Population dynamics of the burrowing ghost shrimp Callianassa filholi on an intertidal sandflat in New Zealand (Decapoda:Thalassinidea)

Population samples of the burrowing ghost shrimp Callianassa filholi Milne-Edwards, 1878 were taken on an intertidal sandflat in Otago Harbour, south-eastern New Zealand, monthly over a period of 16 months. The spatial population distribution appeared to be contagious in autumn and early winter, and random from mid-winter to summer. The population density did not show any significant difference over the sampling period, with a mean density of 16 individuals m(-2). Overall the sex ratio was significantly biased towards females (mean male:female ratio = 1:1.5). Visual inspection of size frequencies indicated the presence of a bimodal population. Large animals (7-14 mm carapace length [CL]) were present throughout the year and no significant difference in CL was observed between sexes. Post-larval shrimp (CL less than or equal to 3 mm) were present in spring and summer, which together with developing juveniles constituted a second group of smaller animals (< 7 mm CL). Regression analysis of allometric growth showed the size of maturation to be 8 mm CL for females and 9 mm CL for males. Only females of CL greater than or equal to 8 mm carried eggs and these ovigerous females were present from mid-winter to mid-summer, with all reproductive females bearing eggs in November (spring). Data obtained suggest that the Callianassa filholi population is stable, with low mortality and recruitment rates and that individuals have a lifespan of 2-3 years.