Copepods of the genus Acartia are found worldwide and play an important role in the zooplanktonic community. Four Acartia species (A. tonsa, A. lilljeborgi, A. danae and A. negligens), are found in Brazilian coastal waters, although only A. tonsa and A. lilljeborgi may occur on the country's northern–northeastern coast. The morphological identification of both species is based on the presence of a spine that develops on the posterior margin of the prosoma in mature A. lilljeborgi. Cryptic genetic diversity has been recorded previously in A. tonsa, but there are no published data on the genetic variability of A. lilljeborgi. In the present study, we used the mitochondrial Cytb gene to evaluate the phylogenetic relationships in samples of these two Acartia species from distinct localities on the northern–northeastern coast of Brazil. Our results indicated that there are two lineages of A. lilljeborgi and one of A. tonsa. To test for the possible existence of an additional species, we sequenced the COI gene from the samples and compared these sequences with those available for other copepods found on the Brazilian coast (A. tonsa, A. danaea, A. negligens). The new lineage was distinct from the other species, but it was previously and erroneously considered to be a cryptic species of A. tonsa. We demonstrate that potential errors of identification may be associated with the difficulties of distinguishing these species in the juvenile stages (copepodites)

The first author is grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the concession of PhD scholarship. This research was supported by CAPES (PRÓ-AMAZÔNIA 047/2012) and Conselho Nacional de Desenvolvimento Científico e Tecnológico -CNPq (473313/2013-8). The authors Marcelo Vallinoto, Nils E. Asp and Rauquírio Marinho da Costa would like to thank CNPq for their research grants (302892/2016-8, 304270/2012-1; 309845/2015-7 and 309527/2014-7, 200622/2014-5)

5 pages, 2 figures, 1 table

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