Aspects of the biology of the big-handed crab,Heterozius rotundifrons (Decapoda: Brachyura), from Kaikoura,New Zealand

Abstract

From January 1975 to June 1976, samples of Heterozius rotundifrons A. Milne Edwards, 1867 were taken monthly from the intertidal zone at First Bay, Kaikoura Peninsula (42°25′S, 173°42′E). Small crabs (< 8 mm carapace width) were never common; they were either too cryptic to be collected in quantity, or have a different habitat from larger crabs. Ovigerous females were present in all months except February (in both 1975 and 1976), but generally formed less than 50% of the total sample of females each month. The number of newly deposited eggs (0.75×0.81 mm) carried by females increased with increase in carapace width according to the equation y = -1123.56+102.97x (r ² = 0.8213). Egg development lasted 3–5 months, and egg mortality during this period was almost 10%. The overall sex ratio for the 18-month sampling period was 1846 ♀ : 993 ♂, which suggests that females were approximately twice as numerous as males. However, when crabs were sorted into size classes it was evident that the sex ratio was not significantly different from 1:1 in crabs of 6–12 mm carapace width, whereas females significantly (P<0.001) outnumbered males in the 13–21 mm size range. The right and left chela were approximately equal in length in females of all sizes, but the right chela of large males was greatly enlarged compared with the left, and with the chelae of females of comparable carapace width. Increase in the growth rate of the males’ right chela commenced at a carapace width of approximately 11.0 mm. Since the smallest ovigerous female collected also had a carapace width of 11.0 mm, it is concluded that both males and females attain sexual maturity at this size.