ホタルイカと光

摘要

A bioluminescent squid, Watasenia scintillans, has a specially developed eye with three vi-sual pigments of absorbance maxima of 470nm, 484nm and 502nm, respectively. In spring the largegroup of the squid come to Toyama Bay for spawning. They live in 200m - 250m depth in daytime andmove to the shallow water at night where some of them are caught by the fisherman' s set-net. In thispaper, the intensity spectra of the environmental light were monitored by the spectrophotometer, operatedon photoncounting mode, using the 10 - bundled lightguide of a 0.2mm diameter and 400m length. Theintensity spectra of the downwelling light at 200m depth at noon of clear day were comparable to thespectra of the light around 30 min. before sunset and at 75 m depth where the squid were passing tomigrate upto 10 - 20 m depth. The maximum intensity at 470 nm of 1.5 - 2 k counts/sec was ranged withinthe limit of the measuring system and about 1/5 x 105 of the downwellig light at 1 m depth under clearsky. The spectral shape of the downwelling light well matched to absorbance spectrum composed of acouple of the visual pigments. Thus the squid has advantage to utilize the wide spectral range of theenvironmental light. The daily yield of the squid caught in the set-nets in Toyama Bay was examinedstatistically, and the tendency was found that the small peak of the yield appeared first in early March andthe maximum yield occurred in the day of full moon between late April and early May. The subsidiarypeaks appeared during March - June in the days of every full moon and new moon before and after theday of the maximum yield. Semimonthly rhythm seen in the squid yield of the setnet seems to demon-strate the rhythmic spawning of W. scintillans. Weather conditions such as rain and heavy cloud in theafternoon, brought a significant decrease in the squid yield of the next morning. Because bad weathercaused the abnormal decrease in the environmental light intensity earlier than the sunset, possibly thedaily rhythm of vertical migration of the squid was disturbed.