricornis (Linnaeus) colonized freshwater lakes of glacial genesis. The presence of freshwater forms of Triglopsis has been noted for 11 lakes in Europe, including Lake Ladoga. We conducted a comparative study of morphological, ecological and genetic differences between the marine form of T. quadricornis from the Baltic Sea and the freshwater form from Lake Ladoga. These forms differ significantly in osteological features related to skeletal lightening: reduction of bony protuberances on the head, thinning of bones and increased size of cavities in the bones of the skull, reduction in the number and size of symmetrical and asymmetrical bone plates on the trunk. In addition to osteological features, the Ladoga form undergoes miniaturization of the body, which is associated with a decrease in the reproductive potential of fish. This is expressed in a significant decrease in the number of eggs in the ovaries of the Ladoga form females compared to those of the Baltic fishes. Successful colonization of Lake Ladoga was supported by the presence of such abundant and universal food in its bental zone as amphipods and mysids. These organisms dominate in the diet of the Ladoga fourhorn sculpin.