Life history of Tanais dulongii (Tanaidacea: Tanaidae) in an intertidal flat in the Southwestern Atlantic

Abstract

We studied the life history, reproductive biology and spatial distribution of Tanais dulongii on an intertidal flat near Mar del Plata, Argentina. The animals were obtained by systematic sampling in three intertidal zones (high, mid and low), from October 2008 to September 2009. The population density was low during most of spring and summer, increased during autumn and reached its maximum values at the end of this season (35 000 individuals/m2); a second, but lower, density peak occurred at the end of winter (15 000 individuals/m2). Male density remained below 1000 individuals/m2during most of the year in the three zones, although in September it was higher than 1800 individuals/m2in the high and mid intertidal zones. Female and juvenile density was below 5000 individuals/m2in spring and summer, with little variation between areas, but it differed among areas during autumn and winter, when both groups reached their maximum densities (20 000-40 000 individuals/m2) in the low and mid intertidal zones. Ovigerous females were always present; their maximum occurred in spring and summer but earlier in the low and later in the high intertidal zone. Recruitment was higher in autumn and early winter. The sex ratio was strongly female biased (0.08 ± 0.01). Individual life time was estimated to be 8-9 months and females developed through more instars than males. This study suggests that the different environmental conditions that T. dulongii faced in the 3 intertidal zones caused an important effect on the population dynamics.