Annual cycle of growth and population structure of the estuarine crab Hemigrapsus crenulatus (Brachyura: Varunidae) off central Chile

Abstract

Estuaries and saltmarshes play a fundamental role in the life cycle of many crab species. Diverse studies show that temperature and salinity modulate abundance, size frequency distribution (SFD), sex ratio and growth in crustaceans. These population parameters are usually challenging to estimate due to the high environmental variability of estuaries. Monthly samples of the estuarine crab Hemigrapsus crenulatus were taken from October 2003 to October 2004 (except July 2004) in the Tubul estuary, central Chile. We quantified temporal changes in abundance, size distribution, sex ratio and monthly growth through the annual cycle. A total of 1025 individuals were collected. Sizes ranged from 7.72-33.51 mm carapace length (CL) with a growth rate ranging between 2.13-30.5% mm CL mo-1. Size and growth rates were greater in spring-summer, suggesting a faster growth of younger crabs correlated with increasing sea temperatures in the austral summer. Overall, sex ratio was 1.75:1 in favour of males. Modal analysis identified at least seven cohorts cohabiting throughout the annual cycle. Growth parameters for males and females were the following, respectively: L∞ = 33.6 and 29.6, k = 0.69 and 0.91, t0 = -0.39 and -0.28. Changes in size distribution suggested a recruitment period during autumn and winter seasons when there are lower salinities and temperature fluctuations stresses. Generalized linear models indicated that sea temperature, salinity and chlorophyll were the environmental variables that better predicted the annual patterns in the population structure.