The objective of this study was to gain histological insights into the retinomotor response of swordtip squid (Uroteuthis edulis) to determine the state of retinal adaptation under fishing light. In this study, the retinomotor response of swordtip squid under a light condition and dark condition was examined histologically. Squid under the light condition were irradiated by a halogen lamp from above the experimental tank for 2 hours. On the other hand, squid under the dark condition were in an experimental tank that was closed and completely covered with a blackout curtain for 2 hours. For both conditions, the thicknesses from the limiting membrane to the retinal surface and the black pigment layer were measured from tissue specimens, and the ratio (%) was calculated, which was used as an index of retinal adaptability. Results showed the index of retinal adaptation for the dark-adapted condition was 33.3% for a mantle length of 223 mm and 25% for a mantle length of 164 mm. On the other hand, the index of retinal adaptation for the light-adapted condition of two tested individuals with mantle lengths of 200 mm and 218 mm was 100% for both. The results suggested that the retinal adaptation state can be judged by observing the layer of black pigment in the retina as in the case of Neon flying squid and Japanese flying squid, which are species of Oegopsida.
retinomotor response
swordtip squid
light and dark adaptation
black pigment
Fishing technology