Growth patterns and population dynamics of the kelp crab Pugettia vulgaris (Decapoda, Brachyura, Epialtidae) on the coast of Sagami Bay, Japan

Abstract

Investigating the possibility of diverse patterns in habitat use of spider crabs requires to distinguish the growth phase of each sampled individual. In this study, ontogenetic stages (OS) were set on the basis of the different combinations of several secondary sexual characteristics, and used to describe the population dynamics of a small species kelp crab,Pugettia vulgarisOhtsuchi, Kawamura & Takeda, 2014. Morphometric analysis, together with an estimation of the size at 50% maturity, confirmed that each OS is a statistically independent group in both sexes, and that this species reaches Stage III (fully grown stage at terminal anecdysis) at around 4-5 mm CW in both sexes. Monthly quantitative sampling during two years, which was carried out in 11 different subtidal habitats (<0.1-8 m deep), revealed thatP. vulgarispreferred lower subtidal (2-8 m deep) red algal turfs with physically complex structures as their habitat, and will stay there throughout their entire post-settlement life history. The densities ofP. vulgarisrapidly increased during summer to late autumn due to the successive recruitments of juveniles, and the highest mean density (315 ± 72 ind./m2) was recorded at lower subtidal gelidiacean turfs (Rhodophyta, Gelidiaceae) in September 2009. Females were ovigerous at >4.28 mm CW, during spring to autumn. Stage III individuals died out by autumn. Thus, the longevity ofP. vulgarisis estimated to be approximately one year. Although the CW class frequency composition did not differ clearly, the OS frequency composition differed among populations in three algal turfs, suggesting settlement selectivity, and a different mortality among these different populations.