A NEUROANATOMICAL STUDY OF THE BLACK DRAGONEYE'S RETINA AND OPTIC TECTUM IN COMPARISON WITH THE RED CRUCIAN'S

The black dragoneye, a kind of dragoneye goldfish (Carassius auratus), has evolved from the red crucian. Four black dragoneyes and four red crucians, each being 10—12 cm long and weighing 35 g, were taken randomly and dissected. Their brains and eye-balls were observed using routine paraffin section and HE staining. The microscopic structures of the retinas and optic tectums of black dragoneye and the red crucian were studied under an optical microscope. We found that, compared with the red crucian, the retina of black dragoneye was 29.9% thinner, of which the outer plexiform layer increased 2.5%, and the inner plexiform layer increased 11.8%, while the inner nuclear layer decreased 21.6%, and the outer nuclear layer decreased 35.6%. Besides, the ganglion cell layer and the photoreceptor layer were thinner and irregular. However, optic tectum in black dragoneye was 28.9% thicker than that of the red crucian, except that the stratum periventriculare layer decreased 22.6%, all layers were incrassate, with the stratum album central layer increased 12.8%, the stratum griseum central layer increased 30.6%, the stratum fibrosum et griseum superficiale layer whose fibers were 21.9% denser than that of the red crusian, the stratum opticum layer was 91.7% thicker and the stratum marginal layer was 35.6% thicker. The results showed that the central nervous system of black dragoneye varied widely due to artificial selection and breeding. It can be inferred that the larger eyeball diameter and retina area of the black dragoneye resulted in an increase of fibers transmitted from the retina to the optic tectum, which probably was the main cause for the thicker layers of neurons and nerve fibers that transmit nervous impulse. To reduce the damage caused by strong light, the pigment epithelium layer moved to the photoreceptor layer interlacedly, and the inner nuclear layer of and outer nuclear layer the retina as well as the stratum periventriculare layer in its optic tectum became thinner. In addition, the thickness of the stratum periventriculare layer decreased, which can help to explain why the black dragoneye is not so good at swimming and balancing as the red crucian.