Abstract The present investigation aimed to estimate the individual growth and mortality of an invasive species found in the estuarine region of Patos Lagoon (Southern Brazil). For 20 months crabs were sampled, and in the laboratory specimens of Rhithropanopeus harrisii (Gould, 1841) were measured (carapace width, CW, and weight). Carapace width data was utilized for a modal progression analysis and to estimate the individual growth curve (von Bertalanffy growth model), and a size-converted catch curve. Von Bertalanffy parameters estimated were CW∞ = 16.92 mm and k = 1.28 year-1. Maximum longevity was estimated at 3.5 years. Instantaneous total mortality was estimated at 2.17 ± 0.30 year-1. It has been suggested that longevity may be explained by a low predation impact on R. harrisii, which, as a consequence, may lead to slow individual growth of this species in the estuarine region of Patos Lagoon.

Natural diet of Neohelice granulata in two salt marshes of Lagoa dos Patos, RS were studied. Sampling was performed seasonally and crabs were captured by hand by three persons during one hour, fixed in formaldehyde (4%) during 24 h, transferred to alcohol (70%). Each foregut was weighed and repletion level was determined. Differences between sexes in the frequencies of occurrence of items were tested by χ2test. A total of 452 guts were analyzed. Quali-quantitative analyses were calculated following the method of relative frequency occurrence and relative frequency of the points. At both sites, for both sexes and in all seasons, the main food items were sediment, Spartina sp. and plant detritus. The highest values of mean repletion index were estimated for the spring and summer. Analysing both salt marshes, in different seasons significant shifts in the natural diet of Neohelice granulata was not observed throughout the period of study.

Kalliapseudes schubartii (Crustacea: Tanaidacea) é um invertebrado tubícola que habita fundos moles de regiões estuarinas e que distribui-se ao longo da costa sudeste e sul do Brasil e na costa uruguaia. Este tanaidaceo é comedor de depósitos e filtrador, e é presa de vários consumidores secundários. Taxas de mortalidade sazonal foram examinadas através de amostras feitas durante um ano num local da região estuarina da Lagoa dos Patos. Os resultados não demonstraram diferenças significativas entre as taxas de mortalidade nas diferentes estações do ano, o que sugere que populações de K. schubartii são fortemente predadas ao longo de todo o ano e que a premissa de maior mortalidade no verão/outono pode não ser correta em fundos de águas rasas não vegetados.

Studies were carried iut on seasonal mortality by taking samples of Kalliapseudes schubartii (Crustacea: Tanaidacea) for a year in the estuarine region of the Lagoa dos Patos (southern Brazil). Results demonstrated no significant difference in seasonal mortality rates, which suggest that populations of K. schubartii was heavily preyed on year round and that an assumption of a significantly higher mortality in summer/autumn than in spring/winter might not be held in unvegetated soft bottoms of the estuarine regions of the Lagoa dos Patos.