This paper deals with the number of times of molting and the mean carapace length in instars estimated from the frequency distribution of carapace length of the edible horse crab Erimacrus isenbeckii collected in the coastal sea area of Eastern-Hokkaido, and with the experimental data of increase at molting.
From size frequencies of female and male of 1091 individuals, under 50 millimeters, nine groups are recognized. The mean carapace length for each group being 5.2, 7.0, 8.7, 12.5, 15.6, 20.4, 27.4, 33.5 and 46.4mm. From 5572 male individuals, ranging from 50 to 124mm, five groups are recognized. The mean carapace length for each group being 59.1, 73.3, 88.2, 103.5 and 117.5mm. From 3884 female individuals, range 50-88mm, four groups are recognized. The mean carapace length for each group being 55.9, 65.8, 74.1 and 84.0mm.
Applying these mean sizes to Hiatt's graphic method in which the post-exuvial lengths (Ln+1) were plotted against the initial lengths (Ln), the following formulas are obtained: male and female under 50 millimeters, Ln+1=1.336Ln-0.40; mature female, Ln+1=0.998 Ln+9.49; mature male, Ln+1=1.036Ln+11.68.
Growth of the carapace length in instars fits the growth equation as follows: male, lt=177/(1+e3.765-0.269t-0.0035t2); female, lt=125/(1+e3.451-0.293t-0.0017t2).
