The freshwater copepod Odontodiaptomus thomseni (Brehm, 1933) (Calanoida: Diaptomidae) is a rare species that has been reported only once - in its original description (BREHM 1933). The lack of subsequent records led to its inclusion in the Red List of threatened species (IUCN). Here we present a new record for O.thomseni. It was discovered in Salto Grande reservoir, which is located in the lower stretches of the Uruguay River, between Uruguay and Argentina, at the River Plate basin. In January 2010, three specimens (two males and one female) were found, and these were studied in detail using scanning electron microscopy (SEM). We only had material of Odontodiaptomus paulistanus (Wright, 1936) for comparison, but the position of the lateral spine in right P5 of the male, and the shape and size of lateral wings of the female are especially distinctive. Odontodiaptomus thomseni remains a rare species and we recommend keeping it on the IUCN Red List.

Durante estudos sobre a diversidade de copépodes harpacticóides meiobentônicos marinhos, um novo representante da família Tachidiidae foi encontrado na plataforma continental interna do estado de São Paulo, entre o canal de São Sebastião e a área costeira em frente á praia de Ubatumirim, Ubatuba (23º24'S, 44º57,6'W). Material complementar foi coletado próximo à praia da Enseada, Ubatuba (23º30'S, 45º05'W). Embora esta nova espécie compartilhe com M. laurenticum (Nicholls, 1940) a redução das antênulas, ela pode ser facilmente distinguida de seus congêneres pela presença de apenas 2 artículos no endópodo da perna 4 e pela redução no número de cerdas internas do endópodo-3 das pernas 1-4 e do exópodo-3 da perna 3. Os Tachidiidae, que tipicamente habitam sedimentos finos de águas rasas salobras e marinhas do Hemisfério Norte, são considerados bioindicadores de ambientes ricos em material orgânico. Este é o primeiro relato da família no hemisfério sul.

During studies on the diversity of meiobenthic marine harpacticoid copepods, a new representative of the family Tachidiidae was found on the inner continental shelf of São Paulo State, between São Sebastião Channel and Ubatumirim Bay, Ubatuba (23º24'S, 44º57,6'W). Complementary material was collected near Enseada Beach, Ubatuba (23º30'S, 45º05'W). Although the new species shares with M. laurenticum (Nicholls, 1940) the reduced antennules, it can be easily distinguished from its congeners by leg 4 endopod with only two segments and the reduction of the number of inner setae of legs 1-4 endopod-3 and leg 3 exopod-3. Tachidiids, typically inhabiting fine sediments of brackish and marine shallow waters in the Northern Hemisphere, are good indicators of heavy organic pollution. This is the first record of the family Tachidiidae in the southern hemisphere.