1Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo, Tokyo 113–8657, 2Kanagawa Prefectural Fisheries Research Institute, Miura, Kanagawa 238–0237, Japan
To validate the daily formation of growth increments on the shell surface of the giant abalone, Haliotis madaka, double time-reference marks were incorporated on the shells of the juvenile abalones reared in aquaria by immersing the abalones in alizarin complexone (ALC) solution at an interval of 14 days. Double fluorescent marks of ALC were observed on the cross section of the shells under fluorescent microscopy. The mean number of growth increments on the shell surface between the two marks was 13.7 ( N=21, S.D.=1.7), which corresponds with the time interval in days between the two marks. Release-and-recapture experiments into the natural sea showed that the shell lengths at release estimated by counting growth increments from the edge toward the apex of the shell of recaptured juveniles were in accordance with the actual shell lengths at release. It was concluded that the growth increments on the shell surface of the abalone were formed daily.