Age determination and growth rate of Mactra chinensis (Bivalvia: Mactridae) by external rings and chondrophore growth bands

Age, growth and mortality of Mactra chinensis were investigated during the period from October 2012 to September 2013 in Busan, South Korea. The monthly variation of the marginal index (MI) of the shell and chondrophore showed that the ring of this species was formed once a year during July. We estimated the age of M. chinensis by reading the external rings on the shell and the growth bands of the chondrophore to compare growth parameters between the two growth characters. The age of this species ranged from 0 to 8 years (shell-based age reading) and from 0 to 10 years (chondrophore-based age reading). Based on external rings and growth bands of chondrophore for the same period, the von Bertalanffy growth functions were expressed by the equation, Lt = 101.53[1-exp {-0.15(t + 0.75)}] and Lt = 90.03[1-exp {-0.20(t + 0.50)}], respectively. The likelihood test showed that there was a significant difference in L∞ (P < 0.001), K (P < 0.001), to (P < 0.001) estimated from non-linear regression between the two growth characters.